Forces and Motion Teacher Guide
Be sure motion and force sensors are operating properly by testing the USB interface before class starts. You will need to install the Concord Consortium Sensor Connector by clicking the free Download button in any of the three activities. Some districts may require an IT technician to set permissions for the download. You can find USB interface devices and probeware from several vendors. For a list, see our Supplier Index link: probesight.concord.org/vendors This module uses a force sensor and motion sensor. The cost for the sensors plus USB connector will range from $225-250. Some vendors offer package deals that include an array of sensors and reduce the per-unit price.
Four class periods: 1 for each activity and one to complete wrap-up or Further Investigation
Materials: List of required materials are displayed in each activity. Both a motion sensor and force sensor are needed to complete the full module.
Thinking about Discovery Questions
This module uses sensor devices to explore both motion and force. Each activity is designed to help students relate the shape of the graph to either motion or to applied force and will be able to see graphically why simple machines produce a mechanical advantage. Students will investigate the following driving questions:
Accelerating Bodies: What is the shape of the graph of an accelerating toy car?
Inclined Planes: How much does an inclined plane reduce the force needed to move an object from one level to a higher level?
Levers: How does using a lever change the force needed to lift an object?
Students commonly confuse the idea of accelerating as meaning "to go fast". The Accelerating Bodies activity is a great way to reinforce the correct meaning -- that acceleration is a change in motion. In physics and physical science, "accelerate" can mean either speeding up or slowing down. It's also common for students this age to confuse acceleration with velocity. Velocity is a rate of change in position. Students may best understand velocity as the speed of something "in a given direction". When an object travels the same distance every second, it is moving with constant velocity. Acceleration is the rate at which an object CHANGES its velocity. At this age, most students won't be able to understand the concept of constant acceleration -- when an accelerating object changes its velocity by the same amount each second.
Students at this age can expand their understanding of pushes and pulls. A force is a push or pull upon an object that results from its interaction with another object. Forces only exist as the result of an interaction. A persistent misconception is that objects at rest have no forces acting on them. It requires explicit instruction to help children overcome this erroneous idea. Another very common misconception is that moving objects have a force within them that keeps them moving. Elementary school students may tend to believe (incorrectly) that it will be easier to pull an object up a steep ramp than a gentle ramp because objects roll faster down a steep ramp. These erroneous ideas are addressed very effectively in the Inclined Plane Activity.
- Performance Expectations
- 3-PS2-2. Make observations and/or measurements of an object's motion to provide evidence that a pattern can be used to predict future motion.
- 3-5-ETS1-3. Plan and carry out fair tests in which variables are controlled and failure points are considered to identify aspects of a model or prototype that can be improved.
- MS-PS2-2. Plan an investigation to provide evidence that the change in an object's motion depends on the sum of the forces on the object and the mass of the object.
- MS-ETS1-3. Analyze data from tests to determine similarities and differences among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success.
- Disciplinary Core Ideas
- Developing Possible Solutions
- Tests are often designed to identify failure points or difficulties, which suggest the elements of the design that need to be improved.
- There are systematic processes for evaluating solutions with respect to how well they meet the criteria and constraints of a problem.
- Forces and Motion
- The patterns of an object's motion in various situations can be observed and measured; when that past motion exhibits a regular pattern, future motion can be predicted from it. (Boundary: Technical terms, such as magnitude, velocity, momentum, and vector quantity, are not introduced at this level, but the concept that some quantities need both size and direction to be described is developed.)
- The motion of an object is determined by the sum of the forces acting on it; if the total force on the object is not zero, its motion will change. The greater the mass of the object, the greater the force needed to achieve the same change in motion. For any given object, a larger force causes a larger change in motion.
- Relationship Between Energy and Forces
- When two objects interact, each one exerts a force on the other that can cause energy to be transferred to or from the object.
- Analyzing and Interpreting Data
- Analyze and interpret data to provide evidence for phenomena.
- Constructing Explanations and Designing Solutions
- Apply scientific principles to design an object, tool, process or system.
- Engaging in Argument from Evidence
- Support an argument with evidence, data, or a model.
- Planning and Carrying Out Investigations
- Collect data to produce data to serve as the basis for evidence to answer scientific questions or test design solutions under a range of conditions.
- Science and Engineering Practices
- Science disciplines share common rules of obtaining and evaluating empirical evidence.
- Using Mathematics and Computational Thinking
- Use mathematical representations to describe and/or support scientific conclusions and design solutions.
- Crosscutting Concepts
- Cause and Effect
- Cause and effect relationships are routinely identified and used to explain change.
- Cause and effect relationships may be used to predict phenomena in natural systems.
- Interdependence of Science, Engineering, and Technology
- Knowledge of relevant scientific concepts and research findings is important in engineering.
- Engineering advances have led to important discoveries in virtually every field of science, and scientific discoveries have led to the development of entire industries and engineered systems.
- Similarities and differences in patterns can be used to sort, classify, communicate and analyze simple rates of change for natural phenomena.
- Graphs and charts can be used to identify patterns in data.
- Patterns in rates of change and other numerical relationships can provide information about natural systems.
- Scientific Knowledge Assumes an Order and Consistency in Natural Systems
- Science assumes consistent patterns in natural systems.
- Science assumes that objects and events in natural systems occur in consistent patterns that are understandable through measurement and observation.
- Stability and Change
- Small changes in one part of a system might cause large changes in another part.
- Structure and Function
- Structures can be designed to serve particular functions.
- Systems and System Models
- A system can be described in terms of its components and their interactions.
NSES Physical Science – Motion and Forces
The motion of an object can be described by its position, direction of motion, and speed. That motion can be measured and represented on a graph.
NSES Physical Science – Motions and forces
If more than one force acts on an object along a straight line, then the forces will reinforce or cancel one another, depending on their direction and magnitude. Unbalanced forces will cause changes in the speed or direction of an object’s motion.
Discussion: Setting the Stage
Basic Vocabulary: Students need simple, but working definitions of the following words:
Position: An object's location in relationship to a reference point. If this seems tough for kids, have them try choosing four things in the classroom and completing a table like the one below:
Speed: Speed is the rate an object covers a distance. For kids, an easy definition is "speed is how fast an object is moving". Explain that you can measure speed many ways, like miles-per-hour or meters-per-second. Imagine a person running laps on an outdoor track. The person's speed measures how fast they are going around the track. Instantaneous speed can be measured at any given second. Average speed measures how fast they went averaged over time. We calculate average speed as Distance/Time. Our car speedometers show us our instantaneous speed, but not our average speed.
Velocity: The rate at which an object changes its position. Make sure kids realize velocity is dependent on direction (physicists refer to velocity as having magnitude and direction). Here's a way to help kids get it: Student 1 -- walks at a quick pace 12 steps east. Student 2 -- takes one step back and one step forward, six times (total of 12 steps). Student 1's velocity can be determined by Displacement/Time. This should produce an average velocity of ~ 2 feet/sec East. Student 1's velocity is zero because she will end up at her starting point with no displacement.
Acceleration: The rate an object increases or decreases its velocity. An object is accelerating if it is changing its velocity, so an object is accelerating if it is either speeding up or slowing down. Examples: A sprinter accelerates off the blocks to get up to top speed. A driver brakes to slow down for a stoplight. We calculate acceleration this way: Change in velocity divided by Time.
Force: For children this age, we go beyond the simple definition of force as a push or a pull. We introduce the idea of balanced and unbalanced forces. Objects at rest typically have multiple forces acting on them, but these combined forces add to zero. Unbalanced forces (that do not sum to zero) can cause changes in speed or direction of motion. It's important for students to understand that all objects in contact exert forces on each other. Each force has both strength and direction.
As teachers, we want to introduce acceleration in bite-size pieces for children in this age group. This activity looks ONLY at Position vs. Time graphs of moving objects. Teachers may want to take a quick look back at motion graphing concepts. Position vs. Time graphs and Velocity vs. Time graphs are quite different in appearance (see above).
For a refresher on the physics of speed, velocity, and acceleration, try The Physics Classroom interactive tutorial: http://www.physicsclassroom.com/Physics-Tutorial/1-D-Kinematics
Position vs. Time graphs can be tricky because the shape of the graph doesn't match how the motion looks. Here's a comprehensive tutorial for teachers that looks deeply at the P/T graph: http://www.physicsclassroom.com/class/1DKin/Lesson-3/The-Meaning-of-Shape-for-a-p-t-Graph
Discussion: Formative Questions
Accelerating Bodies: Predict what a graph of position vs. time of a toy car traveling at a steady (constant) speed would look like. Draw your prediction of the position graph (using the Draw tool). Teachers: Any drawing is acceptable here and predictions can vary greatly. Most of your students will have little prior experience with P/T graphs. In fact, when they see the graphs generated by the activity, it will be a discrepant event for many of them. That's an important step in inquiry-based learning. Ask students to take a snapshot of their graph prediction and type their reason for drawing it as they did.
Inclined Planes: Which do you think would make it easier to slide a load from the floor up to a chair: a steep ramp or a gradual ramp? The accurate answer is that a gradual ramp provides a greater mechanical advantage because it allows a smaller amount of force to be applied over a greater horizontal distance. Many kids will incorrectly predict that the steep ramp makes it easier because they think more about a steep slide making objects go faster as they descend. Elicit student reasoning, but let the kids discover the correct answer by doing the activity.
Levers: Where should you put the fulcrum of a lever so the force needed to lift an object is less than the weight of the object itself? Most students have been exposed to levers but recognize only a few (crowbars, the claw of a hammer). You may want to provide some examples of levers -- wheelbarrows, a human forearm, scissors, pliers, a seesaw. In this activity, kids will assemble a Class 1 Lever system, where the fulcrum is placed between the load and the effort (as in a balance scale, seesaw, or crowbar). The answer to the question is that less force will be required to lift the object if the fulcrum is closer to the load (weighted object). Let kids discover this with the force sensor.
A Word About Motion Sensors
Have fun playing with the sensors! It's almost impossible to move a toy car at a constant speed, but with practice, learners can produce a graph that's close to what they could see on graphing calculators. For the graphs above, the toy car was controlled by a 5th grader after lots of trials. Using a ruler, he could see that the lines are pretty straight, once the car started moving at the 1-second marker. It was also obvious that the line was much steeper in the second graph, where he moved the car faster.
Accelerating Bodies: Collect Data 1
Above are 4 simple graphs that show the slope of the line for objects moving at constant velocity. Your students are asked in Data Collection #1 to move their toy cars "at a steady speed" (aka constant velocity). This is nearly impossible, but if they practice, the graphs generated by the software should look roughly similar to those above. The faster the speed, the steeper the slope of the line. If the cars are moving AWAY FROM the motion sensor, the graph will produce a line that slopes up and to the right. If the car is moving TOWARD the motion sensor, the graph produces a line that slopes downward.
Collect Data 1:
Question 1: See Graph above left. How would you describe the shape of the graph when you moved the car toward the sensor at a steady speed? Answer: The line was pretty much straight and sloped downward. There are some squiggly sections where the speed wasn't controlled perfectly.
Question 2: See Graph above center. How was the graph different when you moved the car at a faster speed? The faster speed resulted in a steeper line. (See images at center and right above)
Question 3: See graph above right. How would you describe the shape of the graph when you moved the car AWAY from the sensor at a steady speed? Answser: The line slopes up and to the right.
Accelerating Bodies: Collect Data II
Important Concepts about Acceleration: In physics, speeding up and slowing down are both forms of acceleration. The term "decelerate" will be taboo when kids reach high school. You may want to simply use the phrases "speeding up" or "slowing down" and review the meaning of acceleration (a change in an object's velocity). A second important idea goes back to the word "position". This activity plots graphs of Position vs. Time. We are looking at a plot of an object's position in relation to the motion sensor. Faster acceleration produces a steeper curve; gradual acceleration is a more gentle curve.
Question 1: See graph above. How would you describe the shape of the graph when you sped up the car going towards the sensor? Answer: Student responses can vary depending on their success in manually accelerating the toy car. In general, their graph should show a curve similar to the one directly above.
How would you describe the shape of the graph when you slowed down the car going towards the sensor? See Graph above. It will be harder to capture the motion as the car slows down in moving toward the sensor. Let the kids discover how the two graphs will differ. You should see a slope that looks similar to the drawing above, but remember -- it's VERY hard to manually produce a smooth deceleration in a toy car. To get a smooth motion, you can try using air track carts, available at all science supply stores.
Question 2: How would you describe the shape of the graph when you sped up the car going away from the sensor? See graph below.
How would you describe the shape of the graph when you slowed down the car going away from the sensor? See graph below.
Collect Data III -- Toy Car on Ramp
Question 1: How would you describe the shape of the graph when the car rolled down a gentle ramp? Answer: See graph ABOVE. The line is curved like an upside-down parabola. It goes from left-to-right and slopes downward.
Question 2: How did the shape of the graph change when the ramp got steeper? Answer: The graph will show a much steeper decline, with a thinner parabola.
1. How does a graph of an object at constant speed look? Answers will vary. Acceptable responses will include: The line is pretty much straight. It slopes upward moving away from the sensor and downward moving toward the sensor. When the line is closest to the bottom, that is where the car is closest to the sensor. The line wasn't perfectly straight because my hand couldn't control the speed perfectly.
2. How does a graph of an accelerating object look? Acceptable answers will include: The graph isn't a straight line -- it's a curve. If the car speeds up very rapidly, the graph's curve will be steeper.
3. How does a graph of a decelerating object look? Acceptable answers will include: The graph isn't a straight line -- it's a curve. If the car is rapidly slowing down, the curve will be steeper.
Place the sensor at the top of the ramp and let the toy car roll down the ramp AWAY from the sensor. How would you describe the shape of the graph when the car rolled down the ramp? Responses will vary, depending on the height of the ramp. Higher ramps will produce a steeper curve on the graph. Generally, the graphs should appear similar to below:
Inclined Planes Activity
Materials: List of materials is displayed near the top of the Inclined Planes student activity page.
Discovery Question: How much does an inclined plane reduce the force needed to move an object from one level to a higher level?
Opening Discussion: Setting the Stage
Students should've been exposed to simple machines in the prior two years. Prompt them to recall their study of inclined planes and explain how they make our work easier. Ask them to think of ways people use ramps and other kinds of inclined planes. Possible responses: sloped roadways, playground slides, water slides, ramps on moving trucks, ramps used to construct the Egyptian pyramids. Tell them today's activity explores how ramps make it easier to move something from a lower to a higher level.
Vocabulary Needed: Mechanical Advantage -- the advantage gained by using a machine to make tasks easier. Teachers: Explain that physics shows us how to use an easy way to calculate mechanical advantage and they will be doing that in the Inclined Planes activity.
Collect Data I
Try lifting the load (block of wood) straight up from the floor to the chair. Give students time to practice to get a smooth motion. When they have a graph that looks relatively smooth, they can save it as a snapshot. SEE IMAGE BELOW: The image was produced by a 5th grader, who used the graphing tools to explain different points on the graph. Tip: you may want to ask students to release the tension on the string after the block reaches the chair top. It's interesting to see that even a slight tension in holding the string will produce 4-5 Newtons of force on the block.
Collect Data II
Move the block from the floor to the seat of the chair, using a steep ramp.
Question 1-Study the graph and determine the average force you used to get the block up to the chair using the ramp. Response: The steep ramp isn't going to give much, if any, mechanical advantage over vertically lifting the small wood block. Expect students to be surprised by this. Results will vary depending on the ramp surface and the incline of the plane. In the example above, the average force was around 13.5 Newtons. This compares to the average force of 15 Newtons to lift the block vertically. NOTE** The average force is different from the mechanical advantage. This calculation will be applied in the Further Investigation.
Question 2- How did the average force needed to pull the block up the ramp compare to the force of lifting it straight up? Answer: It was slightly lower, but not much lower.
Collect Data III
Repeat the activity by building a longer ramp that will create a gentler slope from the floor to the chair top. Wrap a piece of wax paper around the bottom of the block to reduce its surface friction. Teachers: We recommend you separate this into two activities. First, just use the gentler ramp with the same block to generate a graph of Applied Force vs. Time. Then conduct the experiment again using the wax paper to reduce friction on the ramp. The graph below shows ONLY what happened with the longer, gentler ramp.
Questions: Study the graph and find the average force you used to pull the block up this ramp. Responses will vary based on the length of ramp built by the students, but the graph should generally reflect significantly reduced applied force needed to pull the block up the ramp. In this case, the average force for the more gradual ramp was about 7.5 Newtons.
Question 2: What is the difference in the force required to move the load up the gradual ramp, compared to the steeper ramp and lifting the block straight up: Answer: the gradual ramp required a lot less force. The steep ramp required a little less force to raise the block, as compared with a vertical lift.
1. How did using a ramp affect the amount of force needed to move a block from the floor up to a chair? Students should state that the gentler the slope, the less force was required to move the block up the ramp. The tough part will be trying to explain "how" the ramp causes work to be easier. The accurate answer is that a gradual ramp provides a greater mechanical advantage because it spreads the force over a greater horizontal distance. The ramp makes task easier not by changing the amount of work done, but by altering the way the work is done. There are two important components of work as a physics term -- force and distance. They illustrate an inverse relationship: As distance goes up, the force goes down. As distance goes down, the force goes up.
2. What effect does the steepness of the ramp have on the amount of force needed to move the block up the ramp? Answer: The gentler the slope of the incline, the less force is required to move the block up the ramp. The steeper the slope of the incline, the greater force is required.
3. Even though the ramp might have made it easier to move a load from one level to a higher level, there was a trade-off or disadvantage. What do you think that trade-off might be? Responses will vary, but acceptable answers will include, "It takes more time to take the load over a long ramp," or "Gentle ramps take more time to build and take up more space than a steep ramp."
Use math to calculate the mechanical advantage of using a ramp to move a load from a lower to a higher level.
Activity: Students will use the formula for calculating mechanical advantage of an inclined plane: Length of ramp divided by height (L/h). You can set up the experiment in several ways. The images above show how the mechanical advantage compares between a steep and a gentle ramp. Recommendation: Change only one variable. For example, we kept the ramp length the same for the experiment graphed above. Our ramp was 42" in length and we moved it from a higher to a lower vertical height. Our variable was the vertical height.
Materials: List of materials is displayed near the top of the "Levers" activity
Discovery Question: How does using a lever change the force needed to lift an object?
- Mechanical Advantage -- the advantage gained by using a machine to make tasks easier. Teachers: Explain that physics shows us how to use an easy way to calculate mechanical advantage and they will be doing that in the Inclined Planes activity.
- Lever -- A stiff bar, board, or stick that you push or pull against a fulcrum to move something. People have been using levers for 30,000 years to move heavy objects like rocks more easily.
- Fulcrum -- The support the lever moves on when it is used to move an object See image below
Opening Discussion: Setting the Stage A lever is a simple machine composed of a rigid bar that pivots about a fulcrum. As with an inclined plane, using a lever gives you a mechanical advantage in exerting a force. As you open the discussion, ask the students to recall the six simple machines: inclined plane, lever, screw, wheel & axle, pulley, and gears. Ask them how long people have been using simple machines. Answers will vary, but examples would be that the invention of the wheel dates back more than 5,000 years (we think the first wheel users were the ancient Mesopotamians). Native Americans were using pump drills with screw mechanisms to drill holes and start fires in the 10th century. The Mesopotamians were using pulleys as early as 1500 BC. And the lever has been around since prehistoric times.
To get a baseline, you might want to use the force sensor to simply lift the load (weighted cup) straight up to the same height as the lever system. This will give your students a better idea of the mechanical advantage of using levers. A straight-upward vertical lift as shown above required roughly 3 Newtons of force for the entire period of work.
Collect Data II
Task: Move the load closer to the fulcrum and describe what happens to the graph. The graph should look reasonably like the one pictured below:
Task: Set up the lever system with the fulcrum in the precise center, the string from the force sensor at one end, and the weighted cup supported at the other end. Start collecting data with the cup at the farthest end of the lever. Keeping the graph running, slide the cup 10 cm toward the fulcrum, then stop. Last, slide the cup an additional 10 cm toward the fulcrum and stop. Important tip: Have students hold the weighted cup in their hand and place it on the lever AFTER the graph starts running.
What the graph should look like: It will be difficult for young students to create a smooth graph, but their graphs should show the pattern in the image above. Teachers: 1 Newton is equal to .2248 pounds (truncated to 4 decimal places). It is equal to ~ .102 kilograms. When you weighed your load in "Collect Data I", that weight should be close to the force required to lift the weighted up straight up. In our examples here, our cup weighed about .3 kilograms (about 2/3 of a pound). Look for 2 key ideas to emerge:
- With the lever, you can clearly see the mechanical advantage of using a fulcrum.
- You can also see the advantage in a Class 1 Lever of moving the load closer to the fulcrum.
Collect Data III
Task: Using the same lever apparatus, move the fulcrum from a location close to the load to a location close to the force end. The graph should appear similar to the one below:
Question 1: How does the force compare when the fulcrum is close to the load end? When the fulcrum is close to the force end. Accepted response: The graph shows that keeping the fulcrum closer to the load end requires less force. The closer you move the fulcrum to the force end, the more force is required.
Question 2: Is there any pattern in the change of force each time the fulcrum was moved? Accepted responses will vary, but should mention that the pattern shows increasing force as you move the fulcrum farther and farther away from the load end. Astute students may notice, by analyzing the amount of force at each interval, that there is a predictable relationship between effort force and distance from the load.
1. What effect did moving the load closer to the fulcrum have on the force needed to support or lift the load? Answer: (See graph for Data Collection II) Moving the load closer to the fulcrum resulted in less force required to lift the load.
2. What effect did moving the fulcrum closer to the force have on the force needed to support or lift the load? Answer (See graph for Data Collection III) Moving the fulcrum closer to the force (away from the load) resulted in greater force being needed to lift the load. In other words, the farther you move the fulcrum away from the load, the greater force required to lift it.
3. Refer to the picture of the lever in the Procedure section. The distance from the force to the fulcrum is called the "lever arm". Recall the quote from Archimedes: "Give me a place to stand on, and I will move the Earth." Assuming Archimedes could find a suitable place to put a fulcrum, what length of lever arm would he need in order to lift a very heavy load like the Earth? Teachers: Kids may need some scaffolding. Ask them, "Let's pretend we could build a lever this huge. How could we determine the length we need to just to place the Earth on one end?" Response -- The Earth is 7,918 miles in diameter (12,742 kilometers). Next, ask them to "help figure out how much longer the lever arm should be to do the lifting job". Ask kids to look back at their graphs. The levers will need to be longer than the diameter of Earth, but how much longer? Key Takeaways: Students will notice from their investigation that the lever arm could've been shorter to lift their weighted cups. They should be able to analyze how long their lever arm could be to still retain its efficiency in lifting. Their answers will vary because nobody's graph will be perfect. But generally speaking, they can use this information to help gauge how long an ideal lever arm would need to be to lift the Earth. Any response that states a length 4-8 times Earth's diameter would be acceptable.
How does using a lever change the force needed to lift an object? Answer: The lever makes it easier to lift loads over a small distance.
Task: Set up the lever system again with fulcrum at 50 cm, force sensor at one end, and cup with the load at the other end. Perform two tasks: 1) Use the force sensor to pull down on the lever so it is balanced, and 2) Use the force sensor to push down on the force end of the lever until the load is balanced.
Questions: What do you notice about the two graphs? What is the difference between a push and a pull? The push will require somewhat less force because the lever will have the advantage of the combined weight of the sensor and the force of your push. To help students understand this, ask them to consider the difference between lifting a weight by pulling and lifting the same weight using a pushing motion. See photos below. The weightlifter can lift more weight by doing the pushing motion on the right, but the machine on the left may be preferred by some because it requires greater intensity.