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.

**Additional Background**

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. *