October 7 - Motion: Inertia; Newton's Laws of Motion

We continued our study of motion this week by covering inertia (Newton's First Law of Motion) as well as Newton's Second and Third Laws of Motion.

Inertia is the tendency of an object to resist any change in its motion. If an object is at rest, it will remain at rest until it is acted upon by a force. If the object is moving, it will continue to move until acted upon.

We started our session with a lab. This lab was "Crash!" from Janice VanCleave's Physics for Every Kid. Each student had a small piece of modeling clay, 2 rulers, a toy car, a pencil, and 2 books. For the first part of the lab, the students set up a ramp using one book and a ruler. They taped the pencil to their desk about 2 car lengths from the ruler. The students then placed the modeling clay onto the hood of their toy cars and allowed the car to roll down the ruler. The students observed the modeling clay fly off the hood and used the second ruler to measure how far from the car the clay fell. We kept track of the measurements.
After three rolls, the students added a second book to the stack. They made predictions about whether or not the clay would now fly further from the car. Again, they had three rolls and we tracked the measurements.

Once we completed both sets of rolls we discussed how all this related to inertia: when the clay flew off the front of the car, it continued to move even though the car had stopped. As the car rolled down the ruler, its speed increased. The clay was moving at the same speed as the car so, even though the pencil stopped the car, the clay continued to move until gravity acted upon it causing it to fall to the table.

After completing the work with two books, we decided to try it with four.

We moved on to talk about mass and how it relates to inertia. I asked the students if they thought objects with different weights have the same inertia. I used the example of a ping-pong ball, a tennis ball, a basketball, and a bowling ball. The greater an object's mass, the greater its inertia. I also asked them if they would be able to change the velocity of a bowling ball by swatting at it with a ping-pong paddle. We discussed the fact that a greater force would be needed to change the velocity of the bowling ball than would be needed to change that of the ping-pong ball.

The students then completed the lab "More." This lab demonstrated the effect of weight on inertia. The students were given a pair of plastic soda bottles. One bottle was empty; the other was about half full of water. We tied a 12-inch piece of string to a rubber band and then slipped the rubber band onto the bottom of the empty soda bottle. Before we began, I had the students predict which one would have a greater inertia or a greater resistance to motion. The students each pulled on the string to drag the empty bottle across the table. We measured how far the rubber band pulled away from the bottle and recorded our results. We then did the same with the half full bottle. The students discussed that since the bottle with water was heavier than the empty bottle, it had a greater inertia that caused the rubber band to pull away further from this bottle.

We then discussed Newton's Second and Third Laws. I used the example of hitting a baseball versus a bowling ball to describe Newton's Second Law: The Law of Force (Force = Mass x Acceleration). They understood that since the bowling ball has a greater mass, it would require a greater force to hit the bowling ball with a baseball bat.

Newton's Third Law is the law of action and reaction. Whenever an object pushes on another object, the first object gets pushed back in the opposite direction equally hard. We used two labs to demonstrate this concept.
"Balloon Rockets" - Students were given a long piece of string that the threaded a piece of drinking straw onto. They then tied the string to two chairs. Each student then inflated a balloon and taped the balloon onto the straw. The students observed what happened when the balloon was released.
When the balloon was released, it pushed the air out. However, that air pushed back on the balloon (action-reaction), causing the balloon to move forward.

"Paddle Boat" - The students used cardboard to create a simple boat shape. They cut a paddle out of the cardboard and used a rubber band to attach it to the boat. The students then wound up the paddle and placed the boat in a container of water.
When the students wound up the paddle, it turned and hit against the water. The paddle, then, pushed against the water and the water pushed back. This action and reaction caused the boat to move.

We closed up class with one more lab that demonstrated inertia. For this one, I placed five textbooks on the edge of a rolling chair. I pushed the chair across the classroom and then stopped abruptly. Due to inertia, the books remained in motion when the chair stopped. 

To look forward to next week:

We will learn more about Sir Isaac Newton by studying gravity.
We'll find out what some common objects would weight on different planets and we'll complete several experiments about the force of gravity.

The labs "Crash!", "More," "Balloon Rockets," "Plop!" and "Paddle Boat" are from Physics for Every Kid.

VanCleave, J. (1991). Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound. San Francisco: Jossey-Bass.