Main->Readings->4th Grade Readings->Matter in Motion- >Part 2: Collisions

Part 2: Collisions

Remember Newton's First Law: An object in motion tends to stay in motion, and an object at rest tends to stay at rest, unless it is acted on by an unbalanced force. Things keep going the way they are already going, in other words. Matter tends to resist change. Inertia (in-UR-sha) is the word we use for the resistance.

How much does matter resist? Well, it turns out to depend on two things: how much mass the object has, and how fast it is moving. Momentum (moe-MEN-tum) is the word we use to describe an object's combination of mass and velocity.

Momentum is important. For instance, the Earth is constantly being hit by meteorites. Most of them are very small and light. They are traveling very fast, but they do not have enough momentum to cause damage because they have so little mass. Yet 65 million years ago, a meteorite the size of a mountain probably hit the Earth. It had a great deal more momentum. Did it cause damage? You bet. It caused the extinction of the dinosaurs.

Even when you are talking about things that are smaller than Earth-destroying meteorites, momentum matters. A bicycle going 20 kilometers an hour is easier to stop than a car going 20 kilometers an hour. That is why a bicycle has hand brakes and a car may have power brakes. You could not stop a car with hand brakes!

Newton's second law talks about this:

The amount of force needed to affect an object's motion depends on two things: the object's mass, and the acceleration.

Action and Reaction

If matter keeps doing what it is already doing, and if it takes force to stop or start an object, what happens when two objects hit each other? In order to talk about this, you need to know Newton's third law as well. It is very simple:

For every action, there is an equal and opposite reaction.

Sometimes this is said as "You can't touch without being touched." When you hit a wall, it hits you back. Sometimes it is very easy to notice this law at work.

1. If you blow up a balloon and let it go without tying it, the balloon pushes the air out the neck. The air pushes back, and the balloon goes flying.
   
2. If you are on a skateboard and you throw a basketball, the basketball pushes back, and you roll in the opposite direction.
3. If you are on a rocket in airless space, the engine shoves hot gases out of the tail of the rocket, and the gases push back. The rocket moves!

Sometimes it is harder to notice:

1. A batter hits a baseball with a bat. The ball pushes back, but it is so light that you don't notice the bat slowing down (unless you use a high-speed camera).
2. You sit on a chair with the full force of gravity. The chair pushes back. You don't notice it is pushing back, though you would notice if it didn't - you'd be on the floor and the chair would be toothpicks!
3. A moving ball (A) hits another ball (B) of the same mass which is not moving. Ball B pushes back with an equal force, which is just enough to make ball A stop. Meanwhile, because ball A pushed it, ball B moves off at the same speed that ball A had to begin with.

Real Life Collisions

You can see that when you get two or more objects together, things can get complicated fast! Now we are going to put all of Newton's Laws together in a real lfe example and see how much more complicated it can get.

You know an object in motion tends to stay in motion. What if the object is a car going about 50 km/hr (kilometers per hour)? That is a normal speed for many cars going along Lancaster Avenue (remember we are using the metric system!).

We will guess at its mass. Maybe it is 1000 kilograms. And do not forget the people in the car. Let's say there is a kid who weighs about 35 kilograms, and a driver who weighs 70 kilograms. Each of these things - the car, the driver, and the passenger, are driving down Lancaster Avenue at 50 km/hr. What is their momentum? The car has a momentum of 50,000 kilograms per kilometer per hour (kg/km/hr). The driver has a momentum of 3500 kg/km/hr. The passenger has a momentum of 1750 kg/km/hr.

Why do the people in the car have their own momentum? You will see in a minute.

Suppose a large moving van is stopped at the traffic light, and the car hits the moving van. The moving van has a very large mass, so large that it is like the baseball bat hitting the baseball. The van does not move much. However, because for every action there is an equal and opposite reaction, the van pushes back on the car and the car is stopped dead by the impact.

You will notice I said the car is stopped. But it was not the only object that was in motion. The passenger and the driver are also objects in motion. If they are not wearing their seat belts, they will keep moving, probably through the solid windshield of the car and onto the hard road. This is very damaging to soft human bodies.

Even if they are wearing seat belts, the car stopped awfully fast when it hit. Slowing something down is a special kind of acceleration called deceleration. When you decelerate quickly, there is a great deal of force all at once. If the force is applied by a dashboard to a head, even if the dashboard is padded, it is not fun.

Air bags are often in the front seat of a car to make this problem less. They inflate quickly when there is an impact, and they deflate when a person hits them. What they do is slow down the impact. That way the force of the collision is not felt all at once. You still hit with the same amount of force, but it is spread out over time.

It is just the same when you catch a baseball with a glove, or a lacrosse ball with a lacrosse stick. You must give a little when the ball hits the webbing. That way the impact does not happen all at once and the ball does not bounce away.

Please wear your seat belts. Even at 5 km/hr, you still have momentum.

In this section, you have learned:

1. Matter resists change
2. The resistance is called inertia
3. Objects in motion have momentum.
4. Momentum equals mass times velocity.
5. The more massive something is, the more momentum it has.
6. Newton's 2nd Law states that force equals mass times acceleration
7. Newton's 3rd Law says for every action there is a reaction.

In this section, you learned how objects act when they hit one another. In the next section, you will learn how a force can affect an object when nothing is hitting anything at all..

Homework

Questions: For your first assignment of the week, answer these questions in complete sentences on a sheet of loose-leaf paper, with a proper header:

1. Tell which object in each pair would have more momentum, and explain your answer.
   a. A baseball going 150 km per hour or a bowling ball going 150 km per hour.
   b. A baseball going 150 km per hour or a baseball going 75 km per hour.
   
2. If you throw a ball, it pushes back with the same amount of force. Which of Newton's Laws tells you this?
   
3. Why should people in cars wear seat belts?
4. Would it be possible for a ping-pong ball to have more momentum than a bowling ball? Explain your answer.

Notes: For your second assignment of the week, in your journal on the next clean page, write the vocabulary words from this section and their definitions.

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Go on to Part 3: Gravity

This page last modified August 15, 2002

Copyright ©2000 Delia Marshall Turner. All rights reserved.

Questions? Send me a note at dturner@haverford.org