Oh boy! It’s time for the last law.
Newton’s Third Law of Motion states that for every action there will be an equal and opposite reaction. In other words:
Newton’s Third Law of
Motion Cake: If you push cake, it pushes back.
Now that may seem like a weird idea. How can cake push? It doesn’t even have arms.
Well, let’s look at Newton Bunny. Here he is skating on Galileo’s Pond:
What would happen if we placed a giant chocolate cake right in front of him?
SMACK! The cake hits him.
Now wait a minute! Does he hit the cake or does the cake hit him? The answer is both. It takes two sides to create a reaction. If Newton didn’t hit the cake, he wouldn’t put a dent in the cake. And if the cake didn’t hit him back, he would just somehow pass through it like it was a ghost-cake.
So Newton hits the cake and the cake hits him back. Like this:
Newton is pushing a cake and the cake pushes just as hard back. So the cake stays where it is and so does Newton.
But cake is just cake, and eventually it reaches a point where it can’t push back any harder. What if Newton pushes cake beyond its pushing limit?
Anytime there is an unbalanced force something changes to balance it. The extra force is converted into motion. In Newton’s case, he falls into the cake.
Uh, oh! Newton pushed too hard. He fell into the cake.
So if you push something, it pushes back, otherwise you would fall through it.
Here’s a little review. See if you can fill out these activity sheets:
(Answers: Top – any path is fine, but once the bunny hits the cake he will be forced to stop. Bottom – The bunnies on the left hand side of the page will push the wall over because the bunny is pushing harder than the wall pushes back.)
In my last post I mentioned we would be discussing sneaky forces. These are forces you can’t see. Newton’s Third Law says that every action requires a reaction. So if something changes (reacts), you need an action. According to Newton’s First Law of Motion, if you threw a cake into the air, it would keep sailing up, up, and away. But cake won’t do that. Eventually it comes crashing down to the ground. Splat! Even though you can’t see it, something is pulling the cake down. Do you know what it is? Which of the following sneaky forces do you think is acting on the cake in the air?
Gravity is the reason a cake falls to the ground. Here are some other stories. See if you can identify the sneaky force at work.
1) A cake is sliding along a table. No one touches it, but it comes to a stop anyway. What stopped the cake?
2) Buster Bunny delivered a cake to his cousin in jail. He baked a saw into the cake. A police officer with a magnet took the cake away from him without touching the cake. What force took the cake?
3) A cake resting on ice is placed next to a fan. The fan is turned on, and the cake slides away from the fan. What moved the cake?
Here are the answers:
1) Friction, 2) Magnetism, 3) Wind
So there you have it. Now you know all about Newton’s Three Laws of
Motion Cake. Here are the three laws in review:
Newton’s 1st Law of Cake: Cake on a plate will not get up and start moving by itself. If the cake is moving, it will keep going until something stops it.
Newton’s 2nd Law of Cake: If you want to punch a cake harder, you must either hit it with something bigger or punch it faster.
Newton’s Third Law of Cake: If you push cake, it pushes back.
And now for a really nerdy aside: I’m a civil engineer and my husband is a physicist. We were having a discussion/disagreement about Newton’s Third Law. Now my husband is the kindest man I’ve ever met, but he didn’t like the way I was treating Newton’s Third Law, so he said, “Well, you just don’t think about motion because you’re a civil engineer. Civil engineers never do any work.” I had to laugh because he’s right. The whole point of civil engineering is avoiding work… Work is Force multiplied by Distance (W = F×d), and civil engineers build stationary objects. We make things that bend an wiggle but hopefully nothing that moves too much. Movement is the domain of mechanical engineers. So civil engineers build airports and bridges, and mechanical engineers build airplanes and cars. Also here is a free body diagram of cake being eaten by an alligator.
Don’t worry, my husband and I resolved our disagreement and the above is physicist approved.
Have fun with physics! And go smash some cake!