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Asked by William on 25 Jun 2024.
Question: I still don't get Newtons's Third Law. It doesn't make sense. If any two objects intract they exert equal and opposite force on each other. But surely this means that if I push a pencil across a table, the pencil should exert an equal force back, stopping me. So how can anything ever move?
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Martin McCoustra answered on 25 Jun 2024:
This is an interesting question… and shows some of the issue related to understanding physics from a purely human scale observation. Let’s take the example you give of pushing a pencil across a table. Imagine, you start by pushing the pencil very gently. It doesn’t move as the frictional force that the pencil generates on the table surface opposes and balances out the force your finger is apply as per Newton’s Third Law. When you overcome that frictional force, the pencil moves at constant speed and is now subject to Newton’s First Law or if getting faster as you push harder then Newton’s Second Law. You need to break down the forces involved to really understand what’s going on.
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Pete Webb answered on 25 Jun 2024:
When you push a pencil across a table, your finger tip exerts a small force on the pen, and the pen exerts a small force on your finger. But immediately, your finger tip and the pen are joined together into one object. To keep the pen moving, you now have to move your hand and the pen together by keeping your feet anchored to the floor and using your back and core muscles to move the pen against the friction between it and the table top. Newton’s 3rd law is now applied to your body, because as you move your hand and pen across the table, your back will be leaning backwards a bit. Now imagine you’re pushing not a pen but a large rock. You will most definitely feel a sensation of being pushed backwards – Newton’s 3rd law! So in order to push the rock, you stand with your back against the wall…and PUSH! You’re pushing the rock and the wall. The rock is pushing back at you, and the wall is also pushing you – Newton’s 3rd law again! If you have an unlimited supply of energy, either the rock will slide across the table, or you will break the wall down and fall backwards…end of experiment!
Comments
Sheridan commented on :
Just watch snooker balls hitting each other. If 2 collide they both react. If Newton’s 3rd law didn’t apply then one of them would not react on impact.
Pete Webb commented on :
Imagine a rifle being fired: the bullet moves forward but the rifle moves backwards (otherwise known as recoiling). That’s Newtons 3rd law. Reread my 25 June piece. And don’t regard your finger and the pen as the only 2 objects in the universe. Your finger is connected to the rest of you, and the rest of you is connected to the floor, not hovering in a vacuum.
Luke commented on :
F=ma: force equals mass times acceleration. If two objects have different masses (like you and the pencil), the experience different acceleration for the same force.
A pencil has more a lot less mass (m) than you, so for the same force (F) it will experience a lot less acceleration. The amount of force required to push a pencil is a lot lower than needed to move you, so in this case you’ll only feel a small force and you can avoid being pushed backwards, transferring the forces to your surroundings.
Since we live on a very massive planet with an atmosphere, it’s generally quite easy to direct these forces into the Earth, can push back on you while barely receiving any acceleration. If you try pushing something heavy while you’re stood on a slippery object or skateboard, you may find that you start traveling backwards instead!
If you were floating in space and pushed the same pencil, with no air or ground to transfer the energy, you would start floating away from the pencil from the small acceleration you received. So if you’re ever floating in space above the earth and want to get down, try throwing something away from the earth. (I’ll leave surviving reentry and landing safely to you. 🤷)
Hope this helps!