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### Course: AP®︎/College Physics 1>Unit 2

Lesson 2: Newton's third law and free body-diagrams

# More on Newton's third law

Newton's third law states that objects exert equal and opposite forces on each other in every interaction. However, these forces do not cancel out because they act on two different objects. The magnitudes of the forces on both objects will be always be equal, but their resulting accelerations will not be equal if the objects have different masses. Created by David SantoPietro.

## Want to join the conversation?

• Why is the force instantaneous? Why does the wall not generate constant force, even without Chuck Norris drop kicking it? It seems to me that there is more going on than just the wall exerting a force... why is it ONLY exerting this force at the instant when Chuck Norris comes into contact with it? Also... why does the wall break? Thanks
• You have to think that everything moves, even if the movement is too small for you to notice.
In the example of the earth-moon interaction, the wrong common perception is that the earth is stationary while the moon goes around the earth. The moon in fact exerts the same amount of force in the earth, but because the earth is so massive (the mass of the earth is 81 times greater than the moon), the wobbling motion in the earth caused by the moon is very very small (however you can observe the effect force of the moon in the ocean's tides - but this is besides the point)
In the example of the wall, when no force is being applied, the wall is completely straight. When you kick it, the wall actually bends (although this movement is so very small you will only notice it if you use precision instruments). The cause of the wall bending is the force of the kick, and the reaction is the force by the materials being bent, or deformed - pretty much like the force when you push a spring (you cause the spring to deform by pushing it and the materials are struggling to get back to their natural position).
The wall breaks when the material can no longer withstand the deformation you are causing - you went beyond the maximum deformation of the material. The maximum deformation depends of the molecular structure of the material.
• Hi, I didn't understand one thing. Imagine we exert 10^5 newtons on a soft object by kicking it. For example a tissue or a paper. If we kick a wall or a hard object, our foot will break. But when we kick a paper as much as hard, we don't even feel any pain. Why is that? I mean paper isn't even able to exert 10^5 newtons!
(Excuse my awful grammar)
• My teacher explained this today through the lens of the second law, he said that the reason that a fly does not destroy our cars when hitting them at such high speeds is because of the relative differences in mass

ie using the Fnet=(m)(a) equation:

the acceleration value hitting the fly (0.000012 kg) may be 45000 m/s^2 (creating a liquid smear of the little guy) this mass and acceleration indicates a force of 0.54 N

This same 0.54 N for a 2000 kg car is only 0.00027 m/s^2. Almost entirely unnoticeable for the car
• if everything applies equal and opposite force to every force then how come we are able to break stuff??
• The structural strength of an object is unrelated to Newton's third law. If you have a stationary piece of Styrofoam and you push on it it will push back with as much force as you push on it until you reach its breaking point and then it will break. Once it is broken it will not be pushing back and you won't be pushing on on it.
• I got a bit confused around .

When David said that the upwards force is greater than the downward force, what exactly is the upward force? Also, is there a presence of a net force hence the upward acceleration?

• Ultimately the upward force is provided by the motor pulling the rope attached to the elevator, and it's gonna cause the normal force exerted on the box by floor to increase. This extra force coming from motor provides a "net force" upward causing the upward acceleration

When the motor pulls the rope with a certain force and accelerates it upwards, the atoms in the rope will begin to pull each other upwards (with speed of sound) thanks to electrostatic bonds between the molecules of the rope. If these intermolecular bonds are strong enough, rope won't break and it will transmit the motor's force to the material of the elevator, and from there the force will be transmitted all the way to the floor under the box. The floor will begin to push the box upwards with this extra force coming from the motor, as a result the normal force on the box will increase and overcome the weight of the box.

This extra pressure from increased normal force is the reason we feel heavier when the elevator accelerates upwards. İncreased force will put more pressure under your feet and you will feel that as becoming heavier.

When you're free falling in the air there is nothing under your feet that would cause the pressure provided by the normal force; that's the reason you feel weightless when free-falling and also the reason you feel less heavy in a downward accelerating elevator where the floor is "escaping" from under your feet in the moment of acceleration.
• i cannot understand. i think i do and then it just vanishes. what does this all mean?
• Ikr, like I watch the video then the info stays then vanishes in an hour. Physics is complicated
• At , I don't understand what is the green force. What is that force? Where does it come from? Does it mean that the box is pulling the earth with somekind of gravitationnal field? Thanks
• It is the normal force. So, a box on a table does not accelerate through the table because there must be an equal force pushing the box upward as down (because the box is not accelerating or moving downward). The table is not "actively" pushing the box, but it is simply the force that prevents the box from further moving down
(1 vote)
• I have a question that I hope someone can clarify. is there a force from an object even if said object is not moving?
• Yes, put a heavy book or rock in your hand you can feel the force pushing your hand down. If it is not too heavy you can hold the object up and keep it still, it is putting a force on your hand even if it is not moving.
• In the elevator example: what if I were to accelerate downwards the elevator, would F(AT) became negative and therefore F(TA) positive? And what does it mean when F(TA) is positive? Object A is pulling upwards the table like they were glued together?