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## High school physics - NGSS

### Unit 1: Lesson 2

Introduction to momentum

# More on Newton's third law

AP.PHYS:
INT‑3.A (EU)
,
INT‑3.A.3 (EK)
,
INT‑3.A.3.3 (LO)
,
INT‑3.A.4 (EK)
,
INT‑3.A.4.1 (LO)
,
INT‑3.A.4.3 (LO)
David explains some of the common misconceptions in dealing with Newton's Third Law. He also shows how to correctly and reliably identify Third Law force pairs. 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
• 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.
• 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?
• If the elevator accelerates downward (and slowly increases the acceleration over time) then the force on the table gets smaller until the downward acceleration of the elevator has the same magnitude as the acceleration due to gravity. At that point the force exerted on the table is zero and the box (and everything inside the elevator) is in "free-fall." The passengers might stop screaming for a while and enjoy a few moments of weightlessness. If the downward acceleration increases further then the free falling box is now accelerating down less fast than the elevator and this continues until the box hits the elevator's ceiling. At that point the ceiling is pushing down on the box and the box is then pushing back on the ceiling.
• 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 upward reaction force acted on the book.
• So if I were to, lets say walking on a cliff and I started walking off of it, I would be applying a force on the air right? So according to the third law, the air should be exerting an equal force on me so shouldn't I be able to float or levitate? Sorry if this is a stupid question, maybe I'm just missing something.
• Air is a fluid so the atoms/molecules are not tightly connected to each other so while your foot does apply a force on the air but the amount of force is very small and causes the atoms/molecules to move out of the way.

The mass of air is about 0.001225 g per cubic centimeter so it doesn't take much force to move it.

While there is an equal and opposite force from air on your foot is is so small in comparison to your weight that it can be ignored.
• at , can't we consider Fg and Ft are equal and opposite because of the 1st law as well (Basically: An (external) unbalanced force exerted on an object causes its acceleration in the direction of the unbalanced force)
In fact, what Is the difference between the 1st and 2nd law, or I the 2nd law a mathematical expression of the 1st law?
Have I got something messed up? If so please do clarify :)
• The first law says that an object in motion will stay in motion at a constant speed in a straight line until acted on by a net external force and an object will stay at rest until acted on by a net external force. The first law describes what happens when there is no net force. The second law describes what happens when there IS a net force. Hope this helps!
• At , he says that as soon as the force is exerted on one object, that object exerts an equal-magnitude force in the direction that the force is coming from. He also says at that these forces happen instantaneously. Once the "action" comes, the "reaction" comes instantly.

But what if there are two gravitational bodies in space, like he mentioned in his first example? As soon as the two gravitational fields meet, one of the bodies is going to exert a force on the other, and the other is going to instantly "respond" with a reaction force, regardless of the distance between them.

Assuming that the action force is a signal, that means that the receiving body receives the signal instantly, regardless of distance. This means a signal has been sent and received faster than the speed of light, which is not possible.

What is actually happening in this type of scenario? If someone could answer, it would really help out in my understanding of this topic. Thanks!
• Gravitational disturbances travel at the speed of light. In most cases you can consider that "instantaneous", but sure, if the distance is large, the delay can become meaningful.