# Newton's 3rd law -- Trying to understand the forces on objects with different masses

xWaldorf
So there's something I'm missing when I think of Newton's 3rd law of motion.
If all forces between two objects exist in equal magnitude and opposite direction, how is it that, for example, when I'm driving my car, the car "runs through" all the air molecules, and they seemingly don't exert the same force back on my car (although i know that they do), a force of a full iszed car running at X kph?
how is it that hitting a wall with my car has different results from "hitting" air molecules, when the reaction is supposed to be the same - same force on my care on the opposite direction.
In other words, in what way the mass of an object affects the reaction, and what am I missing here?

I realize that the forces don't cancel out because they act on different objects, that is not the misconception I am struggling with

Homework Helper
When the car interacts with the air, there is not much force on the air and not much force on the car. Equal and opposite.

When the car interacts with a wall, there is a lot of force on the car and a lot of force on the wall. Equal and opposite.

Last edited:
• russ_watters and Rishabh Narula
Homework Helper
Gold Member
2022 Award
Summary:: in what way the mass of an object affects the reaction, and what am I missing here?

So there's something I'm missing when I think of Newton's 3rd law of motion.
If all forces between two objects exist in equal magnitude and opposite direction, how is it that, for example, when I'm driving my car, the car "runs through" all the air molecules, and they seemingly don't exert the same force back on my car (although i know that they do), a force of a full iszed car running at X kph?
how is it that hitting a wall with my car has different results from "hitting" air molecules, when the reaction is supposed to be the same - same force on my care on the opposite direction.
In other words, in what way the mass of an object affects the reaction, and what am I missing here?

I realize that the forces don't cancel out because they act on different objects, that is not the misconception I am struggling with

Let's take the standard example. First you punch a wall with a force of, let's say, ##100N##. The wall is perfectly capable of returning that force. It doesn't move and the equal and opposite reaction is ##100N##.

Now, suppose you punch a paper screen. The paper screen is only capable of a force of ##5N##, say. The equal and opposite reaction is ##5N## and your fist goes through the paper.

The maximum force you can exert on something depends on the thing you hit.

In the example of the car, the air provides a limited resisting force. This decelerates the car and the car pushes on the air with the same force. If the car hits a wall, the wall can provide an enormous resisting force and the car decelerates much more quickly. In both cases, the magnitude of the resisting force determines the equal and opposite forces.

A car, moving at a certain speed, does not imply any particular force. Your car is traveling with a speed and a momentum, but there is no implied force. The magnitude of the force is determined by what is resisting its motion.

• Rishabh Narula, hmmm27, hutchphd and 1 other person
xWaldorf
Thanks!
so does that mean that the paper in your example has a force of 5N applied on it by my fist? or it has 10N but only capable of returning 5N?
does the mass determine the resistance capability?

Homework Helper
Gold Member
2022 Award
Thanks!
so does that mean that the paper in your example has a force of 5N applied on it by my fist? or it has 10N but only capable of returning 5N?
does the mass determine the resistance capability?
The force applied by your fist might increase rapidly to ##5N## but at that point the paper cannot sustain further force, so it tears and your fist goes through. You cannot get beyond ##5N## with something that gives way to that value.

You might like to think about what determines the maximum force you can apply to a wall. Hint: assume the wall is completely rigid.

xWaldorf
The force applied by your fist might increase rapidly to ##5N## but at that point the paper cannot sustain further force, so it tears and your fist goes through. You cannot get beyond ##5N## with something that gives way to that value.

You might like to think about what determines the maximum force you can apply to a wall. Hint: assume the wall is completely rigid.

how is the maximum force defined? when the object shatters? if so I'd assume the molecular bonds of that particular matter determine the maximum force wouldn't it? - because i guess you're talking of a situation when the wall or paper is stationary due to something holding it.
and if it is so, what couldn't we go beyond that force if we took a floating block of wall in space, and attach an imaginary spaceship to it and star accelerating until the force applied on the wall by our accelerating spaceship go beyond that maximum?
I hope I am making my qustions clear hahah

Homework Helper
Gold Member
2022 Award
how is the maximum force defined? when the object shatters? i

Yes, that's it.

Gold Member
Summary:: in what way the mass of an object affects the reaction, and what am I missing here?

when I'm driving my car, the car "runs through" all the air molecules, and they seemingly don't exert the same force back on my car
This is a very simplified model for air resistance but it will do for the purpose. As your car speeds up, the resistive force due to all the air particles that it hits will increase (at a rate that approximates to a square law). Eventually, the resistive force will equal the driving force and you will reach the terminal velocity - after which there can be no more acceleration.

This is not just a Third Law phenomenon. The Third Law always applies but the Third Law aspect is that each particle experiences an equal and opposite force to the force it exerts on the car.

It's the Second Law that describes the acceleration during the process and, in this case, the rate at which you are pushing particles along determines how many of those 'Third Law Pairs' are involved in each second.

Then it's the First Law that tells you there will be no further acceleration when the net force is zero.

You need to read such laws very carefully and literally and not try to use them out of context.

• Lnewqban
Homework Helper
Gold Member
Summary:: in what way the mass of an object affects the reaction, and what am I missing here?
Your question is more related to the Second Law than to the Third one.
I believe that what you are missing is the concept of the Laws of Motion, energy-wise, and why they happen to be.

How would you define "force"?
According to
https://en.wikipedia.org/wiki/Force

"In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of Newtons and represented by the symbol F."

https://en.wikipedia.org/wiki/Newton's_laws_of_motion

https://en.wikipedia.org/wiki/Impulse_(physics) Last edited: