# Is Newton's 3rd Law Cancelled in This Friction Problem?

• Lisciu
In summary, the author found that the fork lift could only push forward a certain number of crates if the wheels were powered, but that the coefficient of static friction between the wheels and the ground was μ s and between each crate and the ground was μ 's.
Lisciu
Missing template. Moved from technical forums.
Hi Guys,

I was highly studied the Newton's law once again. Probably to recall the concept from the past of my education. Anyway I find out the exercise during solving the friction problems from Hibbeler book.

The exercises is state as :
The fork lift has a weight W1 and center of gravity at G. If the rear wheels are powered, whereas the front wheels are free to roll, determine the maximum number of crates, each of weight W2 that the fork lift can push forward. The coefficient of static friction between the wheels and the ground is μ s and between each crate and the ground is μ 's.

The whole system looks like this:

And then it's separate on two objects:

FBD of the 1:

FBD of 2:

And there is nothing suprised about that except the action and reaction force from the crates and front wheel.

I expected that those force will be equal like the 3rd Newton law said. But during the solving and after see those images from book solution looks like 3rd Newton law doesn't work anymore...

Could someone explain what is the reason of that? And where is actually the action and reaction force? I'm confused here.

Lisciu said:
And where is actually the action and reaction force?
The only 3rd law pair here is ##P=-P'##. If you got anything else then you made a mistake.

Seriously? :) when you figured it out that I don't know that?

So why P is equal 707,37lb : and P' is equal : 105lb?

Lisciu said:
So why P is equal 707,37lb : and P' is equal : 105lb?
That is not correct, but there is no way to tell where you went wrong without more details about how you attempted to solve it.

It's okay. I figured it out what is going on here. Because in the solving book author calculated the P' force for just one crate to calculate how many he can push by this fork lift. I realize that if I will calculated it with diffrent approach like typical 3rd law Newton without calculating reaction force for just one crate I will have the same result. I just took the calculate force P from fork lift and then using this force in X direction net force summing (2rd law) get the force of normal acting on. When I have it I just divided it by one crates weight and actually I get the number of crates this system can handle.

That's good. I think I probably would have followed something closer to your approach than to the books approach.

Yeap, me too. I was just confused for whole day. Then I just sit once again on the problem and realize where was the hatch.

Anyway I will need to create the topic around 3rd Law because I not undestand in 100% this concept specially if someone talk about cancelling force not cancelling force and then i was thinking about this action and reaction force as a unended storry with domino effect in whole body if we consider for example hammer and nail.

## 1. What is Newton's 3rd Law?

Newton's 3rd Law states that for every action, there is an equal and opposite reaction. This means that when an object exerts a force on another object, the second object will also exert an equal force in the opposite direction on the first object.

## 2. Why is Newton's 3rd Law important?

Newton's 3rd Law is important because it helps us understand how forces work and how objects interact with each other. It also allows us to predict the motion of objects and design systems that utilize these forces.

## 3. Can Newton's 3rd Law be observed in everyday life?

Yes, Newton's 3rd Law can be observed in many everyday situations. For example, when you push against a wall, the wall pushes back with an equal force. When you walk, your feet push against the ground and the ground pushes back, propelling you forward.

## 4. Does Newton's 3rd Law apply to all types of forces?

Yes, Newton's 3rd Law applies to all types of forces, whether they are contact forces or non-contact forces. This means that even forces that act at a distance, such as gravity or magnetism, follow this law.

## 5. Are there any exceptions to Newton's 3rd Law?

There are no exceptions to Newton's 3rd Law. It is a fundamental principle of physics that has been tested and proven to be true in countless experiments. However, in some cases, it may appear as though the forces are not equal and opposite, but this is due to other factors at play, such as friction or air resistance.

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