# Power of an object

1. Oct 28, 2013

### Hepic

Lets say that my desk can stand until 50 newtons.
So I put my hand up to desk with 20 newtons,so third newton's law says that desktop will put force with 20 newtons my hand. After some seconds,I put force with 50 newtons,so my hand will feel 50 newtons too from the desk. After I put force of 55 newtons(so desk will broke). Theoritically my hand will feel 50,or 55 Newtons ??

Thanks.

2. Oct 28, 2013

### A.T.

If it breaks at 50N you can't apply 55N to it.

3. Oct 28, 2013

### Hepic

Lets say that desktop can stand until 50 newtons.
Suddenly I put force of 55 newtons. My hand will feel 50 newtons or more?

4. Oct 28, 2013

### Staff: Mentor

How were you able to apply 55 Newtons to a desk that can only withstand 50 Newtons?

The desk always pushes back with a force equal to whatever force you exert on it. So if you managed to push it with 55 N for a moment before it broke, then the desk would have exerted 55 N back on you for that moment.

5. Oct 28, 2013

### A.T.

I thought you meant static forces. For a short time you can apply greater forces because of the desks inertia.

6. Oct 28, 2013

### Staff: Mentor

That depends on the way the desk breaks. You can get a force above 50 N if you accelerate parts of the desk downwards.
How do you do that? In general, you can choose the position of your hand, but it is problematic to choose the force in non-static environments (like breaking desks).

7. Oct 28, 2013

### Staff: Mentor

In an idealized model, you won't be able to apply a force of more than 50N. You'll push harder and harder, and the desktop will push back on you with equal and opposite force, until you get up to 50N. When you do, the desktop will break, your hand will go through it and keep moving without anything to oppose it. Imagine that there was a spring scale sitting on the desk, and you're piling weights onto the scale. Every time you add a weight, the reading on the scale goes up. But the moment you get the scale to read 50N, the desk will break, the scale will go into free free on the way to the floor, and it will read zero while it's falling.... So you can't apply a force of more than 50N in this idealized model.

It's a different story if you're striking the desk with your fist or a hammer - then we're thinking about accelerating the pieces of the broken desktop, and potentially much larger forces may be involved.

8. Oct 28, 2013

### Hepic

I do not mean about static power,but a sudden one.
So,If I put force 100+ I will feel the same power in my hand?(synchronous)

9. Oct 28, 2013

### Staff: Mentor

Static or sudden doesn't matter: Newton's 3rd law still holds. If you manage to exert a force of X Newtons on the desk, then the desk exerts an equal and opposite force of X Newtons on your hand.

10. Oct 28, 2013

### Hepic

So does not matter how much force an object can stand.
If I ask an infinite power,I will get the same.(theoritically).
Right?

11. Oct 28, 2013

### Staff: Mentor

If you can exert the force against the object, then the object will exert the same force against you.

Sometimes it's not so easy to exert that force. Example: It's easy to punch a concrete wall and create a (relatively) large force. Try punching a fly or a piece of tissue paper with that same force. Good luck!

12. Oct 28, 2013

### Staff: Mentor

No one has corrected this yet, but force and power are two completely different things.