# What is force really

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1. Mar 1, 2016

### Aman Trivedi

I'm getting confused with the concept of force. I know f=ma. this could be interpreted in this way "the force required to make an object accelerate by a given amount depends on its mass" or the "if an object which with a mass m has a force f being applied on it its acceleration will be a. But could this also mean that an object with mass m and acceleration a is moving with a force of f? For example if I hit a person with a ball, I used to force to throw it, but when the ball begins to accelerate, does it mean that the ball as its moving, is also increasing its force? If yes, when in contact how would the effects of
The force be different from the objects momentum?

2. Mar 1, 2016

### Staff: Mentor

I wouldn't say that. A force is not a property of a single object, it is an interaction between two objects. So a ball doesn't "have" a force, there is a force between a ball and a hand and a force between a ball and the earth.

3. Mar 1, 2016

### ZapperZ

Staff Emeritus
You have a bit of a problem with "cause-and-effect" here.

When an object is seen to be accelerating, then yes, one can deduce that there is a net force acting on that object. However, you shouldn't say that as a mass is accelerating, it is "increasing its force", because it seems to imply that the increasing speed is causing a force.

BTW, just because an object is accelerating, it doesn't mean that it is "increasing its force". One can have a constant acceleration, and thus have a CONSTANT, non-zero force (i.e. the force isn't increasing).

You also need to look at the meaning of "Impulse", because here, force is defined as the time-rate of change of momentum, i.e. F = dp/dt. This is NOT a different definition, because you get back F = ma for a constant mass.

Zz.

4. Mar 1, 2016

### HallsofIvy

"moving with a force of f" simply has no meaning.

The instant you release that ball, there is no longer any force on the ball. "Force" does not move with an object. The force is applied during the time it is in your hand, accelerating it to a given speed. Once the ball leaves your hand, the only forces on it are gravity and, possibly, air resistance. That is why the problems, which I am sure you have done, of determining how far a ball, bullet, etc. will go give the initial velocity and say nothing about any force that may have been applied to accelerate it that "initial velocity". Once the object has reached that velocity, the force is no longer applied and is not relevant to the problem.

5. Mar 1, 2016

### CWatters

Its important to remember that the F in F=MA is the Net Force. It's possible to have a force or forces without acceleration.

6. Mar 1, 2016

7. Mar 1, 2016

### Aman Trivedi

Oh okay I think I get it now, so if the ball collides with a wall, then after collision when the ball begins to decelerate and move in the opposite direction (neglecting the fact that the ball bounces just for the sake of understanding) would we say that it's because of the force exerted by the wall on the ball upon the collision?

8. Mar 1, 2016

### ZapperZ

Staff Emeritus
This is why I said that you should look up the concept of "Impulse". This is a combination of impulse and Newton's 3rd law. The force exerted by the wall onto the ball causes a change in the ball's momentum within the time of impact, i.e. F=dp/dt. The impulse, dp, is equal to F dt ,the force exerted by the wall in time dt. This is what is causing the ball to change its momentum.

Zz.

9. Mar 1, 2016

### Aman Trivedi

I may sound silly but I still have one confusion, when a moving object collides with a stationery object, we'd say that the force caused by the moving object made the stationery object move? Right? Now that object is moving with steady velocity (ignoring gravity and overall friction) how would I deduce the force that the moving object exerted on the stationery object? Would it be by the accelrstion caused

10. Mar 1, 2016

### Aman Trivedi

Sorry I pressed reply by mistake... So I was asking would it be deduced by calculating the acceleration and massed of the object which was once stationery? I'm sorry but I'm really confused

11. Mar 1, 2016

### jbriggs444

For a large brief force, neither the duration of the force nor its magnitude are individually relevant. It is their product (actually an integral) that determines how the target object moves. That product/integral is called "impulse".

The momentum of a stationary target object after a collision is equal to the "impulse" delivered during the collision.

12. Mar 1, 2016

### Aman Trivedi

You mentioned Newton's third law, so when two objects moving towards each other collide, according to newtons law the collision would cause an equal and opposite reaction, so for example if a ball collides with a wall with the force of 50 N, does it mean that the wall too would push the ball back with same force?

13. Mar 1, 2016

Yes.