# Average force of a steel ball

1. Apr 28, 2008

### chocolatelover

1. The problem statement, all variables and given/known data
A 3.00 kg stell ball strikes a wall with a speed of 9.0m/s at an angle of 60° with the surface. It bounces off with the same speed and angle. If the ball is in contact with the wall for .200 s, what is the average force exerted by the wall on the ball?

2. Relevant equations
ΣFavg=1/Δt integral ΣFdt from ti to tf

3. The attempt at a solution

F avg=1/.200(3.00kg)(9.8m/s^2)x|0 to .200
=29.4N

Could someone please tell me if this looks correct and if now could someone please show me where I went wrong?

Thank you very much

2. Apr 28, 2008

### Nabeshin

The force you use here is the force of gravity, which isn't what the question is asking. The question is asking for the average force of the water on ball. You don't need to worry about gravity for this problem, because it is merely an impulse problem. Remember:
$$\int F dt=\Delta P$$

3. Apr 28, 2008

### chocolatelover

Thank you very much

Does this look right?

integral from 1.25 to .800 (.110kg)(9.8m/s^2)dt

=.8624-1.35
=-.4876

Thank you

4. Apr 28, 2008

### chocolatelover

Edit:

Does this look right?

integral 3.00(9.8)dt from 0 to .200=
3.00kg(9.8m/s^2)x=
3.00(9.8)(.200)=
5.88

Thank you very much

5. Apr 28, 2008

### alphysicist

Hi chocolatelover,

I think you misunderstood Nabeshin's post. We are searching for the force, we don't know what it is. (When you set the force as 3 kg * 9.8 m/s^2, that is the force of gravity on the ball; but you want the force from the wall.)

If they are looking for the average force, the integral form reduces to

$$\vec F_{\rm avg}(\Delta t)= \Delta \vec p$$

How do you find $\vec F_{\rm avg}$? Can you evaluate the other parts of the equation?

6. Apr 28, 2008

### chocolatelover

Does this look right?

Favg(.200s)=3.0kg(9.0)
Favg=-135N

Thank you

7. Apr 28, 2008

### alphysicist

You need to take into account the vector nature of the formula. In the x direction, and for a one object system, the formula is:

$$F_{{\rm avg},x} (\Delta t)= m v_{f,x} - m v_{i,x}$$

and a similar equation for y.

To evaluate this, you'll need to find the x and y components of the initial and final velocities, and plug them into the x equation and the y equation.

8. Apr 30, 2008

### chocolatelover

Thank you very much

Regards