# Homework Help: Average force exerted on the ball

1. Oct 12, 2016

### nysnacc

1. The problem statement, all variables and given/known data

2. Relevant equations
∑Fav = mv2 - mv1

3. The attempt at a solution
We have to separate v in x dir and y dir?

2. Oct 12, 2016

### Staff: Mentor

Why? (Note that it starts from rest.)

3. Oct 12, 2016

### nysnacc

but are part one and part two asking the same thing?

4. Oct 12, 2016

### Staff: Mentor

No. One asks for the average total force on the ball. The other part asks for just the force exerted by the foot.

5. Oct 12, 2016

### nysnacc

For the average total force on the ball (1st part). what equation should i use?

For the (2nd part) force exerted by the foot, the equation is F = (mv2- mv1 ) / Δt

6. Oct 12, 2016

### Staff: Mentor

No, that will give the average total force on the ball.

7. Oct 12, 2016

### nysnacc

That's total force exerted on the ball?

8. Oct 12, 2016

### nysnacc

For part 1,

F = 1/ Δt * ∫ F dt ?? which is (mv2- mv1 ) / Δt ??

For part 2,

F = (mv2- mv1 ) / Δt - mg (cuz the question said subtract the weight)

9. Oct 12, 2016

### Staff: Mentor

Yep.

10. Oct 12, 2016

### Staff: Mentor

Yes.

Careful. Forces are vectors, so here you need to consider the directions of each.

11. Oct 12, 2016

### nysnacc

For part 2

(mv2- mv1 ) / Δt (going up)

mg (going down)

what should i do with the 60 deg which is given?

12. Oct 12, 2016

### Staff: Mentor

That's the direction of the average total force on the ball.

13. Oct 13, 2016

### nysnacc

Part 1, F = 45 N
Part 2 F = 41.236 N?

14. Oct 13, 2016

### haruspex

I'm not sure whether the hint given in the question is simply wrong or is assuming the weight, being a downward force, will be expressed as a negative quantity. Ask yourself this: does the presence of gravity increase or reduce the force needed from the boot to achieve the given velocity?
Secondly, forces are vectors. You cannot add (or subtract) forces acting in different directions by adding their magnitudes.

15. Oct 13, 2016

### nysnacc

Is #1 correct, 45 N

16. Oct 13, 2016

### nysnacc

#2 i have F = Fav (cos i + sin j) + mg (-j)

= (Fav cos )i + ( Fav sin - mg ) j

17. Oct 13, 2016

### Staff: Mentor

Yes.

18. Oct 13, 2016

### nysnacc

why is my #2 not correct?

19. Oct 13, 2016

### Staff: Mentor

You made a mistake with the sign of the weight vector.

20. Oct 13, 2016

### nysnacc

Fav - mg (-j) ??
so it is a plus?

21. Oct 13, 2016

### nysnacc

So 48.87 N?

#2 is greater than #1

22. Oct 13, 2016

### Staff: Mentor

$\vec{F_{total}} = \vec{F_{kick}} + \vec{F_{weight}}$
To find the force of the kick you must subtract the weight vector, but the weight points down.

23. Oct 13, 2016

Yes.