# Homework Help: Working out speed, kinetic energy and resistive force

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1. Nov 9, 2015

### Meezus

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

2. Relevant equations

3. The attempt at a solution

a) I'm not sure how to calculate speed without having both distance and time but I believe it will revolve around calculating gravitational potential energy and kinetic energy.

bi) I think this is just simply using the formula kinetic energy = 1⁄2 × mass × speed2.

bii/biii) I'm really struggling to work out these. I think It might revolve around using the speed from the first question and then take it away from 22m/s.

2. Nov 9, 2015

### haruspex

You are on the right track for a) and b.i).
I suggest you solve those two first. It might then be more apparent how to solve b.ii)

3. Nov 9, 2015

### Meezus

I believe I have solved them now.

if possible could you help with the last one?

4. Nov 9, 2015

### haruspex

What equation do you know relating work and distance?

5. Nov 9, 2015

### Meezus

Work = Force x Distance.?

6. Nov 9, 2015

### haruspex

Looks promising. How would it apply here? Explain in words.

7. Nov 9, 2015

### Meezus

is it something like:
Work done = change in kinetic energy,
Work = - 11250 J?

8. Nov 9, 2015

### haruspex

Yes, but I meant the work = force x distance equation. Can you express that in respect of frictional force and the circumstances in this question?

9. Nov 9, 2015

### Meezus

The amount of work done is equal to the frictional force times by the distance? I'm sorry i'm not 100% sure.

10. Nov 9, 2015

### haruspex

Yes (given that the force is constant; in general the relationship is an integral).

11. Nov 9, 2015

### Meezus

Am I able to work out the magnitude from this?

12. Nov 10, 2015

### haruspex

Yes, you have all the information. (One clarification: the force is not constant as a vector here, but it is constant in magnitude. This works out ok because the force of friction is always parallel to the motion, so it still reduces to force x distance travelled along the path. You do not know and do not need to know the end-to-end displacement as a vector.)