# Homework Help: Power/Work-Energy HW

1. Jun 8, 2012

### Parzival

1. The problem statement, all variables and given/known data
The motor of a ski boat generates an average power of 7.50 * 10^4 W when the boat is moving at a constant speed of 12 m/s. When the boat is pulling the skier at the same speed, the engine must generate an average power of 8.30 * 10^4 W. What is the tension in the tow rope that is pulling on the skier?

2. Relevant equations
Average power = force * average speed
Average power = change in energy/time
Average power = Work/time

3. The attempt at a solution

I tried setting up an equation:

8.30 * 10^4 W = F*12 m/s
F = 83,000/12 N

= 7 * 10^3 N which is wrong.

2. Jun 9, 2012

### ehild

The motor needs to move both the boat and the skier. The boat is pulled with 7.50 * 10^4 W. What is the power of the force that moves the skier then?

ehild

3. Jun 9, 2012

### harts

Another useful equation that would work in your case is

P=Fv

This comes from the fact that

P = ΔW/Δt = FΔx/Δt = Fv

Last edited: Jun 9, 2012
4. Jun 9, 2012

### Parzival

Is it 83000 - 75000 = 1.2 * 10^4?

5. Jun 9, 2012

### ehild

It is 83000 - 75000, but not 1.2 * 10^4. Check the subtraction.

ehild

6. Jun 9, 2012

### Parzival

Fail. So 8.0 * 10^4?

7. Jun 9, 2012

### e^(i Pi)+1=0

interesting derivation

8. Jun 9, 2012

### harts

Its not 8*10^4 either. That's, 80,000.

And.. thanks?

9. Jun 9, 2012

### Parzival

Fail. 8*10^3. Why thanks?

10. Jun 9, 2012

### ehild

The skier is pulled at v=12 m/s speed by 8000 W power. What is the pulling force then?

ehild

11. Jun 9, 2012

### azizlwl

deleting my post

12. Jun 10, 2012

### Parzival

So: 8000/12 N is the answer. Significant digits yield 6.7*10^2, which is correct!