# Work, Force, Energy and power question about moving car.

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1. Apr 8, 2015

### Ethan_Tab

1. The problem statement, all variables and given/known data
A car has a power output of 45kW and travels at a constant speed of 30m/s. What force is the engine applying on the car?

2. Relevant equations
F=m*a
W=ΔKE+ΔGPE
KE=½mv^2
GPE=mgh
W=F*d
P=ΔE/ΔT
gh_1+½v_1^2=gh_2+½v_2^2

3. The attempt at a solution
I really have no clue. Since there is no change in energy (ΔE) in terms of kinetic or gravitational potential energy, I don't know what to do.

Any help is much appreciated.

2. Apr 8, 2015

### Noctisdark

Does F*v (Force*Velocity) sound familiar to you ?
[Edit: In case not, power is change of energy over change in time, so P = ΔW/Δt = Δ(F*x)/Δt = F*Δx/Δt = F*v]

Last edited: Apr 8, 2015
3. Apr 8, 2015

### Ethan_Tab

Yes, but usually only in the context of force of friction where

P=Ff*d/t and as such P=Ff*v.

4. Apr 8, 2015

### Noctisdark

You're right, but not only for friction,
In general, P = dW/dt so you can work it out here :)

5. Apr 8, 2015

### Ethan_Tab

Hmm.. so with that I get,

45kW=ΔF*d/Δt---since ΔF*d or ΔKE is 0, does that mean the engine is not applying any force on the car?

6. Apr 8, 2015

### Noctisdark

If the engine is applying some force on the car, it would be accelerating and but it's moving at a constant speed (and have a mass) so it has kenetic energy in fact, but it's not the purpose of the exercice I guess, it would be better to consider, in this case that P = F*v and take a force out of that :)

7. Apr 8, 2015

### Ethan_Tab

Aha! I understand. Thanks for the help mate :)

8. Apr 8, 2015

### Noctisdark

Welcome ^^ :)