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# Physics Question Help watch

1. 5char
2. (Original post by Thomas23)
If anyone could help, i'll appreciate it.

An electric car is fitted with a battery, which is used to drive an electric motor that drives the car. The battery has a maximum power output of 12kW, which gives a maximum driving force of 600N.

Calculate (i) the top speed of the car, (ii) the car's maximum range, if the battery lasts for 90 minutes at maximum power output without being recharged.

I worked out both, but wasn't very sure as it seemed to easy. So this is what I did, if anyone could tell me if its ok or if I've gone in the completely wrong track

For (i) P=Fv, 12kW = 600N x v , 12kW / 600N = v (12x10^3W / 600N) = 20 = v = 20m/s, so top speed is 20m/s ?

For (ii) 90mins = 5400seconds, 5400 x 20 = 10,800m = max range?

Thanks in advance.
To me, this is correct, apart from the calculation in bold

EDIT: Now that I've reread the question, there are some things that made me think. First of all, the greater the car's speed is, the less force it can use (f=P/v). This doesn't really make sense for the statement that the maximum driving force is 600N. If it said that it was the force at the maximum speed, then there would be no problem.

The reason cars do not accelerate from rest with infinite acceleration is that the maximum force that can be used by the car is determined by either the friction of the tires (as static friction is greater than kinetic friction, it's more efficient to accelerate - and, likewise, brake - without sliding of the tires) or the power of the car, whichever gives smaller value.

Second thing is that it does not make sense to ask about the maximum speed if there is information about resistive forces. If there are none (apart from static friction, which does not do any work), the car will accelerate infinitely. Again, if 600N was the driving force at the max speed, this problem would have been resolved as this would imply that at that speed there are resistive forces, and 600N is used to keep the car moving at constant speed.

To be honest, to me the question doesn't make much sense as it is worded this way. I can only guess they meant 600N to be the force at the maximum speed; then your solution is correct (recalculate the range).
3. Jaroc is right for the first question, if there is no information about, air resistance or some other sort of resistive force then you can't really speak of a top speed.

For the second one, that looks fine although if you want to be really pedantic you could take in to account the speeding up of the car & at the end you could let it glide without having the motor on. That should only bring a minor difference though.

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