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# Why can't v=u+at be used? watch

1. Hi,

I know this might sound like a silly question but it's worth 3 marks and so I didn't know whether I'd missed something.

The question says:
Car mass = 90kg
Speed = 30kmh^-1
Air resistance = 90N
Forward driving force = 170N

Part One asks you to work out the acceleration which I got to be 0.08m/s^2

It then says...
A student is told that the driving force is kept constant for 5s and is asked to calculate the final speed of the car. Explain why the equation v=u+at cannot be used.

Would I be right (and complete) in saying that we cannot use this equation because we do not know the magnitude of the resistance force at the new speed?

Thanks,
Blake
2. It's not explitly stated in the question but as you increase your speed you'll increase the resistance force. This means that the NET force is non-constant, leading to non constant acceleration. This means you cannot use the suvat equations (including v=u+at) as they are constant accelerations equations
3. You would have to integrate an acceleration expression or differentiate a position expression
4. (Original post by EwanWest)
It's not explitly stated in the question but as you increase your speed you'll increase the resistance force. This means that the NET force is non-constant, leading to non constant acceleration. This means you cannot use the suvat equations (including v=u+at) as they are constant accelerations equations
Awesome, I thought so, I didn't think my answer was worth three marks but I didn't include why the resistance force would change, thanks

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