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

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

(edited 1 year ago)

Reply 1

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