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# Momentum/Energy watch

1. energy.docx

There's the question and mark scheme on the attachment. I'm stuck on part c, the final part.

Power=work done/time

Energy=work done

So energy/power=time. I understand how they got this. But I can't understand how they worked out power as power=force X velocity. They divided the velocity of 1.33 by 2, but why?

Thank you so much
2. (Original post by krisshP)
energy.docx

There's the question and mark scheme on the attachment. I'm stuck on part c, the final part.

Power=work done/time

Energy=work done

So energy/power=time. I understand how they got this. But I can't understand how they worked out power as power=force X velocity. They divided the velocity of 1.33 by 2, but why?

Thank you so much
velocity=distance/time
work done=force(component in direction of motion) x distance

so force=work done/distance

so force x velocity=(work done x distance)/(time x distance)=Power

Without really looking at it I'd say that the division by two would be a result from energy lost in the collision as it is inelastic. You could work this out from conservation of momentum.
3. (Original post by krisshP)
energy.docx

There's the question and mark scheme on the attachment. I'm stuck on part c, the final part.

Power=work done/time

Energy=work done

So energy/power=time. I understand how they got this. But I can't understand how they worked out power as power=force X velocity. They divided the velocity of 1.33 by 2, but why?

Thank you so much
Because the velocity is not constant. The trucks slow down from 1.33m/s to zero and 1.33/2 is the average velocity. You need force x average velocity to calculate the (average) power over that time interval.
4. (Original post by Stonebridge)
Because the velocity is not constant. The trucks slow down from 1.33m/s to zero and 1.33/2 is the average velocity. You need force x average velocity to calculate the (average) power over that time interval.
Oh yeah, that does make sense now

Thanks for the help

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