Impulse: multiple choice question

[zanyzoya]

Hi all
Can anyone please help me with Q14 (attached) as you can see I am getting 167000N however the correct answer is 83000N
Many thanks
ZanyZoya

 

[kuruman]

Your estimate of the time for the collision (t = d/v) is inappropriate. This equation is used if the acceleration is zero. Here this is clearly not the case. What is is the average acceleration of a car that comes to rest over a distance of 0.30 m when it has initial speed 10 m/s?

 

[zanyzoya]

Hmm we can’t use suvat equations as acceleration is not constant, we could use average acceleration =change in velocity/time but we don’t know time... hmm

 

[kuruman]

Hmm we can’t use suvat equations as acceleration is not constant ...

You have to assume that the acceleration is constant, otherwise you cannot solve the problem. What you cannot assume is that the acceleration is zero, which is what you did initially. So yes, use the SUVAT equations.

 

[zanyzoya]

Thanks for your help kuruman, I think I’ve got it now.