Posted by **josh** on Thursday, October 1, 2009 at 5:06pm.

What average force in N is required to stop an 1,188 kg car in 9 s if it is initially traveling at 89 km/h?

f=ma

f=1,188(9.8)

f=11642.4N

This looks wrong to me. Can someone explain this to me.

- physics -
**bobpursley**, Thursday, October 1, 2009 at 5:19pm
Why did you choose 9.8 as the acceleration? It is not falling.

its average velocity during stopping is 89/2 km/hr (change that to m/s)

So it traveled in that time a distance of avgvelocity*9s...

force*distance=initial KE= 1/2 m vi^2

change vi to m/s, then solve for force.

- physics -
**Anonymous**, Thursday, October 1, 2009 at 5:32pm
Firstly F=ma but a=(v-u)/t. First convert the 89 Km/h into m/s.

89x1000/3600=24.72m/s

Then using F=m(v-u)/t

F=1188(24.72-0)/9

F=3263.04N

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