Question
A car starts from rest and travels for 5.0 seconds with a uniform acceleration of +1.5 m/s^2. The driver then applies the brakes, causing a uniform acceleration of -2.0 m/s^2. If the brakes are applied for 3.0 seconds, how fast is the car going at the end of the braking period, and how far has it gone from its start?
Answers
bobpursley
There are many ways to do this, some easy, some hard.
Momentum and impulse are conserved.
F*time=final momentum mv
but force=mass*acceleration
m*1.5m/s^2*5sec-m*2.0m/s^2*3sec=m(vf)
solve for Vf
Distance?
distance1=1/2 a t^2=1/2(1.5)5^2
distance2=vi*t-1/2 (2)3^2
where vi=at from the first or vi=1.5*5
so add the distances.
Momentum and impulse are conserved.
F*time=final momentum mv
but force=mass*acceleration
m*1.5m/s^2*5sec-m*2.0m/s^2*3sec=m(vf)
solve for Vf
Distance?
distance1=1/2 a t^2=1/2(1.5)5^2
distance2=vi*t-1/2 (2)3^2
where vi=at from the first or vi=1.5*5
so add the distances.
19.0
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