7. a car moving at 10m/s (about 22.4 mph) crashes into a barrier and stops in 0.25 m.
a) find the time required to stop the car.
b) if a 20-kg child were to be stopped in the same time as the car, what average force
must be exerted?
c) approximately what is the mass of an object whose weight equals the force from part
b? could you lift such a mass with your arm?
d) what does the answer to part c say about holding an infant on your lap instead of
using a separate infant restraint?

a) 0.05s

b) 4000N

Explanation:

a)When car is stopped its final velocity become zero

U- 10 m/s

V- 0 m/s

S - 0.25 m

t -?

S = (v+u)*t/2

0.25 =(10+0)*t/2

t = 0.05s

b) If we happened to calculate the avarage force we have to consider about acceleration

V= 0

U = 10

t = 0.05 s

a =?

V = U + at

0 = 10 -a * 0.05

a = 200 m/s2

F = m *a

= 20 * 200

= 4000N

the answer would be a

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