A child is riding in a child-restraint chair, securely fastened to the seat of a car. Assume the car has speed 47 km/h when it hits a tree and is brought to rest in 0.20 s. The mass of the child is 19 kg. Assuming constant deceleration during the collision, estimate the net horizontal force that the straps of the restraint chair exert on the child to hold her in the chair.

F = 1235 N

Explanation: Newton's Second Law of Motion describes the effect of mass and net force upon acceleration:

Acceleration is the change of velocity in a period of time:

Velocity of the car is in km/h. Transforming it in m/s:

v = 13 m/s

At the moment the car decelerates, acceleration is

a = 65 m/s²

Then, force will be

= 1235 N

The horizontal net force the straps of the restraint chair exerted on the child to hold her is 1235 newtons.

the answer is c. the lower legs are levers, and the knees are fulcrums. the ankles hold the loads.