An 18-kg child is riding in a child-restraint chair, securely fastened to the seat of a car (Fig. 4–77). Assume the car has speed 45 km/h when it hits a tree and is brought to rest in 0.20 s. Assuming constant deceleration during the colli- sion, estimate the net horizontal force F that the straps of the restraint chair exert on the child FIGURE 4–77 to hold her in the chair. Problem 97.

The mass of a car is 1000 kg. The total opposing force for the moving car is 10 % of theweight of the car. Calculate the net force needed to move the car, if it is movingwith constant speed a) on a flat road, b) upwards on an incline of 5 °.with acceleration of 2.0 m/s2 on c) a flat road, d) downwards on an incline of 5 °.

(II) Figure 4–45 shows a block (mass ma) on a smooth hori- zontal surface, connected by a thin cord that passes over a pulley to a second block (mB), which hangs vertically. (a) Draw a free-body diagram for each block, showing the force of gravity on each, the force (tension) exerted by the cord, and any normal force. (b) Apply Newton's second law to find formulas for the acceleration of the system and for the tension in the cord. Ignore friction and the masses of the pulley and cord. FIGURE 4-45 Problems 51, 52, and 53. Mass m rests on a smooth horizontal surface, mg hangs vertically. mg

A box rests on the (frictionless) bed of a truck. The truck driver starts the truck and accelerates forward. The box immediately starts to slide toward the rear of the truck bed. Discuss the motion of the box, in terms of Newton's laws, as seen (a) by Mary standing on the ground beside the truck, and (b) by Chris who is riding on the truck (Fig. 4–35). FIGURE 4-35 a Question 2. Воx

Two blocks m, = 1kg and m2 > connected inextensible massless string which m1 are by an m2 goes over a frictionless pulley. Take g = 10 m/s². If the magnitude of the acceleration of the system is a = 5 m/s“, then the mass m2 would be: O 2.5 kg O 9 kg O 15 kg

A box weighing 77.0 N rests ona table. A rope tied to the box runsvertically upward over a pulley anda weight is hung from the otherend (Fig. 4–45). Determine theforce that the table exerts on the box if the weight hanging on the other side of the pulleyweighs (a) 30.0 N (b) 60.0 N (c) 90.0 N.