PHYS 2211 Module 7 Self Assessment Practice Problems

Module 7 Self Assessment Practice Problems

A shopper pushes a grocery cart 20.0 m at constant speed on level ground, against a 35.0 N frictional force. He pushes in a direction 25° below the horizontal.
(a) What is the work done on the cart by friction?
(b) What is the work done on the cart by the gravitational force?
(c) What is the work done on the cart by the shopper?
(d) Find the force the shopper exerts.
(e) What is the total work done on the cart?
Answer: (a) -700 J (b) 0 J (c) 700 J (d) 38.6 N (e) 0 J
How much work does the force N do on a particle as it moves from x = 2 m to x = 5 m?
Answer: -1.83 J
It takes 500 J of work to compress a spring 10 cm. What is the force constant of the spring?
Answer: 100 kN/m
(a) Calculate the force needed to bring a 950-kg car to rest from a speed of 90.0 km/h in a distance of 120 m (a fairly typical distance for a non-panic stop).
(b) Suppose instead the car hits a concrete abutment at full speed and is brought to a stop in 2.00 m. Calculate the force exerted on the car and compare it with the force found in part (a).
Answer: (a) 2.47 kN (b) 60 x 2.47 kN
An 8.0-g bullet with a speed of 800 m/s is shot into a wooden block and penetrates 20 cm before stopping. Use the Work-Energy Theorem to find the average force of the wood on the bullet. Assume the block does not move.
Answer: 12.8 kN

A small block of mass 200 g starts at rest at A, slides to B where its speed is vB = 8.0 m/s then slides along the horizontal surface a distance 10 m before coming to rest at C.
(a) What is the work of friction along the curved surface?
(b) What is the coefficient of kinetic friction along the horizontal surface?
Answer: (a) -1.5 J (b) 0.4
(a) How long will it take an 850-kg car with a useful power output of 40.0 hp (1 hp equals 746 W) to reach a speed of 15.0 m/s, neglecting friction?
(b) How long will this acceleration take if the car also climbs a 3.00-m high hill in the process?
Answer: (a) 3.20 s (b) 4.04 s
Calculate the power output needed for a 950-kg car to climb a 2.00° slope at a constant 30.0 m/s while encountering wind resistance and friction totaling 600 N.
Answer: 27.7 kW
A girl pulls her 15-kg wagon along a flat sidewalk by applying a 10-N force at 37° to the horizontal. Assume that friction is negligible and that the wagon starts from rest.
(a) How much work does the girl do on the wagon in the first 2.0 s.
(b) How much instantaneous power does she exert at t = 2.0 s?
Answer: (a) 8.5 J (b) 8.5 W
A particle of mass 2.0 kg moves under the influence of the force F(x)=-5x2 + 7x N. If its speed at x = -4 m is v = 20 m/s, what is its speed at x = 4 m?
Answer: 14 m/s