PHYS 2211 Module 8 Self Assessment Practice Problems

Module 8 Self Assessment Practice Problems

A boy throws a ball of mass 0.25 kg straight upward with an initial speed of 20 m/s. When the ball returns to the boy, its speed is 17 m/s. How much much work does air resistance do on the ball during its flight?
Answer: 14 J
Using energy considerations and assuming negligible air resistance, show that a rock thrown from a bridge 20.0 m above water with an initial speed of 15.0 m/s strikes the water with a speed of 24.8 m/s independent of the direction thrown.
A sled of mass 70 kg starts from rest and slides down a 10° incline 80 m long. It then travels for 20 m horizontally before starting back up an 8° incline. It travels 80 m along this incline before coming to rest. What is the magnitude of the net work done on the sled by friction?
Answer: 1900 J
A 1.0-kg ball at the end of a 2.0-m string swings in a vertical plane. At its lowest point the ball is moving with a speed of 10 m/s.
(a) What is its speed at the top of its path?
(b) What is the tension in the string when the ball is at the bottom and at the top of its path?
Answer: (a) 4.6 m/s (b) 60 N, 1 N
Assume that the force of a bow on an arrow behaves like the spring force. In aiming the arrow, an archer pulls the bow back 50 cm and holds it in position with a force of 150 N. If the mass of the arrow is 50 g and the “spring” is massless, what is the speed of the arrow immediately after it leaves the bow?
Answer: 39 m/s

A 100 kg man is skiing across level ground at a speed of 8.0 m/s when he comes to the small slope 1.8 m higher than ground level shown in the figure.
(a) If the skier coasts up the hill, what is his speed when he reaches the top plateau? Assume friction between the snow and skis is negligible.
(b) What is his speed when he reaches the upper level if an 80-N frictional force acts on the skis?
Answer: (a) 5.4 m/s (b) 4.0 m/s

A small block of mass m slides without friction around the loop-the-loop apparatus shown.
(a) If the block starts from rest at A, what is its speed at B?
(b) What is the force of the track on the block at B?
Answer: (a) (6gR)1/2 (b) 6mg

The massless spring of a spring gun has a force constant k = 12 N/cm. When the gun is aimed vertically, a 15-g projectile is shot to a height of 5.0 m above the end of the expanded spring. How much was the spring compressed initially?
Answer: 3.5 cm
In the movie Monty Python and the Holy Grail a cow is catapulted from the top of a castle wall over to the people down below. The gravitational potential energy is set to zero at ground level. The cow is launched from a spring of spring constant 1.1 x 104 N/m that is compressed 0.5 m from equilibrium. If the castle is 9.1 m tall and the mass of the cow is 110 kg,
(a) what is the gravitational potential energy of the cow at the top of the castle?
(b) What is the elastic spring energy of the cow before the catapult is released?
(c) What is the speed of the cow right before it lands on the ground?
Answer: (a) 9.8 x 103 J (b) 1.4 x 103 J (c) 14 m/s
A pogo stick has a spring with a spring constant of 2.5 x 104 N/m which can be compressed 12.0 cm. To what maximum height from the uncompressed spring can a child jump on the stick using only the energy in the spring, if the child and stick have a total mass of 40 kg?
Answer: 33.9 cm