Question 1: (Potential energy) A 0.145 - kg ball is thrown upward with speed v1
ID: 2243455 • Letter: Q
Question
Question 1: (Potential energy)
A 0.145 - kg ball is thrown upward with speed v1 = 16.0 m/s. At a certain vertical height H, the ball has speed v2 = 2.00 m/s. As it rises to height H, the force of air friction does work of magnitude 4.00 J on the ball.
(a) What is the direction of the air friction force on the ball?
(b) Is the work done by the air friction force positive or negative? Explain briefly.
(c) Is the work done by the gravitational force on the ball positive or negative? Explain briefly.
(d) What is the height H? (Use either the work energy theorem or convervation of energy in your solution)
(e) Assume the air friction force is constant. In parts (a) and (b), you addressed the direction of the friction force. In this part, find the magnitude fk of the constant force of air resistance exerted on the ball during its upward motion.
Question 2: (Conservation of energy)
A block of mass M = 1.000 kg has been released from a position above a mass-less, un-deformed spring with platform. Just before the block lands on the spring platform, its speed is V1. After the block lands on the platform, it compresses the spring downward a distance h = 0.500 m before momentarily coming to rest. the force constant of the spring is k = 60.00 N/m.
(a) What is the kinetic energy of the block just before landing on the spring platform?
(b) What is speed V1 of the block just before landing on the spring platform?
(c) What is the work done (in Joules) by the spring during the compression? Is the spring work positive or negative? Explain briefly.
(d) What is the work dne (in Joules) by the gravitational force during the compression? Is the gravitational work positive or negative? Explain briefly.
Explanation / Answer
1. a) Down (opposite to v)
b) Negative (force is down while actual displacement is up, Product is negative)
c) Negative (same reason as above, gravity acts down)
d) m*16*16/2 = m*g*H + m*2*2/2 + 4
Put m and solve for H ; H = 10m
e) Force = W/dis = 4/10 = 0.4 N
2. a) KE at top + PE at top = Spring energy at bottom
KE + mgh = k*h*h/2 ; solve, KE = 2.6 J
b) v1 = sqrt(2KE/m) = 2.28 m/s
c) Work done = kx2/2 = 7.5 J (Negative, force is up, displacemetn down)
d) Work = mgh = 4.9 J (positive, both force and displacement in same direction)
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