11/14/2017 Name Section TEST 3 AU 17 PHYSICS 4A 2. (35 POINTS) A block of mass m

Question

11/14/2017 Name Section TEST 3 AU 17 PHYSICS 4A 2. (35 POINTS) A block of mass m 1.000 kg slides count clockwise between points A and B along the inside of a quarter circular track of radius R= 20.0 m. The block starts out at point A at rest at the end of a horizontal diameter. When it reaches point B at the other end of the quarter-circle, the speed is only V = 18.00 ms, indicating the presence of friction on the track. After point B, the block then continues on a flat, horizontal part of track until it comes to rest at point C. (a) (15) What is the work done by the friction force - in Joules between points A and B? Is your answer positive or negative? Explain. (b) (5) What is the heat energy in Joules- generated during the motion between points A and B? Is your answer positive or negative? Explain. (c) (15 points) Assume the coefficient of kinetic friction on the flat, horizontal part is =0.300, what is the distance D between points B and C? Note that the diagram-schematic below may not reflect the scale corresponding to your computations. rest

Explanation / Answer

(a) Using work energy theoram

Energy at A - Energy at B = Work by friction

Mgh - 1/2 mv^2 = Wf

(1 x 9.8 x 20) - (.5 x 1 x (18)^2) = Wf

Wf = 34 J

(b) The friction force is convering in heat energy so

Heat = Wf = 34 J

(c) Friction (f) = mu x N

here N = mg = 1 x 9.8 = 9.8

f = .3 x 9.8

f = 2.94 N

acceleration (a) = F/m = f/m = 2.94 / 1 = 2.94 m/s^2 = -2.94 m/s^2 ("-" deaccleration)

Now using 3rd equation of motion

V^2 = u^2 + 2aS

here V^2 = 0

0 = (18)^2 + (2 x - 2.94 x S)

5.88 S = 324

S = 55.10 m

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