13, (From an old test): A point particle of mass m = 20 g is moving on a flat su
ID: 1778936 • Letter: 1
Question
13, (From an old test): A point particle of mass m = 20 g is moving on a flat surface when it reaches a slope which brigs it up by h = 0.8 m. The speed of the particle in position A is v 5 m/s Page 7 of 22 63 (a) Suppose that there is no friction. What is the velocity of the particle when it reaches position B? Hint: see conceptual checkpoint 8-2 page 232 b) Suppose that there is friction, and that the velocity of the particle in B is 0. What is the work done by the friction from A to B? Hint: Remember that friction is a non-conservative force, and so Wfriction E- E, where E is the mechanical energy.Explanation / Answer
(a)
at point A
initial mechanical energy EA = (1/2)*m*vA^2 + m*g*hA
at point B
final mechanical energy EB = (1/2)*m*vB^2 + m*g*hB
Work done by friction = 0
Wf = 0
Ef = Ei
(1/2)*m*vB^2 + m*g*hB = (1/2)*m*vA^2 + m*g*hA
(1/2)*vB^2 + g*hB = (1/2)*vA^2 + g*hA
hA = 0 hB = h = 0.8 m
vA = 5 m/s
(1/2)*vB^2 + (9.8*0.8) = (1/2)*5^2 + 0
vB = 3.05 m/s <<<-----ANSWER
==================
Wfriction = Ef - Ei
Wfriction = (1/2)*m*vB^2 + m*g*hB - (1/2)*m*vA^2 - m*g*hA
hA = 0 hB = h = 0.8 m
vA = 5 m/s vB = 0
mass m = 0.02 kg
Wfriction = (1/2)*0.02*0 + (0.02*9.8*0.8) - (1/2)*0.02*5^2 - 0 = -0.0932 J <<--ANSWER
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