Download Physics 8A MT 1 SU20 Solutions (1)

Physics 8A
Summer 2020
MIDTERM #1
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1. A football is thrown at an unknown speed and angle (although we know the vertical
component of the initial velocity is upward). It then lands at a location 15.0 m below the
level at which it was thrown. Someone observes the football to be traveling at a speed of
25.0 m/s and in a direction 65.0 below the horizontal just before striking the ground.
(a) What was the initial speed of the football? At what angle was it thrown above the
horizontal? (b) How much time did the ball spend in the air? (c) How far horizontally did
the football travel?
15 m
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Physics 8A
Summer 2020
MIDTERM 1
2. Block A has a mass of 20.0 kg. It is attached to a rope, which is slung around a massless,
frictionless pulley and attached at the other end to block B, which has a mass of 25.0 kg. The
incline makes an angle of 15.0 with the horizontal. There is no friction between block B and the
incline. The blocks are initially at rest. (a) What minimum static friction coefficient must there
be between block A and the surface to prevent the blocks from slipping? (b) Suppose the blocks
do slip, and accelerate. If the kinetic friction coefficient between block A and the surface is 0.10,
what is the tension in the rope?
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Physics 8A
Summer 2020
MIDTERM 1
3. A block of mass 5.00 kg is pushed against a horizontal elastic spring with a spring constant
k = 75.0 N/m. The block is then released from rest, and moves up a 25.0 incline that is 15.0 m
long. It then travels along another horizontal surface before entering a loop of radius 10.0 m. All
surfaces are frictionless. Find the minimum spring compression needed in order for the block not
l e c n ac i h he l
hen i a he
f the loop.
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work is done by nonconservative forces
mechanical energy is conserved
But what speed does the block need to have in order
not to lose contact at the top
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