1-A 500 N block sits on a horizontal surface. The coefficient of static friction
ID: 1260072 • Letter: 1
Question
1-A 500 N block sits on a horizontal surface. The coefficient of static friction is 0.7 while the coefficient of kinetic friction is 0.4. You pull on a rope attached to the block so that the tension force is horizontal. Start each part with a free body diagram.
a-What is the force which will just overcome static friction and get the block to move?
b-Now that the block is moving, what is the force which will keep it moving at a constant velocity of 2 m/s?
c-What is the force needed to get the block to accelerate at a constant acceleration of 1.5 m/s2?
2-Consider the previous problem. The free body diagram in part 2b and 2c should have four forces on it. Say the block moves 20 m.
a-What is the work done by each of the four forces for 2b?
b-What is the net work done on the block for 2b?
c-What is the work done by each of the four forces for 2c?
d-What is the net work done on the block for 2c?
Explanation / Answer
For free body diagram, just show one force towards right (thats the tension force), one towards left (thats friction), one vertically down (thats weight of the block) and one vertically up (thats the normal reaction force from the horizontal surface)
Maximum static friction on the block = 500*0.7 = 350N
THerefore the minimum force htat would get the block moving = 350N
Once the block starts moving, kinetic friction acts on it. whose maginitude is 500*0.4 = 200N
Therefore, applying a force of 200N would be sufficient to keep it moving with any constant velocity
To make it accelerate at 1.5m/s2, the force required would be 500*0.4 + 50*1.5 = 200+75 = 275N
where 50 is the mass of the block
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