Unit: Mechanics
Year: 1993 Question#: 17
Question: Each diagram below shows a different block being pushed by a force across a surface at a constant velocity.
In which two diagrams is the force of friction the same?
(1 ) A and B
(2 ) B and D
(3 ) A and D
(4 ) C and D
What is this question really asking?
Explanation by: Jessica S
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Answer 1
A and B
INCORRECT
Since the object is moving at a constant velocity, the net force is zero. Therefore, the force of friction must be equal in magnitude but opposite in direction to the pushing force. This is the only way that the net force could be zero. In Diagrams A and B, the pushing forces are not equal. Therefore, the forces of friction are also unequal. Do not get tricked by the masses in these pictures--In diagrams A and D, the masses of the objects are the same. However this is trivial; it has no effect on the force of friction. return to top
Answer 2
B and D
INCORRECT
Since the object is moving at a constant velocity, the net force is zero. Therefore, the force of friction must be equal in magnitude but opposite in direction to the pushing force. This is the only way that the net force could be zero. In Diagrams B and D, the pushing forces are not equal, therefore, the forces of friction in these diagrams are also unequal. Do not get tricked by trying to make this complicated. If you multiplied the force and the mass of the objects in these diagrams, you would yield the same number. However, this is unimportant to the answer in this question, which depends only on the pushing force. return to top
Answer 3
A and D
CORRECT
Since the object is moving at a constant velocity, the net force is zero. Therefore, the force of friction must be equal in magnitude but opposite in direction to the pushing force. This is the only way that the net force could be zero. In diagrams A and D, the pushing forces are equal, therefore, the forces of friction in these diagrams are aso equal. return to top
Answer 4
C and D
INCORRECT
Since the object is moving at a constant velocity, the net force is zero. Therefore, the force of friction must be equal in magnitude but opposite in direction to the pushing force. This is the only way that the net force could be zero. In Diagrams C and D, the pushing forces arenot equal, therefore, the forces of friction in these diagrams are also unequal. return to top
What's this question really asking?
This Question is testing your knowledge of forces, by being able to recognize that the net force is zero when the velocity is constant. return to top
