Unit: work and energy
Year: 1994 Question#: 07
Question: The diagram below represents a 10.-newton block sliding down a 30-degree incline at a constant speed.
The force of friction on the block is approximately
(1 ) 5.0 N
(2 ) 10. N
(3 ) 49 N
(4 ) 98 N
What is this question really asking?
Explanation by: Christine A (partially)
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Answer 1
5.0 N
CORRECT
There is actually no need to do any math on this problem. You must realize that "constant speed" means the net force is zero. So the friction must be equal but opposite to the component of the weight that is parallel to the ramp (see diagram).
In order to narrow it down to two answers, all you need to know is that the force
of friction is not likely to be greater than the weight of the object itself. Right
there you can disregard answers 3 and 4, as they are both greater than 10N. After
that, you can decide on the correct answer by realizing that the force of friction on the
block will not be the same as the weight because it is on an inclined plane. Thus,
answer 2 is disregarded and answer 1 is the only choice left.
return to top
Answer 2
10. N
INCORRECT
There is actually no need to do any math on this problem. You must realize that "constant speed" means the net force is zero. So the friction must be equal but opposite to the component of the weight that is parallel to the ramp (see diagram).
In order to narrow it down to two answers, all you need to know is that the force
of friction is not likely to be greater than the weight of the object itself. Right
there you can disregard answers 3 and 4, as they are both greater than 10N. After
that, you can decide on the correct answer by realizing that the force of friction on the
block will not be the same as the weight because it is on an inclined plane. Thus,
answer 2 is disregarded and it must be an incorrect choice.return to top
Answer 3
49 N
INCORRECT
There is actually no need to do any math on this problem. You must realize that "constant speed" means the net force is zero. So the friction must be equal but opposite to the component of the weight that is parallel to the ramp (see diagram).
In order to narrow it down to two answers, all you need to know is that the force
of friction is not likely to be greater than the weight of the object itself. Right
there you can disregard answers 3 and 4, as they are both greater than 10N. After
that, you can decide on the correct answer by realizing that the force of friction on the
block will not be the same as the weight because it is on an inclined plane. Thus,
answer 3 cannot be the correct choice.
return to top Answer 4
98 N
INCORRECT
There is actually no need to do any math on this problem. You must realize that "constant speed" means the net force is zero. So the friction must be equal but opposite to the component of the weight that is parallel to the ramp (see diagram).
In order to narrow it down to two answers, all you need to know is that the force
of friction is not likely to be greater than the weight of the object itself. Right
there you can disregard answers 3 and 4, as they are both greater than 10N. After
that, you can decide on the correct answer by realizing that the force of friction on the
block will not be the same as the weight because it is on an inclined plane. Thus,
answer 4 cannot be the correct choice.
return to top What's this question really asking?
a) Do you know that when an object is moving, the force of friction acts in the opposite direction of the motion? b) What does "constant speed" tell you about net force? return to top
