1. Two speedboats are moving at constant speeds on a straight stretch of a racec
ID: 2058139 • Letter: 1
Question
1. Two speedboats are moving at constant speeds on a straight stretch of a racecourse. At the instant shown (Boat A is behind Boat B), Speedboat A has more momentum than Speedboat B.Is the net force on Speedboat A greater than, less than, or equal to the net force on Speedboat B? Explain your answer.
?2. A 2-kg object accelerates as a net external force is applied to it. During the 5-second interval that the force is applied, the object’s velocity changes from 3 m/s east to 7 m/s west.
Several students discussing the impulse on this object state the following:
Andre: “The impulse is equal to the change in momentum, which is (2 kg)(3 m/s + 7 m/s) = 20 kg·m/s.”
Bela: “But the change in velocity is 4 m/s. We multiply by the mass to get the change in momentum, and also the impulse, which is 8 kg·m/s.”
Carleton: “The change in momentum of this object during these 5 seconds was 8 kg·m/s so the impulse applied to this object during these 5 seconds was 8/5 kg·m/s.”
Dylan: “The impulse is the force F times the time t and since we don’t know the force, we can’t find the impulse for this situation.”
?
Which, if any, of these three students do you agree with?
Andre _____ Bela _____ Carleton _____ Dylan _____ None of them______
Please explain your reasoning.
Explanation / Answer
1. They have the same net force: 0. We know this because both objects are moving at constant speed and not changing direction. There is therefore no acceleration and no force. (Recall F = ma).
2. Andre would be correct but for a sign. The impulse is the change in momentum. Bela is incorrect because the velocity also changes direction. The total change in the velocity is (taking west to be negative): -7-3 =- 10 m/s. So 2 g* -10 m/s = -20 kg*m/s. The answer is none of them.
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