Question 9
An object weighing 5 N has a momentum of 6 kg m s−1 at a certain moment. Because of a force applied against its motion, its velocity fell to 4 m s−1 in 4 s. What force was applied to the object? (Take g = 10 m s−2.)
The right answer is the one marked above.
1 N. This is a four-step problem — it needs three separate ideas from the chapter together.
1. mass: m = weight ÷ g = 5 ÷ 10 = 0.5 kg
2. initial velocity: v = momentum ÷ m = 6 ÷ 0.5 = 12 m s−1
3. acceleration: a = (4 − 12) ÷ 4 = −2 m s−2
4. force: F = m a = 0.5 × 2 = 1 N
The negative sign says the force was applied against the motion — which the question told us. So the magnitude of the force is 1 N.
Two common slips: using the weight of 5 as the mass, and forgetting to get the initial velocity from the momentum, taking 4 m s−1 as the change in velocity instead.