Physics Review Question

Unit: Mechanics Year: 1991 Question#: 12
Question: A 0.025 -kilogram bullet is fired from a rifle by an unbalanced force of 200 newtons. If the force acts on the bullet for 0.1 second, what is the maximum speed attained by the bullet?

(1 ) 5 m/s
(2 ) 20 m/s
(3 ) 400 m/s
(4 ) 800 m/s
What is this question really asking?

Explanation by: Rachel M
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Answer 1
5 m/s


If you multiplied 200 by .025, you would get 5. This is incorrect because force times mass does not give you m/s which is what you are looking for. You need to take into account the time factor, as well as use the correct formulas. Force is never multiplied by mass in the problems you will be doing. return to top

Answer 2
20 m/s


You could obtain 20 if you multipled 200 by .1, but this is incorrect because force * time does not give you m/s, which are the units you are looking for. Force * time creates an impulse, not a speed which is what the question asks for. Try again using the formulas:
F=ma and a= change in velocity / change in time. return to top

Answer 3
400 m/s


Most likely, you did all the calculations correct up until the end (you can check yours with #4). But then after finding that the final velocity was 800 m/s, you probably plugged it into the formula: average velocity = (Vf + Vi) / 2. Thus, getting 400 m/s as your answer. The reason this is wrong is because the question does not ask for the average velocity of the bullet, it asks for the maximum speed of the bullet. This is the same as the final velocity. return to top

Answer 4
800 m/s


you know: m= .025 kg , F= 200 N , t= .1 s , Vi= 0 m/s you don't know: a , Vf
You should have done the following calculations:

F=ma 200N = .025 kg * a a= 8000 m/s2 a= change in velocity / change in time

8000 m/s^2 = V / .1 s change in velocity= 800 m/s change in velocity = Vf - Vi

800 m/s - 0 m/s = 800 m/s Vf= 800 m/s

* note: it should really be speed and not velocity, since there is no direction on the bullet, but since speed and velocity are obtained through the same calculations, we use the one given on the reference table, V return to top

What's this question really asking?

What is the final velocity ( speed) reached by the bullet? Do you know how to plug these numbers into the right formulas in order to find the correct answer? return to top

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