Assessment Questions
1. Suppose that it takes a simple pendulum 1.2 seconds to swing from its leftmost point to its
rightmost point. What is the period of the pendulum?
A:
B:
C:
D:
0.6 s
1.2 s
2.4 s
3.6 s
The correct answer is: C. The period of a pendulum is the amount of time required for the
pendulum to complete one full back-and-forth motion – from its rightmost point back to its
rightmost point again, for example. The time required to swing from its rightmost point to its
leftmost point is half of a period. It will require the same amount of time to swing back, so the
period is 2.4 s.
2. Suppose a simple pendulum consists of a 3.5 m string and a 0.3 kg bob, and its period is
3.75 s. What would be the period of the pendulum if the mass of the bob were doubled?
(Assume that gravity remains unchanged.)
A:
B:
C:
D:
1.875 s
3.75 s
7.5 s
Cannot be determined.
The correct answer is: B. Changing the mass of the pendulum bob has no effect on the
period of the pendulum. So, if the pendulum had a period of 3.75 s with a 0.3 kg bob, then it will
have a period of 3.75 s with a 0.6 kg bob.
3. A pendulum with a string of length 2 m has a period of about 2.8 s. What would be the
period of the pendulum if the length of the string were increased to 8 m?
A:
B:
C:
D:
1.4 s
2.8 s
5.6 s
11.2 s
The correct answer is: C. The period of a pendulum is proportional to the square root of the
length of the string. In other words, if the length of the string is multiplied by 4, the period of the
pendulum will be multiplied by 2 (the square root of 4).
4. A pendulum with a period of 1.8 s on Earth is moved to another planet. On the new planet,
its period is 2.5 s. What can you conclude about this new planet?
A: The force of gravity on the new planet is less than it is on Earth.
2
B: The force of gravity on the new planet is
the same as it is on Earth.
C: The force of gravity on the new planet is greater than it is on Earth.
D: There is not enough information to answer the question.
The correct answer is: A. The period of a pendulum is inversely related to the acceleration
due to gravity. Therefore, if the period of the given pendulum increases, it must be due to a
decrease in the force of gravity.
5. A simple pendulum swings with a period of 1.5 s. What would the period of the pendulum be
if the length of its string were doubled, the mass of its bob were cut in half, and the force of
gravity were doubled?
A:
B:
C:
D:
0.5 s
1.5 s
3s
There is not enough information to determine the answer.
The correct answer is: B. The mass of a pendulum’s bob has no effect on its period, so this
change is irrelevant. Multiplying the length of the string by two would multiply the period of the
pendulum by the square root of two. Multiplying the force of gravity by two would divide the
period of the pendulum by the square root of two. Together, those two changes cancel each
other out. Therefore, the pendulum would have exactly the same period that it had before, 1.5
seconds.