A heavy boy and a light girl are balanced on a massless seesaw. If they both move forward so that they are one-third their original distance from the pivot point, what will happen to the seesaw?
A. It is impossible to say without knowing the masses and the distances.
B. Nothing, the seesaw will still be balanced.
C. The side the boy is sitting on will tilt downward.
D. The side the girl is sitting on will tilt downward.
The correct answer and explanation is:
The correct answer is C. The side the boy is sitting on will tilt downward.
Explanation:
To understand this, we need to think about how a seesaw works. A seesaw balances based on the concept of torque. Torque is the rotational force about the pivot point and is calculated as the product of the force (which depends on the weight of the person) and the distance from the pivot. The seesaw will be balanced as long as the torque on both sides of the pivot is equal.
In this scenario, the boy and the girl initially balance the seesaw, meaning the torque produced by the boy is equal to the torque produced by the girl. The boy has a larger mass (since he is described as heavy), and the girl has a smaller mass (she is light). To maintain balance, the boy must be sitting farther from the pivot than the girl.
Now, both the boy and the girl move forward so that they are one-third of their original distance from the pivot. This change affects the torque on both sides of the seesaw. Torque depends on both the mass and the distance from the pivot, so when both of them move closer to the pivot, the new torques will be smaller because the distances are shorter.
However, because the boy has a larger mass, even though his distance from the pivot decreases, the reduction in torque on his side will still be less than the reduction in torque on the girl’s side, since her smaller mass generates less torque overall. The seesaw will then become unbalanced, and the side with the boy will have a larger remaining torque, making the side with the boy tilt downward.
Therefore, the boy’s side will tilt downward, and this is the reason the correct answer is C.