1. Physics 101 Learning Object 2:
Buoyancy Explanation Question
Three beakers are filled with water and various objects and placed on a weighing scale.
Beaker 1 contains nothing, Beaker 2 contains a wooden ball, and Beaker 3 contains a metal
ball suspended from the roof with a string.
Question: Compare and contrast the three beakers and identify all of the important forces on
the ball. Which weighing scale reads a greater value?
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Ceiling
1 2 3
2. Physics 101 Learning Object 2:
Buoyancy Explanation Question
Question: Compare and contrast the three beakers and identify all of the important forces on
the ball. Which weighing scale reads a greater value?
Answer: Important forces are drawn on the ball in each case. All three beakers weigh the
same.
Observations:
- All three beakers are filled to the same level, but the first beaker has the most water while
beakers 2 and 3 have the same amount of water
- All three beakers are in an equilibrium situation - all the forces balance out in each case
- The diameter of the metal and wooden ball is the same
- The metal ball in beaker 3 weighs the most, and has the highest density (the string is taut)
but it’s not sinking because the ball is suspended from the ceiling
- The wooden ball in beaker 2 floats on the surface of the water, and has a lower density than
water
- The amount of water displaced in beaker 2 and 3 due to the wooden and metal balls
(respectively) is the same and consequently, the weight of this water is the same as well
Explanation: The most important thing to note in this question is that the wooden ball is
floating on the surface of the water, while the metal ball is suspended in the same position by
a string suspended from the ceiling. Thus, the weight of the metal ball is reduced because of
the tension in the string. Then, the weight of the water displaced by the wooden and metal
balls is equivalent to the weight of the ball (in the case of the metal ball, the displaced water
weighs the same as the reduced weight of the metal ball). Given that the water levels are the
same in all three beakers (there is more water in beaker 1 though), the weight of the
remaining water adds to the weight of the displaced water in beakers 2 and 3 - thus, the total
weight of all three beakers is the same.
Ceiling
TFB
Fg
FB
Fg
