Question

Simone builds a simple spinning engine by inserting straws through holes in a plastic cup. She then suspends the cup from a metal pole using string. After Simone pours water into the cup, the water rushes out of the straws in the direction that the straws are pointed and the cup spins, as shown in the picture below.



Which of the following is true about the force acting on the cup and causing it to spin?
A.
It is smaller in magnitude and in the same direction as the force pushing the water out of the straws.
B.
It is smaller in magnitude but opposite in direction to the force pushing the water out of the straws.
C.
It is equal in magnitude but opposite in direction to the force pushing the water out of the straws.
D.
It is equal in magnitude and in the same direction as the force pushing the water out of the straws.

Answer 1

C.

As the water is forced out of the straws, the cup (and straws) push the water in the direction of the jets; by Newton’s third law the ejected water pushes back on the cup with an equal-magnitude force in the opposite direction. That reaction torque makes the cup spin.