Question

Louisa builds the body of a race car out of cardboard. She tapes a straw to the cardboard so that one end of the straw is tilted up and the other end hangs over the back of the car. Then, she places an inflated balloon over the tilted end of the straw and releases the mouth of the balloon to allow air to flow through the straw. The race car rolls forward as the balloon releases air through the straw, as shown in the picture below.


Image courtesy of NASA

How does Louisa's race car demonstrate Newton's third law of motion?
A.
The balloon pushes against the air, which results in a zero net force on the balloon.
B.
The forward movement of the car forces air to leave the balloon slower.
C.
The balloon pushes the air out, and the air pushes back on the balloon.
D.
The forward movement of the car forces air to leave the balloon faster.

Answer 1

C. The balloon pushes the air out, and the air pushes back on the balloon.

When the inflated balloon forces air out the straw backward (action), the expelled air exerts an equal and opposite force forward on the balloon/car (reaction), propelling the car forward.