The Physics
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Opus in profectus

Action-Reaction

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Practice

practice problem 1

Suppose everyone in China was to jump in the air simultaneously. Estimate the following quantities to the nearest order of magnitude…
  1. the acceleration of the Earth
  2. the initial velocity of the Earth
  3. the maximum displacement of the Earth

solution

Answer it.

practice problem 2

A surfer in a red wetsuit on a red, black, and white surfboard is riding a wave as shown in the diagram below. Draw a free body diagram showing the forces acting on…
  1. the surfer
  2. the surfboard

Cartoon representation

solution

Since this chapter is about Newton's third law of motion you should expect problems that deal with action and reaction. The surfer was wearing read, so the relevant action-reaction pairs will be colored red in our free body diagrams.

  1. Surfers have weight. Weight points down. Get that out of the way first. The surfer is pressing on the surfboard with her feet. The force between solid surfaces in contact is call normal (short for normal force). It points in the normal direction (normal to the inclined surface of the surfboard). Surfers do not slip on their wet surfboards because they apply wax to the board. Wax provides friction in watery environments. The friction force on the feet of the surfer from the surfboard points parallel to the board. We are done with the surfer. There are no other forces acting on her.

  2. Surfboards have weight. I don't find that particularly interesting, but it's true. All things have weight and weight points down. A surfboard is partially submerged in water, so there's a buoyant force acting on it. The surface of a wave is not level, so this force does not point up. It points perpendicular to the surface of the wave at that location. That's what's different about the water when one surfs. It's tilted. Surfers fall "downhill" in a sense when they ride a wave. If the board is moving through the water there will be drag. Drag acts opposite the direction of motion relative to the water's tilted surface.

    So far we've only dealt with the forces of the water (buoyancy and drag) and the earth (weight) on the board. The surfboard is also acted upon by another object — the surfer. If the surfer experiences a normal force from the board, the board will experience an equal and opposite normal force from the surfer. If the surfer experiences a friction force on her feet, the board will also experience an equal and opposite friction force from her feet.

Free body diagram for the surfer

Free body diagram for the surfboard