Why don’t action and reaction forces cancel?
Action and reaction forces act on different objects. They are equal and opposite, but they cannot cancel each other because forces cancel only when they act on the same object.
Kelly sits on a rock. Her weight is an action force. Describe the reaction force.
Earth pulls Kelly downward with the force of her weight. The reaction force is Kelly pulling Earth upward with an equal gravitational force.
Calculate the force on an object that has a mass of 12 kg and an acceleration of 4 m/s2.
F = ma = (12 kg)(4 m/s²) = 48 N.
Describe the action and reaction forces that occur as you walk along a path.
As you walk, your foot pushes backward on the path, and the path pushes forward on your foot with an equal force in the opposite direction. The forward force from the path helps you move forward.
Explain what happens to a wall if you push on it with a force of 10 N.
If you push on the wall with 10 N, the wall pushes back on you with 10 N in the opposite direction. If it is firmly supported, the wall will not noticeably move.
Explain how weight is measured.
Weight is the gravitational force on an object. It is calculated by multiplying mass by gravitational acceleration (W = mg) and is measured in newtons, often with a spring scale.
Are astronauts really “weightless” while in orbit? Explain why or why not.
Astronauts in orbit are not truly without gravity; Earth’s gravity keeps them in orbit. They feel weightless because they and their spacecraft are in continuous free fall together, so there is little or no support force acting on them.
Explain when a falling object is in free fall.
A falling object is in free fall when gravity is the only force acting on it (air resistance is negligible). Near Earth’s surface, it accelerates downward at about 9.8 m/s².
Explain why an object moves if the action-reaction forces are equal.
The action and reaction forces act on different objects, so they do not cancel on either object. Each object can move because its motion depends on the net force acting on it.
Show that the Newton has the units of mass times acceleration.
From F = ma, force equals mass times acceleration. Its units are kg × m/s², or kg·m/s², which is defined as one newton (N).
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