Q4. If you push on a heavy box that is a rest, you must exert some force to start its motion. However, once the box is sliding, you can apply a smaller force to maintain that motion. Why?
You must exert a horizontal force equal to or greater than the force of friction. The force of static friction -- when the box is still at rest -- is greater than the force of sliding friction (or kinetic friction).
Q6. A weight lifter stands on a bathroom scale. She pumps a barbell up and down. What happens to the reading on the scale? Suppose she is strong enough to actually throw the barbell upward. How does the reading on the scale vary now?
The barbell always exerts a downward force on the lifter equal in magnitude to the upward force that she exerts on the barbell. Since the lifter is in equilibrium, the magnitude of the upward force exerted on her by the scale (that is, the scale reading) equals the sum of her weight and the downward force exerted by the barbell. As the barbell goes through the bottom of the cycle and is being lifted upward, the scale reading exceeds the combined weights of the lifter and the barbell. At the top of the motion and as the barbell is allowed to move back downward, the scale reading is less than the combined weights. If the barbell is moving upward, the lifter can declare she has thrown it just by letting go of it for a moment.
Q8. If only one force acts on an object, can it be in equilibrium? Explain.
No. The object cannot be in equilibrium because whenever a force acts on an object, it is out of equilibrium. For example, an object resting on a table is a good example. A block resting on a table is exerting a force on the table by simply resting on it. This very same force is being counteracted by another force. That is, the table is pushing back up on the object to keep it in equilibrium.
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