#6 #10 #16 #22 #24 #26 #34 #42 #46 #54 6. What is the net force on an ocean liner sailing due west at a steady speed of 7 knots?
The ocean liner has a constant velocity, 7 knots WEST, so its acceleration is zero. Since force is mass times acceleration, the net force on the ocean liner is zero.
10. Assume that you're not wearing your seat belt and the car stops suddenly. Why would your head hit the windshield?
There is nothing to stop you when the car stops. Your head, and the rest of you, continues to move at the pre-stop velocity of the car until a force acts on you to slow you down.
16. What are the largest and smallest forces that you can obtain by adding a 3 Newton and a 5 Newton force?
The largest force is obtained when the 3 N and 5 N forces act in the same direction. In that case the net force is 8 N in size.
When the two forces act in opposite directions the smallest net force is achieved. The size of the net force in that case is 5 N - 3 N = 2 N.
22. What happens to the acceleration of a rocket if the net force on it is doubled?
Since the mass of the rocket is unchanged, its acceleration doubles when the net force acting on it doubles.
24. What happens to the acceleration of an object when the mass is cut in half but the force remains the same?
The acceleration doubles in this case since the new mass times the new acceleration is equal to the old mass times the old acceleration.
26. When an astronaut walks on the moon, is either her mass or her weight the same as on earth? Explain.
Her mass remains constant since it is merely a measure of her size. Weight is a force equal to mass times the acceleration of gravity. Since the size of gravitaional acceleration on the earth is different than that on the moon, her weight would not be the same. In fact, her moon weight would be roughly 1/6 of her earth weight.
34. A marble dropped into a bottle of liquid soap quickly reaches terminal speed. What is the acceleration of the marble just before it hits the bottom of the jar?
The acceleration just prior to impact is zero. Recall that terminal speed is achieved when the upward resistive force is equal to the downward pull of gravity. That is, the net force acting on the marble is zero once it achieves terminal speed. Since the net force is zero, the acceleration is also zero.
42. What force is required to pull a dog in a wagon along a level sidewalk, as in the figure on page 49 in your text, at a constant speed if the frictional force is 400 Newtons?
The required force is 400 Newtons forward. Since we are told that the dog and the wagon are to travel at a constant speed, the net force acting on the wagon must be zero. The two forces, the friction and the pull, must be exactly the same size and in opposite directions to achieve a zero net force.
46. Is the force that the earth exerts on the moon bigger, smaller, or the same size as the force the moon exerts on the earth? Explain your reasoning.
By Newton's third law, the force exerted by the earth on the moon is the same size, but in the opposite direction, as the force exerted by the moon on the earth.
54. A ball with a weight of 40 Newtons is falling freely toward the surface of the moon. What force does this ball exert on the moon?
The ball exerts a force of 40 Newtons (directed toward the moon) on the moon. The reasoning here is the same as that for question 46 above - Newton's third law.