1. Is it possible for an object to be in motion if no net force is acting on it? Explain.
Solution: Yes, it is possible for an object to be in motion, even if it experiences no net external force. For example,
consider a car moving down the interstate at constant velocity. There is friction between the tires and the road, air
resistance, and wind acting on the car. However, these all can cancel out, causing the net force to be zero. This
implies that the car is accelerating, but it continues to move at constant velocity.
2. If an object is at rest, can we conclude that no external forces are acting on it?
Solution: No, that is not a reasonable conclusion. Consider a book at rest on a table. The book experiences a force
due to gravity, as well as the normal force from the table. The book remains at rest because the net force is zero,
not because there are no external forces.
3. An object thrown into the air stops at the highest point in its path. Is it in equilibrium at this point? Explain.
Solution: While the velocity is zero at the highest point, it is not at equilibrium. At the highest point of the throw,
gravity is still acting on the object, pulling it downwards. This is the only force, and thus the net force is downward.
If an object experiences a non-zero net force, it is not at equilibrium.
4. The force that attracts the Earth to an object is equal to and opposite the force that Earth exerts on the object. Explain
why Earth’s acceleration is not equal to and opposite the object’s acceleration.
Solution: While the forces are equal, the mass of the earth is much larger than an everyday object (the mass of
the earth is 6.0 × 1024 kg). By Newton’s 2nd law, if it is subject to the same force, it will have a much smaller
F
acceleration due to its larger mass. a = m
shows this, as if you have a very large mass, the denominator will be very
large, and thus a will be very small.
5. Explain why a rope climber must pull downward on the rope in order to move upward. Discus the force exerted by the
climber’s arms in relation to the weight of the climber during the various stages of each ”step” up the rope.
Solution: By Newton’s third law, if the climber exerts a downward force on the rope, the rope exerts an equal
upwards force on the climber. It is this force that propels the climber upwards. The climber must exert enough force
to overcome his or her weight (mg).