Newton’s Laws of Motion
Answers to Even-numbered Conceptual Questions
As the dog shakes its body, it starts water in its fur moving in one direction.
When it then
begins to shake its body in the opposite direction, much of the water continues in the same
direction – due to the law of inertia (Newton’s first law).
As a result, water leaves the fur
with each reversal in direction.
If the tablecloth is pulled rapidly, it can exert a force on the place settings for only a very
In this brief time, the objects on the table accelerate, but only slightly.
Therefore, the objects may have barely moved by the time the tablecloth is completely
The drag racer needs an engine to turn the wheels, which makes them push against the
It is only in this way that the ground is able to exert a reaction force on the car.
The astronaut should push the jet pack away from him, in the opposite direction from the
As a result, the reaction force exerted on him by the pack will accelerate him
toward the ship.
The action-reaction forces are the force exerted on the ball by the bat, and the force
exerted on the bat by the ball.
The force exerted on the ball causes it to change its
direction of motion and move into the field of play (hopefully).
The force exerted on the
bat slows its forward motion.
The forces between the two cars are an action-reaction pair; therefore, the forces have
exactly the same magnitude.
The effect of the forces is quite different, however.
particular, the car with the smaller mass experiences a greater acceleration than the car
with the larger mass.
This could increase the chance of injury for the occupants of the
The floor exerts an upward force on the ball that causes it to rebound to your hand.
To cause a deceleration, the upward force exerted on you by the parachute must be greater
in magnitude than your weight.
In this case, the net force acting on you is upward, as is
Since your acceleration and velocity are in different directions, you
See Figure 2-11 for further details.
The whole brick also experiences twice the
due to gravity.
As a result, these two
effects (more inertia, more force) exactly cancel, and the free fall acceleration is
independent of mass.
If only a single force acts on the object, it will not stay at rest; instead, it will
accelerate in the direction of the force.
Object 1 has the greater net force acting on it, due to its larger mass.
Therefore, object 1
reaches the ground first.
In the upwardly accelerating elevator it is as if you are playing a game of darts on a
planet with a greater acceleration of gravity.
Therefore, you must aim higher than you
would if the elevator were moving upward with constant speed.