The Laws of Motion
The Concept of Force
Newton’s First Law
The Particle Under
a Net Force
The Gravitational Force
Newton’s Third Law
Applications of Newton’s
ANSWERS TO QUESTIONS
The force due to gravity of the earth pulling down on the
ball—the reaction force is the force due to gravity of the ball
pulling up on the earth. The force of the hand pushing up on
the ball—reaction force is ball pushing down on the hand.
The only force acting on the ball in free-fall is the gravity due
to the earth -the reaction force is the gravity due to the ball
pulling on the earth.
Mistake one: The car might be momentarily at rest, in the process of (suddenly) reversing forward
into backward motion. In this case, the forces on it add to a (large) backward resultant.
Mistake two: There are no cars in interstellar space. If the car is remaining at rest, there are
some large forces on it, including its weight and some force or forces of support.
Mistake three: The statement reverses cause and effect, like a politician who thinks that his
getting elected was the reason for people to vote for him.
When the bus starts moving, the mass of Claudette is accelerated by the force of the back of the seat
on her body. Clark is standing, however, and the only force on him is the friction between his shoes
and the floor of the bus. Thus, when the bus starts moving, his feet start accelerating forward, but
the rest of his body experiences almost no accelerating force (only that due to his being attached to
his accelerating feet!). As a consequence, his body tends to stay almost at rest, according to Newton’s
first law, relative to the ground. Relative to Claudette, however, he is moving toward her and falls
into her lap. (Both performers won Academy Awards.)
No. The reaction to your weight is an upward force you exert on the Earth.
First ask, “Was the bus moving forward or backing up?” If it was moving forward, the passenger is
lying. A fast stop would make the suitcase fly toward the front of the bus, not toward the rear. If the
bus was backing up at any reasonable speed, a sudden stop could not make a suitcase fly far. Fine
her for malicious litigiousness.
It would be smart for the explorer to gently push the rock back into the storage compartment.
Newton’s 3rd law states that the rock will apply the same size force on her that she applies on it. The
harder she pushes on the rock, the larger her resulting acceleration.
The molecules of the floor resist the ball on impact and push the ball back, upward. The actual force
acting is due to the forces between molecules that allow the floor to keep its integrity and to prevent
the ball from passing through. Notice that for a ball passing through a window, the molecular forces
weren’t strong enough.