HW on 3. Newton’s Law
1. The "reaction" force does not cancel the "action" force because:
a) the action force is greater than the reaction force
b) they act on different bodies
c) they are in the same direction
d) the reaction force exists only after the action force is removed
e) the reaction force is greater than the action force
2. A book rests on a table, exerting a downward force on the table. The reaction to this force is:
a) the force of the Earth on the book
d) the force of the book on the Earth
b) the force of the table on the book
e) the inertia of the book
c) the force of the Earth on the table
3. A stone falls towards the Earth. The force that the stone exerts on the Earth is
A. zero.
B. less than the force the Earth exerts on the stone.
C. greater than the force the Earth exerts on the stone.
D. equal to the force the Earth exerts on the stone.
4. A car collides with a more massive truck.
During the collision, each vehicle exerts a force on the other. How do the magnitudes of these forces compare?
A. The forces cannot be compared without knowing how the initial speeds compare.
B. The magnitudes of the forces are equal.
C. The force exerted by the truck is the greater.
D. The force exerted by the car is the greater.
5.
a)
b)
A marble is dropped straight down from a distance h above the floor.
LetFm = the magnitude of the gravitational force on the marble due to the earth;
Fe = the magnitude of the gravitational force on the earth due to the marble;
am = the magnitude of the acceleration of the marble toward the earth;
ae = the magnitude of the acceleration of the earth toward the marble.
Which set of conditions is true as the marble falls toward the earth?
Fm = Fe and am < ae
Fm = Fe and am > ae
c)
d)
Fm < Fe and am = ae
Fm > Fe and am = ae
e)
Fm < Fe and am > ae
6. An astronaut on a space walk discovers that his jetpack no longer works; leaving him stranded 50m from the spacecraft. If
the jetpack is removable, explain how the astronaut can still use it to return to the ship.
7. A Cadillac bumps into a Volkswagen. Is the force exerted by the Cadillac on the Volkswagen greater than, less than, or equal
to the force exerted on the by the Volkswagen on the Cadillac? Explain.
8. Suppose you jump from the cliffs of Acapulco and perform a perfect swan dive. As you fall you exert an upward force on the
earth equal in magnitude to the downward force the earth exerts on you.
Why, then, does it seem that you are the one doing all the accelerating? Since the forces are the same why aren’t the
accelerations?
9. Give some everyday examples of situations where friction is beneficial.
10. Butch, the 72.0-kg star quarterback of Belmont High School’s football team, collides with Trask, a stationary left tackle, and
is brought to a stop with an acceleration of —20.0 m/s2. a) What force does Trask exert on Butch? b) What force does Butch
exert on Trask?
11. A 20-g sparrow flying toward a bird feeder mistakes the pane of glass in a window for an opening and slams into it with a
force of 2.0 N. What is the bird’s acceleration?
1. b)
2. b)
3. d)
4. b)
5. b) 6. Turn their back to spacecraft and throw their jetpack away.
7. the same
8. The acceleration of an object is inversely proportional to its mass. The diver and the earth experience
the same force, but the earth – with its much larger mass – has a much smaller acceleration.
9. walking, swimming, driving,....
10. a. The force depends upon the rate at which Butch’s mass is brought to rest.
F = ma = (72.0 kg)( —20.0 m/s2) = —1440 N
The negative sign in the answer implies that the direction of the force is opposite that of Butch’s original direction of motion.
b. Newton’s third law states that for every action there is an equal and opposite reaction. Therefore, if Trask exerts a force of —
1440 N on Butch, Butch will exert the same 1440 N force back on Trask, but in the opposite direction.
11. Since the sparrow exerts 2.0 N of force on the window, the window must provide —2.0 N back in the opposite direction.
Don’t forget to convert grams into kilograms before beginning.
a = F/m = (-2.0 N)/(0.02 kg) = - 100 m/s2
(about 10 g’s!)
Therefore, the bird experiences a very rapid negative acceleration, as the window brings the bird to a sudden stop. Ouch!