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Physics
Newton’s Laws Review Questions
1. The planet Venus has a radius that is almost identical to Earth's. But the planet's mass
is noticeably lower than Earth's mass. If astronauts went to Venus, would they find
themselves weighing LESS or MORE or the SAME as what they weigh on Earth?
Explain.
2. What is the mass of a 50 kg dingo on Venus?
3. You travel to another planet and notice that your weight is 1/8 of its value on earth.
What is the free-fall acceleration due to gravity on this other planet? (In other words,
if you dropped a rock on this planet, what would its acceleration be during the fall?)
4. Students will sometimes object if a teacher tells them that the Gravitational force that
the Earth EXERTS ON THEM is identical to the force that they exert ON THE
EARTH. (Remember, gravitation is a mutual interaction.) They might think, "Hey,
you're telling me that if the earth is pulling on me with 600 Newtons of force, then I
am magically pulling up on the earth with 600 N of force? Inconceivable!" "I can't
pull the Earth around!"
A. Perform a calculation to try to convince the wayward students. Let's assume what
the teacher says above is true and that a single person DOES exert 600 N of
gravitational force on the earth. Figure out how much the earth would
accelerate in response to this force, assuming that this is the only force acting on
it and assuming that Earth's mass is 6.0 x 1024 kg.
B. Do you think the acceleration you calculated is measurably significant? What if
all 6 x 109 people on Earth joined together in one big blob and tried to magnify
the acceleration? Would it be anything substantial even then? (Don't spend too
much time thinking about this one. Use common sense.)
5. A 7 kg bowling ball is dropped from a tall clock tower on Earth.
A. What is the weight of the bowling ball?
B. What is the FORCE of gravity that acts on the bowling ball?
C. Assuming that you can ignore air resistance, what is the ball's acceleration as it
free-falls to Earth?
D. A 14 kg ball, ONE THAT IS NOT NECESSARILY FALLING, would be (select
any of the choices that are true. Could be none, could be all.)
i.
ii.
iii.
iv.
subject to twice as many Newtons of gravitational force as the 7 kg ball.
Subject to the same amount of gravitational force as the 7 kg ball.
Twice as hard to accelerate as a 7 kg ball.
Just as hard to accelerate as a 7 kg ball.
E. Now imagine that air resistance is affecting the 7 kg bowling ball as it falls.
Suppose that after a brief time of falling, the air resistance becomes equal to 30 N.
Figure out the acceleration of the bowling ball at this time. Show a method and
diagram to receive full credit.
F. A different ball, also with a mass of 7 kg, has been falling under the influence of
air resistance, when you happen to notice it. At the time that you notice it, it is
falling with a CONSTANT VELOCITY. Solve for any forces acting on the
bowling ball.
6. Given the pulley system below, there IS friction between the 4 kg mass and the
tabletop. The blocks are moving and accelerating in the direction of motion. The
acceleration of the 4 kg block is to the right and of the 2 kg block is downward. The
acceleration is 2 m/s2. IT IS GIVEN.
4 kg
2 kg
A. Draw two well-labeled and complete free-body (force) diagrams, one for each
block. Near each diagram show the coordinate system you intend to use.
B. Using the variables in YOUR force diagrams, you must set up 3 Newton's
Second Law equations: 1) vertical one for the 4 kg block; 2) horizontal one
for the 4 kg block; and 3) one for the 2 kg block.
These equations should have the ma substituted in for Fnet. No "Fnet" in them.
C. Find the tension in the string.
D. Find the FRICTION FORCE.
E. Explain why for the 4 kg block, the normal force is equal to the block's
weight.
F. Explain whether the horizontal forces on the 4 kg block are equal and
opposite.
G. Explain whether the vertical forces on the 2 kg block are equal and opposite.
H. Solve for the COEFFICIENT OF FRICTION, the measure of roughness,
between the table and the 4 kg block.
Physics Answers
Newton’s Laws Review Questions
1. The planet Venus has a radius that is almost identical to Earth's. But the planet's
mass is noticeably lower than Earth's mass. If astronauts went to Venus, would
they find themselves weighing LESS or MORE or the SAME as what they weigh
on Earth? Explain. LESS, since Fgrav increases with mass, and Weight is Fgrav.
2. What is the mass of a 50 kg dingo on Venus? 50 kg.
3. You travel to another planet and notice that your weight is 1/8 of its value on
earth. What is the free-fall acceleration due to gravity on this other planet? (In
other words, if you dropped a rock on this planet, what would its acceleration be
during the fall?)
1.23 m/s2
4. Students will sometimes object if a teacher tells them that the Gravitational force
that the Earth EXERTS ON THEM is identical to the force that they exert ON
THE EARTH. (Remember, gravitation is a mutual interaction.) They might
think, "Hey, you're telling me that if the earth is pulling on me with 600 Newtons
of force, then I am magically pulling up on the earth with 600 N of force?
Inconceivable!" "I can't pull the Earth around!"
C. Perform a calculation to try to convince the wayward students. Let's assume what
the teacher says above is true and that a single person DOES exert 600 N of
gravitational force on the earth. Figure out how much the earth would
accelerate in response to this force, assuming that this is the only force acting on
it and assuming that Earth's mass is 6.0 x 1024 kg. 1.0 x 10-22 m/s2
D. Do you think the acceleration you calculated is measurably significant? What if
all 6 x 109 people on Earth joined together in one big blob and tried to magnify
the acceleration? Would it be anything substantial even then? (Don't spend too
much time thinking about this one. Use common sense.) 6.0 x 10-13 m/s2
5. A 7 kg bowling ball is dropped from a tall clock tower on Earth. Tests, g = 10
m/s2
A. What is the weight of the bowling ball? About 70 N
B. What is the FORCE of gravity that acts on the bowling ball? About 70 N
C. Assuming that you can ignore air resistance, what is the ball's acceleration as
it free-falls to Earth? About 10 m/s2
D. A 14 kg ball, ONE THAT IS NOT NECESSARILY FALLING, would be
(select any of the choices that are true. Could be none, could be all.) Corrects
in bold.
v.
subject to twice as many Newtons of gravitational force as the 7 kg ball.
vi.
Subject to the same amount of gravitational force as the 7 kg ball.
vii.
Twice as hard to accelerate as a 7 kg ball.
viii. Just as hard to accelerate as a 7 kg ball.
E. Now imagine that air resistance is affecting the 7 kg bowling ball as it falls.
Suppose that after a brief time of falling, the air resistance becomes equal to
30 N. Figure out the acceleration of the bowling ball at this time. Show a
method and diagram to receive full credit. 5.7 m/s2
F. A different ball, also with a mass of 7 kg, has been falling under the influence
of air resistance, when you happen to notice it. At the time that you notice it,
it is falling with a CONSTANT VELOCITY. Solve for any forces acting on
the bowling ball. Fg = 70 N, Fair = 70 N
6. Given the pulley system below, there IS friction between the 4 kg mass and the
tabletop. The blocks are moving and accelerating in the direction of motion. The
acceleration of the 4 kg block is to the right and of the 2 kg block is downward.
The acceleration is 2 m/s2. IT IS GIVEN.
4 kg
2
kg
A. Draw two well-labeled and complete free-body (force) diagrams, one for
each block. Near each diagram show the coordinate system you intend to
use.
B. Using the variables in YOUR force diagrams, you must set up 3 Newton's
Second Law equations: 1) vertical one for the 4 kg block; 2) horizontal
one for the 4 kg block; and 3) one for the 2 kg block.
These equations should have the ma substituted in for Fnet. No "Fnet" in them.
C. Find the tension in the string. 16 N
D. Find the FRICTION FORCE. 8 N
E. Explain why for the 4 kg block, the normal force is equal to the block's
weight. Because ay = 0 and (4 kg)ay = n - W
F. Explain whether the horizontal forces on the 4 kg block are equal and
opposite. If they were, ax = 0, but we know ax = 2 m/s2, so T > f.
G. Explain whether the vertical forces on the 2 kg block are equal and
opposite.
No; explanation similar to F; you try wording it. Show it to a friend.
H. Solve for the COEFFICIENT OF FRICTION, the measure of roughness,
between the table and the 4 kg block. μ = 0.2