Download Newton`s Laws Review (no Forces at Angles) Questions: 1) A

Newton’s Laws Review (no Forces at Angles)
Questions:
1) A person throws a ball into the air. The force exerted on the ball by the hand remains with the ball…
a) until the ball reaches its highest point.
b) until the ball hits the ground.
c) only while the hand is touching the ball.
2) A hovercraft--so well-designed there is no friction--moves down the school hallway at 5 m.p.h. Ignore the
effects of air resistance. In order to maintain its constant speed…
a) There needs to be a small forward force on the hovercraft.
b) There needs to be a small backward force on the hovercraft.
c) There should be no forces applied in the sideways direction on the hovercraft.
3) You push on a large boulder in an attempt to make it move, but it doesn’t budge. Why not?
a) Your force is not greater than the boulder’s inertia.
b) Your force is not greater than the friction force holding the boulder in place.
c) Both of the above.
d) Your force on the boulder is canceled by the boulders force on you (3rd Law).
4) A 2 kg book sits on the floor in an elevator that is accelerating upward at 3m/s/s. Which of the following
are actual forces that are being exerted on the book? (You must choose all of the correct answers)
a) the force of gravity on the book by the Earth
b) the upward force on the book by the elevator floor
c) the net force on the book
d) the quantity “ma” (what the net force is equal to)
5) Which of the forces you identified in question #5 as forces acting on the book must be larger, since the book
is accelerating up?
6-15) A 10-kg suitcase is placed on a scale that is in an elevator. Is the elevator accelerating up or down when the scale
reads:
a) 75 N?
b) 120 N?
Justify your answers.
7-16) A stack of books whose true weight is 165 N is placed on a scale in an elevator. The scale reads 165 N. Can you tell
from this information whether the elevator is moving with a constant velocity of a 2 m/s upward, or 2 m/s downward, or
whether the elevator is at rest? Explain.
8-17) Suppose you are in an elevator that is moving upward with a constant velocity. A scale inside the elevator shows
your weight to be 600 N.
a) Does the scale register a value that is greater than, less than, or equal to 600 N during the time when the
elevator slows down as it comes to a stop?
b) What is the reading when the elevator is stopped?
c) How does the value registered on the scale compare to 600 N during the time when the elevator picks up
speed again on its way back down?
Give your reasoning in each case.
9-19) A box is resting on a platform, and the coefficient of static friction between the two touching surfaces is s. In an
experiment, an astronaut pushes horizontally on the box so it just begins to slide. He practices this maneuver on Earth
and performs it on the moon. In which case, if either, does he exert a greater pushing force? Justify your answer.
10-21) A rope is used in a tug-of-war between two teams of five people each. Both teams are equally strong, so neither
team wins (but does that really matter?). An identical rope is tied to a tree and the same ten people pull just as hard on
the loose end as they did in the contest. In both cases, the people pull steadily with no jerking. Which rope, if either, is
more likely to break? Explain.
Problems:
1) A 2 kg toy car is traveling at a constant 5 m/s.
a) What is the net force acting on the car?
b) What is the weight of the car on Earth?
2) A child exerts a 20 N force North on a 10 kg sled. There is a friction force of 15 N between the sled and the
snow.
a) What is the net force on the sled?
b) What is the acceleration of the sled?
c) What is the magnitude and direction of the force the sled exerts on the child?
3) Look inside the cover of your textbook to get the masses of the Earth and the Moon and the distance from
the center of the Earth to the center of the Moon.
a) What is the magnitude of the gravitational force the Earth exerts on the Moon?
b) What is the magnitude of the gravitational force the Moon exerts on the Earth?
c) What law can be used to justify that the answers to “a” and “b” are the same magnitude?
4) A 50kg man and a 30kg woman are standing facing each other on a frictionless lake. The man uses his
hands to push to the right on the woman’s hips with a force of 100N. Find:
a) The amount of force the woman exerts on the man.
b) The direction of the force the woman exerts on the man.
c) The law used to justify your answers to parts a and b.
d) The acceleration of the woman as a result of the man’s push on her.
e) The acceleration of the man as a result of the woman’s push on him.
5) Calculate the upward force on the book by the elevator floor from QUESTION #4.
6-50) A water skier (mass=89 kg) is being pulled at a constant velocity. The horizontal pulling force is 350 N. Find:
a) the magnitude of the total resistive force exerted on the skier by the water and air, and
b) the magnitude of the upward force exerted on the skier by the water.
7-49) A 12 kg lantern is suspended from the ceiling by two vertical wires. What is the tension in each wire?
8-68) A 1380 kg car is moving due east with an initial speed of 27 m/s. After 8 s the car has slowed down to 17 m/s. Find
the magnitude and direction of the net force that produces the deceleration.
9-73) A parachutist (mass=72 kg) is falling straight down with an acceleration of 1.3 m/s2. What is the magnitude of the
resistive force of the air that opposes the downward motion of the parachutist?
10) A person is pushing horizontally on a 50 kg sofa. If between the sofa legs and the floor the coefficient of static
friction is 0.7 and the coefficient of kinetic friction is 0.6:
a) How hard must this person push to start it sliding?
b) If the person pushes with 700 N of force, what is the acceleration of the sofa?