Download Review Questions - mom, UG, 3rd Law, centri

Review Questions for Newton’s Law of U. Gravitation, and Centripetal Force &Newton’s Third
Law and Momentum
General:
a. What is the definition of a force?
A force is an interaction between two objects having the ability to change motion.
b. State Newton’s 1st, 2nd and 3rd laws and give an example where you might observe each? For
example, the fact that a dinner table setting stays on a table when the tablecloth beneath it is
yanked quickly away is an example of Newton’s First law that states objects at rest tend to stay at
rest unless acted on by an unbalanced force.
Newton’s 1st Law: Objects in motion stay in motion and objects at rest stay at rest unless
acted on by an unbalanced force. Example: When I shake a ketchup bottle and then stop it
abruptly, the ketchup continues forward (which is what I want to so it will leave the bottle).
Newton’s 2nd Law: Fnet = ma. Example: A small object is accelerated greatly by the same
force that might barely accelerate a large object. Acceleration is a force to mass ratio.
Newton’s 3rd Law: For every action force there is an equal and opposite reaction force.
Example: A whales fin pushes on water and the water pushes on the fin moving the whale
forward.
c. Is inertia a force? NO! No interaction is required to have inertia. If something has mass
it has inertia. Inertia is the tendency of an object to resist a change in motion.
d. Be sure you can define vocab on Overview.
Newton’s Law of Universal Gravitation
1. How does the force of gravity between two objects depend on their masses?
Directly proportional (i.e. one mass doubles, FG doubles)
2. How does the force of gravity between two objects depend on the distance between the
two objects?
Inversely related to the square of the distance (i.e if distance doubles, Fg reduced by ¼)
3. What is the magnitude of the gravitational force between a 1-kg object and the Earth (you
don’t need to use Newton’s Law of Universal Gravitation or this one).
(about 10 N . Use W = mg)
4. What happens to the force of gravity between a pair of objects when the distance between
them is doubled? (reduced by ¼)
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5. Hanging from a tree is an apple that weighs 1N. Twice as high above the ground on the same
tree is another apple of the same mass. Why is its weight practically the same as that of
the other apple and not ¼ N?
(remember that distance is from center of mass to center of mass – distance from the center
of the earth is approximately the same for both apples)
6. If force were plotted against distance, which graph best describes the relationship
according to Newton’s Law of Universal Gravitation? _____
A
7. If force were plotted against mass, which graph in 6 above best describes the relationship
according to Newton’s Law of Universal Gravitation? _ D _____
8. What is another name for Newton’s Law of Universal Gravitation that relates the answer in
#6. Inverse Square Law
Circular Motion
9.
What is the force responsible for circular motion? What direction is it?
CENTRIPETAL TOWARD THE CENTER
10. Explain how a ball being whirled around your head by a string is accelerating toward the
center?
IT is constantly changing direction toward the center; its acceleration is toward the center
11. If I double the mass of the object in circular motion, how does the centripetal force
change? It doubles.
12. If I double the speed of an object in circular motion, how does the centripetal force
change? It quadruples (4x)
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13. You are riding your bike along Massachusetts Avenue. The road is wet and icy. When you
approach a turn what changes can you make to ensure that the friction your tires makes
with the road matches the centripetal force required to make the turn?
lower your speed or increase the radius of the turn (make a wider turn) or salt the
road so that adequate centripetal force can be created through friction.
14. Below is a ball on a string moving in circle in the clockwise direction. Draw the force vector
and velocity vector on the ball at position A, B, and C in the diagram below. Label all vectors
clearly.
In what direction is the ball accelerating? Toward the center.
Newton’s Third Law
15. State Newton’s Third Law. For every action force there is an equal and opposite reaction
force.
16. Identify an action reaction pair using the picture below.
Examples: Paddle pushes back on water, water pushes forward on paddle; boat pushes
down on water, water pushes up on boat.
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17. Why can’t you push on a rope? Because it can’t push back – forces are interactions between
two objects – you can only generate a force that can give back an equal and opposite force –
ropes can’t do this.
18. Why don’t N and W constitute an action-reaction pair on the diagram shown below?
Becase they act on the same object. Action reaction pairs act on each other (different
objects)
Momentum
19. Which has a greater momentum, a heavy truck at rest or a moving skateboard moving
skateboard? Which has more inertia? Heavy truck
20. What are the SI units for momentum?
Kg ∙m/s
What are the units for impulse? N·s
What is the definition of 1 N, the SI unit for force? Kg ∙m/s2
21. What is the impulse momentum concept? Explain with an equation and in words. How is it
derived from Newton’s Second Law?
The impulse (the product of force and time) is equal to the momentum change of an
object. The greater the force the greater the momentum change. The longer the
time the force is applied to the object, the greater the momentum change.
See class notes for derivation from Newton’s Second Law.
22. An egg is dropped on the floor and breaks. A second egg is dropped and is brought to a stop
safely and gradually using a blanket.
Explain why the second egg does not break.
In both scenarios the momentum change of the egg is the same and therefore the
impulse is the same (F·t). When the egg hits the floor, a force is applied very quickly
to change the momentum. So time is small and the force must be large. When the
egg falls in a blanket or other soft material, the time of the momentum change is
increased, decreasing the force resulting in less damage to the egg.
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23. A tennis ball and a clay ball have the same mass. In which collision is more force required:
a) the tennis ball collides with the floor and bounces back up. B) the clay ball collides with
the floor and sticks.
In scenario a since there is a greater momentum change (change in direction).
24. What type of collisions are the following:
a.
A train car hits a second train car and continues moving while hooked together.
Inelastic
b. A pencil falls on the floor. elastic
c.
A block of jello falls on the floor. inelastic
d. A tennis ball hits a racket elastic
25. Explain how Newton’s Third Law supports the Law of Conservation of momentum
Forces are interactions and can be thought of something that changes an objects
momentum over time. Because the force of object 1 on 2 is equal to the force of
object 2 on 1 (opposite direction), the momentum change of each object must be equal
and opposite, meaning there is no overall change in momentum of a system.
26. For each scenario, decide if a closed system is described. If the system is not closed,
decide what the external force is.
a.
Two cars collide on a gravel roadway on which frictional forces are large.
closed? ___no____ external force?___road friction________
b. Hans Full is doing the annual vacuuming. Hans is pushing the Hoover vacuum cleaner
across the living room carpet.
closed? __no_____ external force?____friction on carpet_______
c.
Two air track gliders collide on a friction-free air track.
closed? __yes_____ external force?___________
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Sample Problems - Honors:
1.
The centripetal force required to keep car going around a turn is 1000. N. If the radius
of the turn decreases by half, what is the new centripetal force required to make the
turn? What is causing the centripetal force? What happens if that new force cannot
be made?
2. The force between a satellite and Earth is 400.0 N. According to Newton’s Law of
Universal Gravitation, if the mass of the satellite is doubled, and the distance of
separation is doubled, what will the new force be?
3. What is the momentum of a 50. Kg boy running 4.0 m/s?
4. An 8-kg bowling ball rolling at 2 m/s bumps into a padded guardrail and stops.
A) What is the momentum of the ball just before hitting the guardrail?
B) How much impulse acts on the ball?
C) If the time of impact is 0.5 seconds, what force did the ball exert on the guardrail?
D) What force did the guardrail exert on the bowling ball?
5. In a physics lab, 0.500-kg cart (Cart A) moving rightward with a speed of 92.8 cm/s
collides with a 1.50-kg cart (Cart B) moving leftward with a speed of 21.6 cm/s. The two
carts stick together and move as a single object after the collision.
What kind of collision is this?
Determine the post-collision speed of the two carts.
Sample Problems: 1. 2000N; 2. 200 N; 3. 200 kg∙m/s 4a. 16 kg∙m/s b. 16 N ∙s; c. 32 N
d. -32 N; 5. Inelastic; 7 cm/s (see solutions in file called “Review for Test 2 Solutions”
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