Download Newton`s Laws Review Key

Physics
Name: __________________________
Date: _____________ Period: _______
Newton’s Laws of Motion
(Unit Review)
Key Vocabulary
Mass
Inertia
Weight
Momentum
Friction
Newton’s First Law
Newton’s Second Law
Newton’s Third Law
1. What is friction and briefly explain why there is more on some surfaces than on others.
Friction is what happens when any two things rub against each other. These can be solid things, like
your two hands rubbing together, or your skis rubbing on the snow, or a hammer hitting a nail, or
they can be gases, like friction with the air slowing down your car, or liquids, like friction with the
water slowing down a boat.
Nobody completely understands what causes friction. Partly, friction happens when the rough edges
of one object snag on the rough edges of another object, and some of the objects' energy has to be
used to break off those rough edges so the objects can keep moving. And when you rub two soft
things together, like your hands, sometimes they squish into each other and get in each other's way.
But even completely smooth, hard things have some friction. This friction is the result of the
molecules in both objects being attracted to each other.
2. Explain how inertia and momentum are related.
You have to know what inertia and momentum mean. Now, hopefully you recall Newton's first law
of motion, that one which says that if there is no force acting on an object, then it will either remain
at rest or move with uniform velocity (motion at constant speed along a fixed direction of motion)?
The tendency of all objects to remain at rest or move with a uniform velocity is what we call inertia.
That is why Newton's first law of motion is sometimes called the law inertia.
Then, there is Newton's second law of motion which tells us that force is present whenever a
moving object shows acceleration. Because the first law of motion, or law of inertia, clarifies the
meaning of force, Newton defined force in the second law of motion by using inertia. In the
equation of the second law F = ma, the quantity m called the mass of the system under study is also
known as the inertial mass.
In Newton's second law of motion, we find not only the definition of force but also the definition of
mass. It says that mass is the resistance to an applied force. According to F = ma, the larger the
mass for a given acceleration, the larger the force, the more we have to exert a force on the object
because the resistance to the applied force is greater. In that sense, mass and inertia are the same
since the tendency to resist the change in motion brought about by the applied force is none other
than the tendency of an object to remain in its earlier state.
Finally, since the momentum p is just the product of the inertial mass m and the velocity v (p =mv),
we find a direct proportionality relationship between inertia and momentum. The larger the inertia,
the larger the momentum for a given velocity. The smaller the inertia, the smaller the momentum,
also for a given fixed velocity.
3. List Newton’s Three Laws
I.
An object at rest will remain at rest unless acted on by an unbalanced force. An object in
motion continues in motion with the same speed and in the same direction unless acted upon
by an unbalanced force.
II.
Acceleration is produced when a force acts on a mass. The greater the mass (of the object
being accelerated) the greater the amount of force needed (to accelerate the object).
III.
When one object exerts a force on a second object, the second object also exerts a force on
the first object. The two forces are equal in strength and oriented in opposite directions.
4. Force is measured in kg/m/s/s. The name for this unit is a Newton. A unit like kg/m/s/s is known
as a derived unit. Using Newton’s Second Law, show why this derived unit makes sense.
According to Newton’s second law F=m*a. The force (F) is then measured in a combination of the
units used to measure mass (m) and acceleration (a). Mass (m) is measured in kilograms (kg)
according to the SI system of measurement. Acceleration (a) is measured in meters per second
squared (m/s2) as it too is a derived measurement relating a change in velocity (v) over time (t).
Velocity (v) is also a derived measure relating change in distance (d) over time (t). Distance (d) is
measured in meters (m) according to the SI system of measurement and time (t) is measured in
seconds (s) according to the SI system of measurement. Thus, the Newton is a measure of the
ability to change the distance traveled by an object over time at a rate dictated by the force applied.
Identify each of the following statements as true or false and explain why.
Which of the following statements are true of inertia?
1.
2.
3.
4.
5.
6.
7.
8.
9.
Inertia is a force. F
Inertia is a force which keeps stationary objects at rest and moving objects in motion at constant
velocity. F
Inertia is a force which brings all objects to a rest position. F
All objects have inertia. T
A more massive object has more inertia than a less massive object. T
Fast-moving objects have more inertia than slow-moving objects. F
An object would not have any inertia in a gravity-free environment (if there is such a place). F
Inertia is the tendency of all objects to resist motion and ultimately stop. F
In a gravity-free environment (should there be one), a person with a lot of inertia would have the
same ability to make a turn as a person with a small amount of inertia. F
Which of the following statements are true of the quantity mass?
1.
2.
3.
4.
5.
The mass of an object is dependent upon the value of the acceleration of gravity. F
The standard metric unit of mass is the kilogram. T
Mass depends on how much stuff is present in an object. T
The mass of an object is variable and dependent upon its location. F
An object would have more mass on Mount Everest than the same object in the middle of Lake
Michigan. F
6. People in Weight Watcher's are really concerned about their mass (they're mass watchers). T
7. The mass of an object can be measured in pounds. F
8. If all other variables are equal, then an object with a greater mass would have a more difficult
time accelerating. T
9. If all other variables are equal, then it would require less exerted force to stop a less massive
object than to stop a more massive object. T
10. The mass of an object is mathematically related to the weight of the object. T
Which of the following statements are true of the quantity weight?
1.
2.
3.
4.
5.
6.
7.
8.
9.
The weight of an object is dependent upon the value of the acceleration of gravity. T
Weight refers to a force experienced by an object. T
The weight of an object would be less on the Moon than on the Earth. T
A person could reduce their weight significantly by taking an airplane ride to the top of Mount
Everest. F
Two objects of the same mass can weigh differently. T*
To gain weight, one must put on more mass. F*
The weight of an object can be measured in kilograms. F
The weight of an object is equal to the force of gravity acting upon the object. T
When a chemistry student places a beaker on a balance and determines it to be 84.3 grams, they
have weighed the beaker. F
*#5 is mostly true as two objects of the same mass can weigh differently if they are located in different
locations. #6 is mostly false as weight is the product of mass and the acceleration of gravity (g). To gain
weight, one must either increase their mass or increase the acceleration of gravity for the environment
where they are located. So the statement is true if one disregards the word MUST which is found in the
statement.
In each of the following scenarios, consider Newton's second law of motion to determine which of the
following statements are true?
a. If an object is accelerating to the right, the net force on the object must be directed towards the right. T
b. If an object is moving to the right and slowing down, then the net force on the object is directed
towards the left. T
c. Accelerating objects are either slowing down or speeding up. F
d. The acceleration of an object is directly dependent upon its mass and inversely dependent upon its net
force. F
e. An object has an acceleration of 8 m/s/s. If the net force acting upon the object is increased by a factor
of 2, then the new acceleration would be 10 m/s/s. F
f. An object has an acceleration of 8 m/s/s. If the net force acting upon the object is increased by a factor
of 3, then the new acceleration would be 11 m/s/s. F
g. An object has an acceleration of 8 m/s/s. If the mass of the object is increased by a factor of 2, then the
new acceleration would be 16 m/s/s. F
h. An object has an acceleration of 8 m/s/s. If the mass of the object is increased by a factor of 4, then the
new acceleration would be 2 m/s/s. T
i. An object has an acceleration of 8 m/s/s. If the net force acting upon the object is increased by a factor
of 2 and the mass of the object is decreased by a factor of 2, then the two factors would offset each
other and the acceleration would still be 8 m/s/s. F
j. An object has an acceleration of 8 m/s/s. If the net force acting upon the object is increased by a factor
of 2 and the mass of the object is increased by a factor of 4, then the new acceleration would be 4 m/s/s.
T
k. An object has an acceleration of 8 m/s/s. If the net force acting upon the object is decreased by a factor
of 2 and the mass of the object is increased by a factor of 4, then the new acceleration would be 1 m/s/s.
T
l. An object has an acceleration of 8 m/s/s. If the net force acting upon the object is increased by a factor
of 4 and the mass of the object is increased by a factor of 2, then the new acceleration would be 16
m/s/s. T
m. A 2-kg object accelerates from rest to a final velocity of 6 m/s in 3 seconds. The magnitude of the net
force acting upon the object is 12 N. F
n. A 10-kg object slows down from 24 m/s to a final velocity of 9 m/s in 3 seconds. The magnitude of the
net force acting upon the object is 80 N. F
5. Christopher "Big Black" Boykin has a mass of 188 kg on the earth. What is Big Black’s mass on the
moon where the force of gravity is approximately 1/6th that of Earth's? (Show your work)
Answer: 188 kg (Remember that mass doesn’t change but weight changes with gravity)
6. Rob Dyrdek weighs 598 N on Earth. What is Rob’s mass on the moon where the force of gravity is
approximately 1/6th that of Earth’s?
Answer: F = ma; 598 N = m (9.8 m/s2); 61 kg = m, 61 kg is mass on earth and on the moon.
7. Which one(s) of the following force diagrams depict an object moving to the right with a constant
speed? List all that apply.
If an object is moving at a constant speed in a constant rightward direction, then the acceleration is
zero and the net force must be zero. Choice B and D show a rightward net force and therefore a
rightward acceleration, inconsistent with the described motion.
8. According to Newton's third law, every force is accompanied by an equal and opposite reaction
force. The reason that these forces do not cancel each other is _D__.
a. the action force acts for a longer time period
b. the two forces are not always in the same direction
c. one of the two forces is greater than the other
d. the two forces act upon different objects; only forces on the same object can balance
each other.
e. ... nonsense! They do cancel each other. Objects accelerate because of the presence of a
third force.
9. As you sit in your chair and study your physics (presuming that you do), the force of gravity acts
downward upon your body. The reaction force to the force of the Earth pulling you downward is
_E_.
a.
b.
c.
d.
e.
f.
the force of the chair pushing you upward
the force of the floor pushing your chair upward
the force of the Earth pushing you upward
the force of air molecules pushing you upwards
the force of your body pulling the Earth upwards
... nonsense! Gravity is a field force and there is no such reaction force.
10. A golf pro places a ball at rest on the tee, lines up his shot, draws back his club, and lets one rip.
During the contact of the golf club with the golf ball, the force of the club on the ball is _equal to_
the force of the ball on the club and the acceleration of the club is _less_ than the acceleration of the
ball.
a. greater than, greater than
b. greater than, equal to
c. greater than, less than
d. less than, less than
e. less than, equal to
f. less than, greater than
g. equal to, equal to
h. equal to, greater than
i. equal to, less than
Each one of Newton's Laws can play a role in any one particular situation. However, one of the laws is
often most obviously dominant in governing the motion of a situation. Pick which of Newton's most
governs the situations described below.
a. First Law (inertia)
b. Second Law (F = m•a)
c. Third Law (action-reaction)
11. A helicopter must have two sets of blades in order to fly with stability. C
12. If you were in an elevator and the cable broke, jumping up just before the elevator hit the ground
would not save you. Sorry. A
13. You usually jerk a paper towel from a roll in order to tear it instead of pulling it smoothly. A
14. A student desk changes the amount of force it puts on other objects throughout a school day. C
15. Heavy objects are not easier to move around in a horizontal fashion on the Moon than on the Earth.
A
16. The stronger, heavier team in a tug-of-war does not create a larger tension in the rope than the
weaker, lighter team. C
Construct free-body diagrams for the following physical situations at the instant in time for which they are
described. As is always done in free-body diagrams, label the forces according to type and draw the arrows
such that their length reflects the magnitude of the force.
a. A book is at rest on top of a
table.
b. A book is being pushed to the c. A book is being pushed to the
right across a table surface
right across a table surface and
with a constant velocity.
accelerating in the direction of
(Neglect Fair.)
the push. (Neglect Fair.)
d. A ball is dropped from rest
from the top of a building.
(Neglect Fair.)
e. Several seconds after being
thrown, a football is moving
upwards and rightwards
towards the peak of its
trajectory. (Neglect Fair.)
f. Several seconds after being
thrown, a football reaches the
precise peak of its trajectory.
(Neglect Fair.)
g. A falling skydiver is speeding h. A falling skydiver has reached i. After reaching a terminal
up.
a terminal velocity.
velocity, a falling skydiver has
opened her parachute.
j. A car is moving to the right at k. A car is skidding to a stop
a high speed across a level
(with wheels locked) while
roadway surface; the driver's
traveling to the right across a
foot remains on the gas pedal.
level roadway surface.
l. An elevator (held by a cable)
is moving upwards at a
constant velocity. (Neglect
Fair.)
To answer the following questions use Newton’s second law with the kinematics equations. Draw a freebody diagram for each.
17. A 72-kg skydiver is falling from 10000 feet. At an instant during the fall, the skydiver encounters an
air resistance force of 540 Newtons. Determine the acceleration of the skydiver at this instant.
Answer: 2.3 m/s2, down
18. A 5.20-N force is applied to a 1.05-kg object to accelerate it rightwards across a friction-free
surface. Determine the acceleration of the object. (Neglect air resistance.)
Answer: 4.95 m/s2, right
19. A 5.20-N force is applied to a 1.05-kg object to accelerate it rightwards. The object encounters
3.29-N of friction. Determine the acceleration of the object. (Neglect air resistance.)
Answer: 1.82 m/s2, right
20. A 921-kg sports car is moving rightward with a speed of 29.0 m/s. The driver suddenly slams on the
brakes and the car skids to a stop over the course of 3.20 seconds with the wheels locked.
Determine the average resistive force acting upon the car.
Answer: 8350 N
21. A 1250-kg small aircraft decelerates along the runway from 36.6 m/s to 6.80 m/s in 5.10 seconds.
Determine the average resistive force acting upon the plane. (Assume that its engine/propeller
makes no contributes to its forward motion).
Answer: 7.30 x 103 N
22. A tow truck exerts a 18300-N force upon a 1210-kg car to drag it out of a mud puddle onto the
shoulder of a road. A 17900 N force opposes the car's motion. The plane of motion of the car is
horizontal. Determine the time required to drag the car a distance of 6.90 meters from its rest
position.
Answer: 6.46 s
23. A 22.6-N horizontal force is applied to a 0.0710-kg hockey puck to accelerate it across the ice from
an initial rest position. Ignore friction and determine the final speed (in m/s) of the puck after being
pushed for a time of .0721 seconds.
Answer: 23.0 m/s
24. A train has a mass of 6.32x104 kg and is moving with a speed of 94.3 km/hr. The engineer applies
the brakes which results in a net backward force of 2.43x105 Newtons on the train. The brakes are
held for 3.40 seconds. How far (in meters) does the train travel during this time?
Answer: 66.8 m