Download Physics Key to Dynamics Review Sheet

Physics
Key to Dynamics Review Sheet
I. Review/Conceptual Questions
1. Why do you lunge forward when your car suddenly comes to a halt? Why are you
pressed backward against the seat when your car rapidly accelerates? In your
explanation, refer to the most appropriate one of Newton’s three laws of motion.
When the car comes to a sudden halt, the upper part of the body continues forward
(as predicted by Newton's first law) if the force exerted by the lower back muscles
is not great enough to give the upper body the same deceleration as the car. The
lower portion of the body is held in place by the force of friction exerted by the car
seat and the floor.
When the car rapidly accelerates, the upper part of the body tries to remain at a
constant velocity (again as predicted by Newton's first law). If the force provided by
the lower back muscles is not great enough to give the upper body the same
acceleration as the car, the upper body appears to be pressed backward against
the seat as the car moves forward.
2. The net external force acting on an object is zero. Is it possible for the object to be
traveling with a velocity that is not zero? If your answer is yes, state whether any
conditions must be placed on the magnitude and direction of the velocity? If your
answer is no, provide a reason for your answer.
If the net external force acting on an object is zero, it is possible for the object to be
traveling with a nonzero velocity. According to Newton’s second law, Fnet = ma, if
the net external force Fnet is zero, the acceleration a is also zero. If the acceleration
is zero, the velocity must be constant, both in magnitude and in direction. Thus, an
object can move with a constant nonzero velocity when the net external force is
zero.
3. Is a net force being applied to an object when the object is moving downward (a)
with a constant acceleration of 9.81 m/s2 and (b) with a constant velocity of 9.81
m/s? Explain.
According to Newton's second law, a net force is required to give an object a nonzero acceleration.
(a) If an object is moving with a constant acceleration of 9.80 m/s2, we can
conclude that there is a net force on the object.
(b) If an object moves with a constant velocity of 9.80 m/s, its acceleration is zero;
therefore, we can conclude that the net force acting on the object is zero.
4. A father and his seven-year-old daughter are facing each other on ice skates. With
their hands, they push off against one another. (a) Compare the magnitudes of the
pushing forces that they experience. (b) Which one, if either, experiences the larger
acceleration? Account for your answers.
Since the father and the daughter are standing on ice skates, there is virtually no
friction between their bodies and the ground. We can assume, therefore, that the
only horizontal force that acts on the daughter is due to the father, and similarly, the
only horizontal force that acts on the father is due to the daughter.
(a) According to Newton's third law, when they push off against each other, the
force exerted on the father by the daughter must be equal in magnitude and
opposite in direction to the force exerted on the daughter by the father. In other
words, both the father and the daughter experience pushing forces of equal
magnitude.
(b) According to Newton's second law, Fnet = ma. Therefore, a = Fnet/m. The
magnitude of the net force on the father is the same as the magnitude of the net
force on the daughter, so we can conclude that, since the daughter has the smaller
mass, she will acquire the larger acceleration.
5.
A person has a choice of either pushing or pulling a sled at a constant velocity, as
the drawing illustrates. Friction is present. If the angle θ is the same in both cases,
does it require less force to push or to pull? Account for your answer.
6. 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. 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? Justify your answer.
7. A stone is thrown from the top of a cliff. As the stone falls, is it in equilibrium?
Explain, ignoring air resistance.
An object is in equilibrium when its acceleration is zero. When a stone is thrown
from the top of a cliff, its acceleration is the acceleration due to gravity; therefore,
the stone is not in equilibrium.
8. Can an object ever be in equilibrium if the object is acted on by only (a) a single
nonzero force, (b) two forces that point in mutually perpendicular directions, and (c)
two forces that point in directions that are not perpendicular? Account for your
answers.
9. A circus performer hangs stationary from a rope. She then begins to climb upward
by pulling herself up, hand over hand. When she starts climbing, is the tension in
the rope less than, equal to, or greater than it is when she hangs stationary?
Explain.
10. During the final stages of descent, a sky diver with an open parachute approaches
the ground with a constant velocity. The wind does not blow him from side to side.
Is the sky diver in equilibrium and, if so, what forces are responsible for the
equilibrium?
11. What kind of relationship is between force and acceleration? What kind of
relationship is between mass and acceleration?
Force and acceleration are directly related to each other. Mass and acceleration
F
a = net
are inversely related to each other.
m
a. If the mass is doubled, what happens to acceleration? halves
b. If the force is doubled, what happens to acceleration? doubles
c. If the force is doubled, what happens to the mass?
12. If an object is moved from one location to another, its ___ stays the same but it’s
_____ may change.
13. Identify each of the following as an example of Newton’s 1st, 2nd, or 3rd law.
a. Two astronauts explore the moon in a lunar rover, the more force exerted by the
lunar rover’s motor the greater the acceleration of the vehicle.
2nd Law- Law of acceleration
b. An astronaut stands on the moon. The astronaut exerts a force on the moon’s
surface and the the moon exerts an equal but opposite force on the astronaut.
3rd Law- Law of action / reaction
c. A man is able to launch a ping pong ball with greater velocity than he can launch
a bowling ball using the same apparatus.
2nd Law – Law of acceleration
d. As a man steps off of a boat onto the dock, he notices that the boat moves
away from the dock.
3rd Law – Law of Action / Reaction
14. How are the direction of motion the direction of the frictional force related? The
frictional force always acts in the opposite direction
15. What quantity is a measure of the amount of inertia an object has? Mass (more
mass then more inertia)
16. A race car
mass dof=710
from rest and travels
Vihas
=0a
m/s
40 mkg. It starts
t = 3 sec
m = 71040.0 m in 3.0 s. The
car is uniformly accelerated during
FN the entire time. What net force is exerted on it?
kg
Fapplied
Fw
17. In bench pressing 1.0 x 102 kg, a weight lifter applies a force of 1040 N. How large
is the upward acceleration of the weights during the lift?
m = 100 kg
Fapplied = 1040
U − D = ma
1040 − 980 = 100( a )
a = 0 .6 m / s 2
Fw = 100*9.8 = 980 N
18. A trunk with a mass of 200 kg is resting on a moving truck’s frictionless ramp that
makes a 35° angle with the horizontal. The normal force between the trunk and the
ramp is 1606 N. If the trunk begins to slide backwards, what is its acceleration?
Ignore friction.
19. A 12 kg car on a frictionless table is accelerated by a 5 kg mass hanging from the
table. Calculate the acceleration of the system.
20. A boy exerts a 36-N horizontal force as he pulls a 52-N sled across a cement
sidewalk with an acceleration of 2 m/s2. What is the force of kinetic friction between
the sidewalk and the metal sled runners?
21. The instruments attached to a weather balloon have a mass of 5.0 kg. The balloon
is released and exerts an upward force of 82 N on the instruments.
a. What is the acceleration of the balloon and instruments?
b. After the balloon has accelerated for 15 s, the instruments are released. What is
the velocity of the instruments at the moment of their release?
c. What net force acts on the instruments after their release?
d. When does the direction of their velocity first become downward?
22. A 215-N box is placed on a frictionless inclined plane that makes a 35.0 o-angle with
the horizontal The normal force between the box and the inclined plane is 176 N.
a. Find the component of the weight force parallel to the plane’s surface.
b. If a force of 135 N is applied up the plane, what is the acceleration of the box?