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Physics - Dynamics Flashcards

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1
Q

The action force and the reaction force that occur in an interaction.

A

Force Pair

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2
Q

Motion under influence of gravitational pull only.

A

Free Fall

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3
Q

Mutual action between objects during which each object exerts an equal and opposite force on the other.

A

Interaction

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4
Q

Every object continues in a state of rest, or in a state of motion in a straight line at a constant speed, unless acted upon by a net force.

A

Newton’s first law of motion

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5
Q

the acceleration produced by a net force on an object is directly proportional to the net force, is in the direction as the net force, and is inversely proportional to the mass of the object.

A

Newton’s second law of motion

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6
Q

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

A

Newton’s third law of motion

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7
Q

The speed at which the acceleration of a falling object terminates when air resistance balances its weight.

A

Terminal speed

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8
Q

Terminal speed in a given direction (oftern downward).

A

Terminal Velocity

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9
Q

A Newton is equal to one:
kg⋅m^2/s
g⋅cm^2/s
kg⋅m/s^2
g⋅cm/s^2

A

kg⋅m/s^2

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10
Q

A steel beam hangs from a cable as a crane lifts the beam. What forces act on the beam?

Gravity.
Gravity and tension in the cable.
Gravity and a force of motion.
Gravity and tension and a force of motion.

A

Gravity and tension in the cable.

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11
Q

A rock is suspended from a string; and it moves downward at constant speed. Which one of the following statements is true concerning the tension in the string if air resistance is not ignored?

The tension is zero newtons.
The tension points downward.
The tension is equal to the weight of the rock.
The tension is less than the weight of the rock.
The tension is greater than the weight of the rock.

A

The tension is equal to the weight of the rock.

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12
Q

A crate weighs 10 N and is being pulled along the floor at a constant velocity with a force of 2N. The coefficient of static friction is:

  1. 10
  2. 20
  3. 0
  4. 0
A

0.20

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13
Q

The box is sitting on the floor of an elevator. The elevator is accelerating upward. The magnitude of the normal force on the box is

n > mg.
n = mg.
n < mg.
n = 0.
Not enough information to tell.

A

n > mg.

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14
Q

An astronaut, whose mass on the surface of the Earth is m, orbits the Earth in the space shuttle at an altitude of 450 km. What is her mass while orbiting in the space shuttle?

greater than m
m
less than m

A

less than m

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15
Q

A book has a mass of 2.0 kg. Its weight is approximately:

2 newtons
20 newtons
2 dynes
20 dynes

A

20 newtons

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16
Q

A rock is thrown straight up from the earth’s surface. Which one of the following statements concerning the net force acting on the rock at the top of its path is true?

The net force is equal to the weight of the rock.
The net force is instantaneously equal to zero newtons.
The net force’s direction changes from up to down.
The net force is greater than the weight of the rock.
The net force is less than the weight of the rock, but greater than zero newtons.

A

The net force is equal to the weight of the rock.

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17
Q

A physics student in a hot air balloon ascends vertically at constant speed. Consider the following four forces that arise in this situation:
F1 = the weight of the hot air balloon

F3 = the force of the student pulling on the earth
F2 = the weight of the student

F4 = the force of the hot air balloon pulling on the student

Which one of the following relationships concerning the forces or their magnitudes is true?

F4 > F2
F1 < F2
F4 > F1
F2 = –F4
F3 = –F4

A

F2 = –F4

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18
Q

An elevator, lifted by a cable, is moving upward and slowing. Which is the correct free-body diagram?

A

D

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19
Q

I swing a ball attached to a string over my head. If the string breaks, which way does the ball fly off?

Direction 1
Direction 2
Direction 3
Direction 4

A

Direction 3

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20
Q

You brake suddenly. The passengers in your car lurch forward. This demonstrates

gravity
mass
weight
length
good driving
inertia
that a leprechaun was crossing the road

A

inertia

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21
Q

A truck is decelerating. A package is dropped from the midpoint of the ceiling of the truck’s storage compartment. The package hits the floor

immediately beneath the midpoint of the ceiling
closer to the back of the truck than the midpoint
closer to the front of the truck than the midpoint
all of the above
none of the above

A

immediately beneath the midpoint of the ceiling

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22
Q

The force of gravity acting on a 100 kg person is ___ compared to the force of gravity on a 1 kg book

4 times larger
the same
double
zero
10 times larger
100 times larger
lower
none of the above

A

100 times larger

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23
Q

When the net force that acts on a hockey puck is 10 N, the puck accelerates at a rate of 50 m/s2. Determine the mass of the puck.

0.2 kg
1.0 kg
5 kg
10 kg
50 kg

A

0.2 kg

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24
Q

There is only one force acting on an object. For the object to travel at constant velocity, this force must be

a constant non-zero value
a varying non-zero value
a constant zero value
a positive value
a negative value
none of the above

A

a constant zero value

25
Q

An object is moving in a circle with constant speed. The object has

a constant velocity
zero acceleration
constant velocity and acceleration
zero velocity
no forces acting on it
no net force acting on
changing velocity but constant acceleration

A

changing velocity but constant acceleration

26
Q

A man weighs 900 N, and is standing with his feet across three bathroom scales so his weight is equally shared among the three scales. The reading on each scale is

900 N
2700 N
300 N
90 N
30 N
none of the above

A

300 N

27
Q

Which of the following is a situation in which the net external force on a system is not zero, yet its speed remains constant.

The ball at the top of its trajectory after being thrown up
A bowling ball rolling down the aisle toward the pins
A feather slowly falling to the ground
A skydiver at terminal velocity
There are no cases which this happens

A

There are no cases which this happens

28
Q

A mosquito runs head-on into a truck. Splat! Which is true during the collision?

The mosquito exerts more force on the truck than the truck exerts on the mosquito.
The truck exerts more force on the mosquito than the mosquito exerts on the truck.
The mosquito exerts the same force on the truck as the truck exerts on the mosquito.
The truck exerts a force on the mosquito but the mosquito does not exert a force on the truck.
The mosquito exerts a force on the truck but the truck does not exert a force on the mosquito.

A

The mosquito exerts the same force on the truck as the truck exerts on the mosquito.

29
Q

As a meteor falls towards earth, it is attracted by earth’s gravity. What is the reaction to this force?

air resistance
the meteor’s inertia
the earth’s inertia
the earth’s gravity
earth is attracted by the meteor’s gravity
the meteor is attracted by all the particles in the universe
earth is attracted by all the particles in the universe

A

earth is attracted by the meteor’s gravity

30
Q

In a tug-of-war, each man on a 5-man team pulls with an average force of 500 N. What is the tension in the center of the rope?

zero newtons
100 N
500 N
1000 N
5000 N
Depends on the diameter of the rope
None of the above

A

500 N

31
Q

A physics student in a hot air balloon ascends vertically at constant speed. Consider the following four forces that arise in this situation:
F1 = the weight of the hot air balloon

F3 = the force of the student pulling on the earth
F2 = the weight of the student

F4 = the force of the hot air balloon pulling on the student

Which two forces form an “action-reaction” pair that obeys Newton’s third law?

F1 and F2
F2 and F3
F1 and F3
F2 and F4
F3 and F4

A

F2 and F4

32
Q

Every force has an equal and opposite reaction. A force and its reaction

cancel each other with no movement resulting
are two interactions from two bodies
are parts of a single interaction from one body
have double the effect compared to a force without its reaction
none of the above

A

are two interactions from two bodies

33
Q

Two sleds are hooked together in tandem as shown in the figure. The front sled is twice as massive as the rear sled.

The sleds are pulled along a frictionless surface by an applied force F. The tension in the rope between the sleds is T. Determine the ratio of the magnitudes of the two forces, T/F.

  1. 25
  2. 33
  3. 50
  4. 67
  5. 0
A

0.33

34
Q

If you push horizontally on a crate that contains your new desk and it slides across the floor, slightly gaining speed, how does the friction acting on the crate compare with your push?

A

Less friction force is acting on the crate compared to your push.

35
Q

Why is a massive cleaver more effective for chopping vegetables than a lighter knife of the same sharpness?

A

The massive cleaver has more inertia, so stopping it upon impact is harder, and thus cuts deeper.

36
Q

A race car travels along a raceway at a constant velocity of 200 km/h. What horizontal forces act on the car? What is the net force acting on the car? Draw a free-body diagram of the situation.

A

Friction force (between the tires and the road) propels the car forward while air resistance force pushes backward. The net force must be zero for the car to go at a constant velocity.

37
Q

You hold an apple in your hand, palm upward.

(a) Identify all the forces acting on the apple and their reaction forces. Draw a free-body diagram of the situation.
(b) When you drop the apple, identify all the forces acting on it as it falls and the corresponding reaction forces.

A

Gravitational force, normal force
Gravitational force, air resistance force

38
Q

When an athlete holds a barbell overhead, the reaction force is the weight of the barbell on his hand. How does this force vary for the case in which the barbell is accelerated upward? Downward?

A

Weight is greater if barbell is accelerated upward. Weight lessens if barbell is accelerated downward.

39
Q

Two people of equal mass attempt a tug-of-war with a 12-m rope while standing on frictionless ice. When they pull on the rope, each person slides toward the other. How do their accelerations compare, and how far does each person slide before they meet?

A

Accelerations should be same and their distance of travel before meeting should be same as well.

40
Q

You tell your friend that the acceleration of a skydiver decreases as falling progresses. Your friend then asks if this means that the skydiver is slowing down. What is your response?

A

It doesn’t mean the skydiver is slowing down. Decreasing acceleration means, in this case, that the velocity increases slower and slower.

41
Q

Is the force of gravity stronger on a piece of iron than on a piece of wood if both have the same mass? Defend your answer.

A

No. Both have the same mass, and so with F=mg, both will experience the same force.

42
Q

What properties do forces have that allow us to classify them as vectors?

A

They have magnitude and direction.

43
Q

How are inertia and mass related? How do they differ?

A

Inertia is the property of matter, and mass is a quantity to describe amount of matter.

44
Q

What is the relationship between weight and mass? Which is an intrinsic, unchanging property of a body?

A

Mass is intrinsic of the object while weight is the force of gravity acting on the object.

45
Q

Why can we neglect forces such as those holding a body together when we apply Newton’s second law of motion?

A

Due to Newton’s Third Law, all internal forces cancel out the overall motion of the apple, so they will not be involved with tugging the apple one way or another.

46
Q

Explain how the choice of the “system of interest” affects which forces must be considered when applying Newton’s second law of motion.

A

Choosing a system involves knowing which forces are internal to the system and which are external. You must consider only external forces when looking at the motion of the “system of interest”.

47
Q

A rock is thrown straight up. What is the net external force acting on the rock when it is at the top of its trajectory?

A

The net external force at the top of trajectory is the same as any other time the rock was in the air.

48
Q

(a) Give an example of different net external forces acting on the same system to produce different accelerations.
(b) Give an example of the same net external force acting on systems of different masses, producing different accelerations.
(c) What law accurately describes both effects? State it in words and as an equation.

A

Me pushing a crate. A strong person pushing on crate.
Me pushing on crate. Me pushing on lighter crate.
This describes Newton’s Second Law.

49
Q

If the acceleration of a system is zero, are no external forces acting on it? What about internal forces? Explain your answers.

A

External forces may still be acting on it, but their net effects are zero. There may still be internal forces acting on it.

50
Q

If a constant, nonzero force is applied to an object, what can you say about the velocity and acceleration of the object?

A

Velocity changes and acceleration is a non-zero constant.

51
Q

When you take off in a jet aircraft, there is a sensation of being pushed back into the seat. Explain why you move backward in the seat—is there really a force backward on you? (The same reasoning explains whiplash injuries, in which the head is apparently thrown backward.)

A

It’s not a force, but the inertia of your body. Your body wants to stay put while your car is forcing you forward.

52
Q

An American football lineman reasons that it is senseless to try to out-push the opposing player, since no matter how hard he pushes he will experience an equal and opposite force from the other player (as stated in Newton’s third law). Use Newton’s laws and draw a free-body diagram of an appropriate system to explain how he can still out-push the opposition if he is strong enough.

A

The football lineman’s reasoning demonstrates Newton’s Third Law, but that law applies across two bodies. Since the net force on the lineman only is based on Newton’s Second Law, he can use the friction force on the ground to overcome the backward force from the opponent.

53
Q

To simulate the apparent weightlessness of space orbit, astronauts are trained in the hold of a cargo aircraft that is accelerating downward at g . Why will they appear to be weightless, as measured by standing on a bathroom scale, in this accelerated frame of reference? Is there any difference between their apparent weightlessness in orbit and in the aircraft?

A

Apparent weightlessness is based on the lack of support force that counters the force of gravity. So since there is no support force in either case, both situations have apparent weightlessness.

54
Q

Since astronauts in orbit are apparently weightless, a clever method of measuring their masses is needed to monitor their mass gains or losses to adjust diets. One way to do this is to exert a known force on an astronaut and measure the acceleration produced. Suppose a net external force of 50.0 N is exerted and the astronaut’s acceleration is measured to be 0.893 m/s2 .

(a) Calculate her mass.
(b) By exerting a force on the astronaut, the vehicle in which they orbit experiences an equal and opposite force. Discuss how this would affect the measurement of the astronaut’s acceleration. Propose a method in which recoil of the vehicle is avoided.

A

56 kg
The vehicle is also accelerated in the opposite direction when the astronaut’s mass is measured. The measurement of the astronaut’s acceleration would thus look lower than it should be.
To avoid recoil of the vehicle, perhaps the device could rotate the astronaut instead.

55
Q

Suppose a 60.0-kg gymnast climbs a rope. (a) What is the tension in the rope if he climbs at a constant speed? (b) What is the tension in the rope if he accelerates upward at a rate of 1.50 m/s2 ?

A

Tension should be 588N.
Tension should be 678N.

56
Q

The wheels of a midsize car exert a force of 2100 N backward on the road to accelerate the car in the forward direction. If the force of air resistance is 250 N and the acceleration of the car is 1.80 m/s2 , what is the mass of the car plus its occupants? Explicitly show how you follow the steps in the Problem-Solving Strategy for Newton’s laws of motion. For this situation, draw a free-body diagram and write the net force equation.

A

About 1028 kg

57
Q

The figure below shows Superhero and Trusty Sidekick hanging motionless from a rope (photo turned sideways to save room on the page). Superhero’s mass is 90.0 kg, while Trusty Sidekick’s is 55.0 kg, and the mass of the rope is negligible.

(a) Draw a free-body diagram of the situation showing all forces acting on Superhero, Trusty Sidekick, and the rope.
(b) Find the tension in the rope above Superhero.
(c) Find the tension in the rope between Superhero and Trusty Sidekick. Will the tension be the same everywhere in the rope?

A

The tension of the rope above both the Superhero is 1421N.
The tension of the rope between the Superhero and the Sidekick is 539N.
Tension won’t be the same everywhere on the rope.

58
Q

Integrated Concepts When starting a foot race, a 70.0-kg sprinter exerts an average force of 650 N backward on the ground for 0.800 s.

(a) What is his final speed?
(b) How far does he travel?

A

His final speed is 7.43 m/s.
2.97 m.

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