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Physics > Chapter 4 > Flashcards

Chapter 4 Flashcards

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1
Q

What are we studying in chapter 4?

A

Why objects move, what causes them to move, i.e. dynamics.

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2
Q

If an object’s motion changes, what causes it to change?

A

Force - force will cause acceleration

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3
Q

What is a force?

A

A force is a push or pull and a vector quantity.

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4
Q

What 2 basic types can forces be divided into?

A

contact forces and field forces

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5
Q

What are some kinds of forces?

A

springs, elastic, electrical, and magnetic

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6
Q

What’s a contact force?

A

pushes or pulls using physical contact with the object - e.g., lifting a book, pulling on something with a rope

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7
Q

What is a field force?

A

pushes or pulls that happen over a distance and do not require physical contact e.g., gravitational force, magnetic force

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8
Q

Why are field forces called field forces?

A

Because we view a magnet as having a magnetic field around it, and when an object such as a nail is placed in that field, a force is exerted on the nail by the magnetic field, so it moves toward the magnet.

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9
Q

What did Aristotle (350 B.C.) say?

A

The harder you push an object, the further it goes. A greater force means greater distance. Also, to keep an object moving, you must keep applying force, e.g, push a book on a desk.

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10
Q

What did Galileo (1590 A.D) say?

A

No! He said that it was friction that caused a moving object to stop moving. If you reduce friction, it moves further. He reasoned that if you could remove friction totally, the object would simply keep moving.

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11
Q

What is friction?

A

A force that acts on an object opposite to its motion.

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12
Q

What must we start with when analyzing the forces on an object?

A

We must begin with a free-body diagram.

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13
Q

What is a free-body diagram, and how is it drawn?

A
  • A circle or box that represents the object being analyzed, with labeled arrows coming out of it to represent the forces acting on the object.
  • All forces are drawn coming out of the object, so a push on the left side would be drawn as a pull on the right side instead.
  • Forces are simply labeled with an F along with the appropriate subscript.
  • The magnitude of the various forces does not belong on the diagram.
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14
Q

Net Force

A

A term that simply means, “the vector sum of the forces” or “the resultant of all the forces added together”
When dealing with net force we do not generally use complete vector notation, but rather add and subtract magnitudes.

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15
Q

How to calculate net force:

A
  1. Begin by drawing a free-body diagram
  2. Identify which way the object is moving and define that as the positive direction (if there is no motion, then go with the traditional designations: up is positive and down is negative; right is positive, and left is negative.)
  3. Find the net force by adding the magnitudes of all the forces in the positive direction and subtracting the magnitude of the forces in the negative direction.
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16
Q

What did Isaac Newton (1687 A.D.) do?

A

Developed Galileo’s ideas more fully and published his famous Laws of Motion

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17
Q

Newton’s First Law:

A

“If no net force acts on an object, it remains at rest or continues to move with constant speed in a straight line.” (or simply, “with constant velocity”)

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18
Q

What is the first law referred to as?

A

The seatbelt law

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19
Q

Notes with first law:

A
  1. The two cases, “at rest” and “constant velocity” are really the same thing depending on your frame of reference.
  2. It does not say there is no force on the object, just that there is no net force - i.e., the forces are all balanced
  3. When the net force on an object is zero, the object is in equilibrium
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20
Q

Inertia

A

The tendency of a body to remain at rest or to continue to move at a constant velocity.

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21
Q

Newtons Second Law:

A

The acceleration of a body is directly proportional to the net force on it and it’s inversely proportional to the mass of the object. the object accelerates in the direction of the net force.

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22
Q

Notes with the second law

A
  1. One factor that determines how much an object accelerates when a force is applied to it is the amount of force applied - stronger force = greater acceleration
  2. Another factor that determines how much it accelerates is the mass of the object - larger mass = less acceleration
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23
Q

What could you refer to the second law as:

A

Hotrod Law

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24
Q

Mass

A

A measurement of the amount of inertia an object has.
i.e - Newton’s 1st Law - how much it resists a change in its state of rest of motion

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25
Q

Weight

A

The force of gravity on an object. ( If there is no gravity, there is no weight)
Weight is a force! It is measured in newtons, not kilograms.
Fg = mg

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26
Q

How to calculate the weight of an object if it is in free fall:

A

If an object is in free fall, the net force on it is the force of gravity (or, its weight), and it would accelerate at a rate of 9.80 m/s^2 downward. So to calculate weight, or, the force of gravity on an object, Fnet = ma becomes: Fg = mg

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27
Q

What are the units of force?

A

Newtons - derived units

28
Q

What is the equation for the second law?

A

-> ->
Fnet = ma

Vector relationship

29
Q

Why do we not include the vector symbol and direction when calculating weight?

A

We know that the direction of the force of gravity is down, but we do not speak of weight as being a negative number. We simply consider the direction of the force when we need to.

30
Q

What’s the difference between weight and mass?

A

Weight depends on acceleration due to gravity, whether or not there are other forces, and whether or not the object is accelerating, the weight is always calculated this way.

31
Q

Newton’s third law:

A

Forces always come in pairs. It is impossible to exert a force on an object without it exerting a force back. the two forces form an interaction pair.

When one object exerts a force on a second object, the second object exerts an equal force in the opposite direction.

32
Q

What is the third law referred to as?

A

Get Even Law

33
Q

Examples of the third law:

A

Push down on the desk, desk pushes back
Skating - pushing against the wall moves you backward
A ball bouncing
Jumping
Walking

33
Q

Tension Force:

A

The tension force at one end of the rope is the same as at the other end of the rope. The force with which the rope pulls up on the hanging object is the same as the force with which it pulls down on the ceiling. We can simply think of this as an application of Newton’s 3rd Law between the object and the ceiling - the rope is just an agent of the interaction force pair.

34
Q

Normal Force

A

Anytime two surfaces are in contact, they exert a normal force on each other. This force is always perpendicular to the surfaces

35
Q

Why is Fn not always equal to Fg in normal forces?

A

If I push down on a box, the total force pushing down on the table is Fa + Fg, so Fn = Fa + Fg.

36
Q

Elastic Force

A

Materials that resist being stretched, compressed, or twisted. This resistance is called an elastic force.

37
Q

What do elastic forces resist?

A
  1. Compression (pushing together)
  2. Tension (pulling apart)
38
Q

What type of relationship is elastic force:

A

direct relationship - force to a spring is graphed against the amount of stretch, so it is a straight line graph.

39
Q

What is the slope of a Fe vs. x graph?

A

k= Fe/x

40
Q

What is an elastic force relationship known as?

A

Hooke’s Law

41
Q

What does Fa represent?

A

Applied force (such as a person pushing or pulling on an object)

42
Q

What does Ff represent?

A

Friction force

43
Q

What does Fg represent?

A

gravitational force (i.e. weight)

44
Q

What does Fn represent?

A

normal force

45
Q

What does Fe represent?

A

elastic force

46
Q

What does Ft represent?

A

tension force

47
Q

Spring Constant

A

K (N/m)

48
Q

Increase in length

A

x (m)

49
Q

Terminal Velocity

A

When the drag force equals the force of gravity. When this happens, there is no net force, and so there is no
acceleration. The constant velocity that is reached
when the drag force equals the force of gravity is the terminal velocity

50
Q

Drag Force

A

When an object moves through any fluid, such as air or
water, the fluid exerts a drag force on the moving object in the direction opposite to its motion. A drag force is the force exerted by a fluid on the object moving through the fluid.

51
Q

When do light objects reach their terminal velocity?

A

When light objects with large surface areas are
falling, the drag force has a substantial effect on
their motion, and they quickly reach the terminal
velocity.

52
Q

How can skydivers or skiers decrease or increase their terminal velocity?

A

Skydivers can increase or decrease their
terminal velocity by changing their body orientation and shape. horizontal, spread-eagle shape produces the slowest terminal velocity, about 60 m/s. Because a parachute changes the shape of the skydiver when it
opens, a sky diver becomes part of a very large object with a correspondingly large drag force and a terminal velocity of about 5 m/s.

53
Q

Interaction pair

A

Is two forces that are in opposite directions and have equal magnitude. An interaction pair must consist of two forces of equal magnitude pointing in opposite directions. These opposing forces must act on two different objects that can exert a force against each other.

54
Q

A rock is dropped from a bridge into a valley. Earth
pulls on the rock and accelerates it downward.
According to Newton’s third law, the rock must
also be pulling on Earth, yet Earth does not seem
to accelerate. Explain.

A

The acceleration caused by the force of an object interacting with Earth is usually a very small number. Under most circumstances, the number is so
small that for problems involving falling or stationary objects, Earth can be treated as part of the external world rather than as a second system.

55
Q

If your textbook is in equilibrium, what can you say
about the forces acting on it?

A

If an object, such as your textbook, is in equilibrium, it means that the object is not accelerating. In other words, the net force acting on the object is zero.

56
Q

If you are in a car that is struck from
behind, you can receive a serious neck injury called
whiplash.
a. Using Newton’s laws, explain what happens to
cause such an injury.
b. How does a headrest reduce whiplash?

A

a. Explanation using Newton’s Laws:

When a car is struck from behind, the passengers inside experience a sudden acceleration forward. According to Newton’s first law of motion, an object at rest will remain at rest, and an object in motion will remain in motion with a constant velocity unless acted upon by a net external force. In the case of a car collision, the external force is applied by the striking vehicle, causing an abrupt change in the velocity of the car being struck.

This sudden change in velocity is where Newton’s second law comes into play. The acceleration:

The passengers in the car experience this acceleration, and their bodies tend to continue moving forward due to inertia. However, their heads, being part of the body, also have inertia. As a result, the head lags behind the rest of the body during the sudden acceleration.

B) A headrest in a car is designed to limit the backward motion of the head during a rear-end collision. When the car is struck from behind, the headrest provides a surface that the occupant’s head can come into contact with, preventing excessive backward movement.

57
Q

Before a skydiver opens her parachute, she may be
falling at a velocity higher than the terminal velocity
that she will have after the parachute opens. (4.2)
a. Describe what happens to her velocity as she
opens the parachute.
b. Describe the sky diver’s velocity from when her
parachute has been open for a time until she is
about to land.

A

A)
As the parachute opens, it dramatically increases the surface area that interacts with the air. This significantly increases the air resistance acting on the skydiver. The increased air resistance slows down the skydiver’s descent, causing a rapid decrease in velocity. The skydiver undergoes a period of deceleration as the forces become unbalanced, transitioning from a high-speed free fall to a slower descent.

B) After the parachute is open, the skydiver enters a phase where the forces of gravity and air resistance become balanced again, resulting in a new, lower terminal velocity. During this phase, the skydiver continues to descend, but at a more constant and reduced velocity compared to the initial free fall.

In summary, opening the parachute causes a rapid decrease in velocity as air resistance increases. After the parachute is open, the skydiver reaches a new, lower terminal velocity, and her descent becomes more controlled until she is ready to land. The parachute serves to slow down the fall and ensure a safer and more controlled descent to the ground.

58
Q

Suppose that the acceleration of an object is zero.
Does this mean that no forces are acting on it?

A

No, the fact that the acceleration of an object is zero does not necessarily mean that no forces are acting on it. In physics, when the acceleration of an object is zero, it indicates that the net force acting on the object is zero, not that there are no forces present.

59
Q

Why do you have to push harder on the
pedals of a single-speed bicycle to start it moving
then to keep it moving at a constant velocity?

A

Because of newtons 1st law, objects in motion stay in motion, as you first start pushing the petals you need to overcome the initial inertia or mass keeping it at rest, then once you have the petals going at a constant velocity, the inertia will keep it moving.

60
Q

Basketball When a basketball player dribbles a
ball, it falls to the floor and bounces up. Is a force
required to make it bounce? Why? If a force is
needed, what is the agent involved?

A

Yes, a force is required to make a basketball bounce. When a basketball is dribbled, it falls to the floor due to the force of gravity acting on it. When it collides with the floor, the shape and elasticity of the basketball, as well as the floor, cause the ball to deform briefly.

During this deformation, the ball experiences an upward force exerted by the floor. This force is the result of the elastic properties of both the ball and the floor. The ball compresses and then quickly returns to its original shape, and this process generates an upward force.

According to Newton’s third law of motion, for every action, there is an equal and opposite reaction.

61
Q

Explain why the tension in a massless rope is
constant throughout it.

A

The rope pulling on the object is the same as the force with which the object is pulling on the bottom piece of rope But the bottom piece of the rope is not moving, so the piece of rope above it must be pulling back with the same amount of force. The result is that the tension force must be the same in each part of the rope.

62
Q

Should astronauts choose pencils with hard
or soft lead for making notes in space? Explain.

A

Astronauts typically use pencils with softer leads (B or 2B) in space rather than harder leads (H or 2H). Because then the graphite won’t break and since there is no gravity, then when the astronaut writes the soft pencil won’t push it away.

63
Q

It can be said that 1 kg equals 2.2 lb. What does this
statement mean? What would be the proper way of
making the comparison?

A

1kg actually equals 9.80 Newtons on Earth. Because weight is a measurment of the magnitude of acceleraion due to gravity. So 2.2lb = 9.80N

64
Q

What is the winner tug-of-war determined by?

A

Who can push on the ground harder so that it pushes back (Newton’s 3rd Law) and gets you moving.

65
Q

You drop two table tennis balls from the top of a tall building, one filled with air
and the other with water. Both experience
air resistance as they fall. Which ball reaches terminal velocity first? Do both hit the
ground at the same time?

A

The lighter, air-filled table tennis ball
reaches terminal velocity first. It’s mass
is less for the same shape and size, so
the friction force of upward air resistance becomes equal to the downward
the force of mg sooner. Because the force
of gravity on the water-filled table tennis
ball (more mass) is larger, its terminal
velocity is larger, and it strikes the
ground first.

Physics (13 decks)

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