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* Physics > HW 1-5 > Flashcards

HW 1-5 Flashcards

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1
Q

What relationship between the Sun and Earth did Copernicus formulate?

A

The Earth revolved in a circle about the Sun.

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2
Q

What did Galileo discover in his legendary experiment on the Leaning Tower of Pisa?

A

Galileo found that a heavier stone does not fall significantly faster than a lighter one

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3
Q

What did Galileo discover about moving bodies and force in his experiments with inclined planes?

A

In the absence of a retarding force, a body will keep moving at a constant speed in a straight line forever

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4
Q

What does an object at rest tend to do?

A

The blobs tend to stay where they were.

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5
Q

What property of an object at rest tends to make it stay at rest?

A

They have inertia the tendency of an object at rest to stay at rest.

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6
Q

What type of path does a moving object follow in the absence of a force?

A

It continues to move in a straight line at a constant speed

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7
Q

What is the net force on a cart that is pulled to the right with 100 pounds of force and to the left with 30 pounds of force?

A

The net force is 70 pounds to the right.

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8
Q

Why do we say that force is a vector quantity?

A

A force has a magnitude and a direction.

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9
Q

According to the parallelogram rule, what quantity is represented by the diagonal of a constructed parallelogram?

A

The diagonal is the resultant, or sum, of two vectors

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10
Q

What are the units for force?

A

newtons

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11
Q

What is the net force on a bag pulled down by gravity with a force of 18 newtons and pulled upward by a rope with a force of 18 newtons?

A

The net force is zero newtons.

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12
Q

What does it mean to say something is in mechanical equilibrium?

A

An object in mechanical equilibrium experiences a zero net force.

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13
Q

State the equilibrium rule for forces in symbolic notation

A

∑F=0

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14
Q

When you stand at rest on a bathroom scale, how does your weight compare with the support force from the scale?

A

Your weight is equal in magnitude and opposite in direction to the support force from the scale.

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15
Q

If you push on a crate with a horizontal force of 100 N and it slides at constant velocity, what is the magnitude and direction of the frictional force acting on the crate?

A

The frictional force on the crate is 100 N opposite the direction of motion.

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16
Q

Lucy Lightfoot stands with one foot on one bathroom scale and her other foot on a second bathroom scale. Each scale reads 360 .

A

720N

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17
Q

A different scaffold that weighs 400 supports two painters, one 500 and the other 400 . The reading in the left scale is 800 .

A

500N

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18
Q

What two units of measurement are necessary for describing speed?

A

Distance and time

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19
Q

What kind of speed is registered by an automobile speedometer?

A

Instantaneous speed

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20
Q

If a car is moving at 90 km/h and it rounds a corner, also at 90 km/h, does it maintain a constant speed? A constant velocity?

A

It maintains a constant speed, but does not maintain a constant velocity.

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21
Q

What is the acceleration of a car that maintains a constant velocity of 100 km/h for 10 s?

A

0 km/h·s

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22
Q

Acceleration is generally defined as the time rate of change of velocity. When can it be defined as the time rate of change of speed?

A

When moving in a straight line, the time rate of change of speed is acceleration.

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23
Q

What exactly is meant by a “freely falling” object?

A

An object in free fall falls under gravity alone with no friction.

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24
Q

What relationship between distance traveled and time did Galileo discover for freely falling objects released from rest?

A

Galileo discovered that distance increased as the time squared.

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25
Q

What is the distance fallen for a freely falling object 1 s after being dropped from a rest position? What is the distance for a 4-s drop?

A

It falls 5 m in 1 s and 80 m in 4 s

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26
Q

Select the correct equations that show that the average speed of a rabbit that runs a distance of45min a time of3sis15m/s

A

v=s/t=(45m)/(3s)=15m/s

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27
Q

Select the correct equations that show that the acceleration of a hamster is5m/s2when it increases its velocity from rest to10m/sin2s

A

a=Δv/t=(10m/s)/(2s)=5m/s2

28
Q

Select the correct equations that show that a freely falling rock drops a distance of20mwhen it falls from rest for2s.

A

s=g⋅t2/2=(10m/s2)⋅(2s)2/2=20m

29
Q

What is the velocity of the ball when it reaches its highest point?

A

v= 0 m/s

30
Q

What is a ball’s velocity 1sbefore it reaches its highest point?

A

v = 10 m/s

31
Q

How does acceleration depend on the net force?

A

Acceleration is proportional to the net force.

32
Q

Once the crate is sliding, how hard do you push to keep it moving at constant velocity?

A

You push with a force equal to and opposite the dynamic friction force.

33
Q

Which is normally greater: static friction or sliding friction on the same object?

A

Static friction is normally greater than dynamic friction.

34
Q

How does the force of friction for a sliding object vary with speed?

A

The force of friction is approximately independent of speed.

35
Q

What is the weight of a 1-kilogram brick resting on a table?

A

10 N

36
Q

If the mass of a sliding block is tripled while a constant net force is applied, by how much does the acceleration change?

A

The acceleration is reduced to 1/3 of its original value.

37
Q

Why doesn’t a heavy object accelerate more than a light object when both are freely falling?

A

The ratio of the weight to mass is the same for all objects in the same locality.

38
Q

Object A has three times the mass of object B. Identical forces are exerted on the two objects. What are the accelerations?

A

The acceleration of object B is three times that of object A.

Since F = ma, the acceleration is given by a = F/m, so the object with the lower mass has a higher acceleration.

39
Q

A parachutist is falling toward the ground. The downward force of gravity is exactly equal to the upward force of air resistance. What happens to the velocity?

A

The velocity of the parachutist is not changing with time.

Since the net force is zero, the parachutist’s acceleration is zero, so the velocity is constant in time.

40
Q

A kilogram is a measure of an object’s __________.

A

Mass.

An object far from any source of gravity still has mass, even if no forces of gravity are acting on it.

41
Q

A person who weighs 800 N (about 180 pounds) is standing on a scale inside an elevator. The elevator is moving upward yet slowing down. The reading on the scale is __________.

A

less than 800 N.

Since the scale is pushing upward with a strength less than the force of gravity, the net force is directed downward, causing the person to decelerate

42
Q

A person who weighs 800 N is squatting on a scale. Suddenly he starts jumping, extending his legs as quickly as possible, so that he can jump into the air. As the person is in the process of jumping (accelerating upward) the reading of the scale is ______

A

greater than 800 N.

Since the person is accelerating upward, the scale must be pushing upward more strongly than the force of gravity.

43
Q

A bowling ball and a small marble will fall downward to the surface of the Moon at the same rate. Why?

A

The ratio of the force of gravity exerted on an object to the object’s mass is the same.

The acceleration is given by a = F/m, so all objects fall at the same rate since the force of gravity is proportional to the object’s mass.

44
Q

Two identical twins are falling toward the ground. The twin in the green jumpsuit is going down head first, and the twin in the red jumpsuit is going down belly first. The two twins have the same mass. Which twin reaches the fastest terminal velocity?

A

the twin in the green jumpsuit

Since the twin in the green jumpsuit (going head first) has a smaller surface area plowing through the air, she must fall faster in order for the force of air resistance to have the same strength as the force of gravity.

45
Q

Two parachutists have identical parachutes open and are falling toward the ground. The parachutist with the red jumpsuit is much more massive than the parachutist with the blue jumpsuit. How do the terminal velocities of the two parachutists compare?

A

The red parachutist has a higher terminal velocity.

Since the red parachutist is more massive, he experiences a stronger force of gravity acting downward. In order for the wind resistance to balance this stronger force, he must have a faster terminal velocity.

46
Q

An object cannot remain at rest unless which of the following holds?

A

The net force acting on it is zero.

If there is a net force acting on a body, regardless of whether it is a constant force, the body accelerates. If the body is at rest and the net force acting on it is zero, then it will remain at rest.
The net force could be zero either because there are no forces acting on the body at all or because several forces are acting on the body but they all cancel out.

47
Q

If a block is moving to the left at a constant velocity, what can one conclude?

A

The net force applied to the block is zero.

If there is a net force acting on a body, regardless of whether the body is already moving, the body accelerates.
If a body is moving with constant velocity, then it is not accelerating and the net force acting on it is zero.
The net force could be zero either because there are no forces acting on the body at all or because several forces are acting on the body but they all cancel out.

48
Q

A block of mass2kgis acted upon by two forces:3N(directed to the left) and4N(directed to the right). What can you say about the block’s motion?

A

It could be moving to the left, moving to the right, or be instantaneously at rest.

The acceleration of an object tells you nothing about its velocity–the direction and speed at which it is moving. In this case, the net force on (and therefore the acceleration of) the block is to the right, but the block could be moving left, right, or in any other direction.

49
Q

A massive block is being pulled along a horizontal frictionless surface by a constant horizontal force. The block must be __________.

A

moving with a constant nonzero acceleration

Since there is a net force acting, the body does not move at a constant velocity, but it accelerates instead. However, the force acting on the body is constant. Hence, according to Newton’s 2nd law of motion, the acceleration of the body is also constant.

50
Q

If Dr. Hewitt applies 3 N of force to keep the block moving at a steady rate, what must be the force of friction, and why?

A

Equal to 3 N, because the force of friction should exactly balance out the force Dr. Hewitt applies, so that there is no net force acting on the block.

51
Q

After he gets the block to move, what does Dr. Hewitt do to drag the block across the table at a steady rate?

A

He continues to apply a steady force.

52
Q

You are dragging a block on a surface with friction at a steady speed of 2 m/s and exert a force of 5 N to do so. What is the force of friction? Why?

A

Equal to 5 N, because the force of friction should exactly balance out the force Dr. Hewitt applies, so that there is no net force acting on the block.

53
Q

What general rule can you conclude about the force needed to keep an object in motion at a steady rate?

A

The force that you need to apply to keep an object moving at a steady rate is equal to the force resisting the motion of the object.

54
Q

What is the net force acting on a 1-kgball in free fall?

A

10.0N

55
Q

What is the net force acting on a falling 1-kgball if it encounters 2Nof air resistance?

A

8.00N

56
Q

The action is: Earth pulls on Moon. What is the reaction?

A

Moon pulls on Earth.

57
Q

Which force is greater, the Earth’s pull on the Moon, or the Moon’s pull on the Earth?

A

Both pulls are the same

58
Q

If the cannon’s recoil is the result of an equal and opposite reaction, why is the cannon’s acceleration lower than the shell’s acceleration?

A

The cannon has more mass than the shell.

Since the cannon has more mass than the shell, it will have a lower acceleration because both objects experience the force. This can be understood from Newton’s second law (a=F/m)

59
Q

Since movement of an object or system requires that a net force be applied to it, how can this occur if all action forces have opposite and equal reaction forces?

A

The action force doesn’t act on the same object as the reaction force.

Newton’s second law can be applied to each of the objects. In fact, this explains why the two objects can have different resulting accelerations

60
Q

In Lesson 3 of the tutorial, the horse is pulling a cart. What allows the horse to accelerate forward even though the cart is pulling back on the horse?

A

The force of friction the ground exerts on the horse is larger than the force the cart is pulling back on the horse.

The net force exerted on the horse must be to the right if the horse accelerates to the right. The only two horizontal forces acting on the horse are friction (pushing forward) and the cart (pulling backward), so the friction force must be stronger.

61
Q

How does the force on the rifle compare with the force on the bullet, and why?

A

The magnitude of the force of the rifle is equal to the magnitude of force on the bullet and the forces are in the opposite direction, because for every action there is an equal and opposite reaction.

62
Q

How does the magnitude of acceleration of the rifle compare with the magnitude of acceleration of the bullet, and why?

A

The acceleration of the rifle is smaller than the acceleration of the bullet.

They experience the same magnitude of force, but the mass of the rifle is greater, and so the rifle will experience a smaller acceleration than the bullet.

63
Q

How does the distance traveled by the bullet compare with the distance traveled by the rifle in the same time, and why?

A

The bullet will travel farther than the rifle because it has a greater magnitude of acceleration.

64
Q

If the forces that act on a cannonball and the recoiling cannon from which it is fired are equal in magnitude, why do the cannonball and cannon have very different accelerations?

A

RememberF=ma, and note that the cannon has a much greater mass than the cannon ball, so the cannon accelerates less for the same force.

65
Q

What is the name of the force responsible for keeping a cylinder rolling?

A

There is no force responsible. (don’t mistakenly call it inertia!)

66
Q

Ignoring wind resistance, how far does a ball, dropped from rest, travel after 3 seconds?

A

45m d = 1/2gt

67
Q

how fast is a ball moving three seconds after being dropped from rest?

A

30 ms v =at

* Physics (12 decks)

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