Gen Physics 1 Exam 2 Concepts

If a car tries to go round the bend too fast, it starts to skid. Why? (A)car's engine is not strong enough to keep the car from being pushed out (B) friction between tires and road is not strong enough to keep car moving in a circle (C) car is too heavy to make the turn (D) none of the above

(B) friction between tires and road is not strong enough to keep car moving in a circle

A 0.55-kg ball, attached to the end of a horizontal cord, is revolved in a circle of radius 1.3 m on a frictionless horizontal surface. If the cord will break when the tension in it exceeds 75 N, what is the maximum speed the ball can have?

13.3 m/s

A ping pong ball is shot into a circular tube that is lying flat (horizontal) on a tabletop. When the ping pong ball leaves the track, which path will it follow? 1. keep its circular motion 2. go in a straight line 3. slowly lose its circular motion

2. go in a straight line Once the ball leaves the tube, there is no longer a force to keep it going in a circle. Therefore, it simply continues in a straight line, as Newton's First Law requires!

A golfer making a putt gives the ball an initial velocity of v0,but he has badly misjudged the putt, and the ball only travels one-quarter of the distance to the hole. If the resistance force due to the grass is constant, what speed should he have given the ball (from its original position) in order to make it into the hole?

2v0

You kick a smooth flat stone out on a frozen pond. The stone slides, slows down and eventually stops. You conclude that: 1) the force pushing the stone forward finally stopped pushing on it 2) no net force acted on the stone 3) a net force acted on it all along 4) the stone simply "ran out of steam" chapter 4

3) a net force acted on it all along After the stone was kicked, no force was pushing it along! However, there must have been some force acting on the stone to slow it down and stop it. This would be friction!!

Which is stronger, Earth's pull on the Moon, or the Moon's pull on Earth? 1) the Earth pulls harder on the Moon 2) the Moon pulls harder on the Earth 3) they pull on each other equally 4) there is no force between the Earth and the Moon

3) they pull on each other equally

Which vehicle will be able to drive around a turn at the fastest speed without skidding off the road? 1. Truck 2. Smartcar 3. No difference

3. No difference

How does the work required to stretch a spring 2 cm compare with the work required to stretch it 1 cm

4 times the work, The elastic potential energy is 1/2 kx 2. So in the second case, the elastic PE is 4 times greater than in the first case. Thus, the work required to stretch the spring is also 4 times greater.

Three students, named A through C, each exert a constant force on a block as the block moves from left to right. All six forces have the same magnitude. Which student does zero work on the block (Chapter 6)

B

Paul and Corinne start from rest at the same time on frictionless water slides with different shapes. At the bottom, whose velocity is greater?

Both the same, Conservation of Energy: Ei = mgH = Ef = 1/2 mv 2therefore: gH = 1/2 v 2 Since they both start from the same height, they have the same velocity at the bottom.

By what factor does the kinetic energy of a car change when its speed is tripled? (Chapter 6)

By a factor of 9, speed is proportional to the kinetic energy squared

Three students, named A through C, each exert a constant force on a block as the block moves from left to right. All six forces have the same magnitude. Which student does positive work on the block (Chapter 6)

C

Suppose the two blocks are in contact with each other and speeding up as they move to the right. In this case the magnitude of force that the 10-kg block exerts on the 5-kg block is __________ the magnitude of the force that the 5-kg block exerts on the 10-kg block. A. greater than B. less than C. equal to chapter 4

C. equal to third law pairs

A car rounds a level curve at a constant speed. Which of the following statements regarding the acceleration of the car must be correct. A) The car has a constant zero acceleration B) The car has constant non-zero acceleration pointing towards the center of the curve C) The car has constant non-zero acceleration pointing away from the center of the curve D) The car has constant non-zero acceleration pointing tangential to the curve

B) The car has constant non-zero acceleration pointing towards the center of the curve

Object 1 has mass m1, and object 2 has mass m2, which is smaller than m1. The distance between their centers is r. How does the acceleration caused by the gravitational force for m1 and m2 compare? A. a1 > a2 B. a1 < a2 C. a1 = a2

B. a1 < a2 Smaller m, larger a

conservation of energy

Change in energy (delta e) = 0, the total energy at every point is the same

If a car traveling 60 km/hr can brake to a stop within 20 m, what is its stopping distance if it is traveling 120 km/hr?Assume that the braking force is the same in both cases

80 mF d = Wnet = KE = 0 - 21 mv2, and thus, |F| d = 21 mv2. Therefore, if the speed doubles, the stopping distance gets four times larger.

The work W0 accelerates a car from 0 to 50km/hr. How much work is needed to accelerate the car from 50 km/hr to 150km/hr?

8W

When a car goes around a curve, there must be a net force towards the center of the circle of which the curve is an arc. If the road is flat, that force is supplied by ____________

Friction

A box is being pulled across a rough floor at a constant speed. What can you say about the work done by friction? (Chapter 6)

Friction does negative work

What is Newtons 1st law? (ch4)

If an object experiences no net force, then its velocity is constant: the object is either at rest, or it moves in a straight line with constant speed.

You and your friend both solve a problem involving a skier going down a slope, starting from rest. The two of you have chosen different levels for y = 0 in this problem. Which of the following quantities will you and your friend agree on? A) skier's PE B) skier's change in PE C) skier's final KE

Only B and C, The gravitational PE depends upon the reference level, but the difference ΔPE does not! The work done by gravity must be the same in the two solutions, so ΔPE and ΔKE should be the same.

to calculate work done when force is applied at an angle, only which force does work? (Chapter 6)

Only force in direction of the motion counts towards work

What is the formula for potential energy? (Chapter 6)

PeGrav = mgy or mgh

Positive centripetal forces point __________ the circle, whereas negative centripetal forces point _________ the circle

Pos = towards center of circle neg = away from center of circle

A child on a skateboard is moving at a speed of 2 m/s. After a force acts on the child, her speed is 3 m/s. What can you say about the work done by the external force on the child?

Positive

Two paths lead to the top of a big hill. One is steep and direct, while the other is twice as long but less steep. How much more potential energy would you gain if you take the longer path?

Since your vertical position (height) changes by the same amount in each case, the gain in potential energy is the same

What is Newtons 2nd law? (ch4)

The acceleration a of a body is parallel to and directly proportional to the net force F acting on the body, is in the direction of the net force, and is inversely proportional to the mass m of the body, i.e., F = ma

A car starts from rest and accelerates to 30 mph. Later, it gets on a highway and accelerates to 60 mph. Which takes more energy, the 0→30 mph, or the 30→60 mph? (Chapter 6)

The change in KE ( 1/2 mv^2 ) involves the velocity squared. So in the first case,we have: 1/2 m(30^2 −0^2) =1/2 m(900)In the second case we have: 1/2m(60^2-30^2)=1/2m2700

In the previous question, just before hitting the ground, what is the final speed of the heavy stone compared to the light one?

The same: All freely falling objects fall at the same rate, which is g. Because the acceleration is the same for both, and the distance is the same, then the final speeds will be the same for both stones.

what does the free body diagram of a bucket full of water being spun in uniform circular motion at its highest point look like

W pointing downwards (towards center of circle) N pointing downwards (towards center of circle)

What is Newtons 3rd law? (ch4)

When a first body exerts a force F1 on a second body, the second body simultaneously exerts a force F2 = −F1 on the first body. This means that F1 and F2 are equal in magnitude and opposite in direction.

Astronauts in the space shuttle float because: (Chapter 5)

While gravity is trying to pull them inward, they are trying to continue on a straight-line path

Work done by nonconservative forces

Wnonconservative = ΔE = ΔU + ΔK

Can friction ever do positive work? (Chapter 6)

YES: Consider the case of a box on the back of a pickup truck. If the box moves along with the truck , then it is actually the force of friction that is making the box move.

Is it possible for the gravitational potential energy of an object to be negative?

Yes, Gravitational PE is mgh, where height h is measured relative to some arbitrary reference level where PE = 0. For example, a book on a table has positive PE if the zero reference level is chosen to be the floor. However, if the ceiling is the zero level, then the book has negative PE on the table. It is only differences (or changes) in PE that have any physical meaning.

Work is done when: (Chapter 6)

a force acts on an object and the object moves in the direction of the force

The Moon does not crash into Earth because: a) It has a high tangential velocity b) the net force on it is zero c) it is beyond the main pull of Earth's gravity d) it's being pulled by the Sun as well as by Earth

a) It has a high tangential velocity

A block of mass m rests on the floor of an elevator that is accelerating upward. What is the relationship between the force due to gravity and the normal force on the block? a) N > mg b) N = mg c) N < mg (but not zero) d) N = 0 chapter 4

a) N > mg The block is accelerating upward, so it must have a net upward force. The forces on it are N (up) and mg (down), so N must be greater than mg in order to give the net upward force!

You tie a rope to a tree and you pull on the rope with a force of 100 N. What is the tension in the rope? a) 0 N b) 100 N c) 150 N d) 200 N chapter 4

b) 100 N Since typically, the forces on the rope are much greater than the weight of the rope Tension is the same on both sides of the rope!

Two tug-of-war opponents each pull with a force of 100 N on opposite ends of a rope. What is the tension in the rope? a) 0 N b) 100 N c) 150 N d) 200 N chapter 4

b) 100 N This is literally the identical situation to the previous question. The tension is not 200 N!! Whether the other end of the rope is pulled by a person, or pulled by a tree, the tension in the rope is still 100 N!!

A cart starting from rest rolls down a hill and at the bottom has a speed of 4 m/s. If the cart were given an initial push, so its initial speed at the top of the hill was 3 m/s, what would be its speed at the bottom? a) 4 m/s b) 5 m/s c) 6 m/s d) 25 m/s

b) 5 m/s When starting from rest, the cart's PE is changed into KE: ΔPE = Δ KE = 1/2 m(4)2

A skier goes over a small round hill with radius R. Since she is in circular motion, there has to be a centripetal force. At the top of the hill, what is Fc of the skier equal to? a) Fc = N + mg b) Fc = mg - N c) Fc = T + N - mg d) Fc = N

b) Fc = mg - N Fc points toward the center of the circle, i.e., downward in this case. The weight vector points down and the normal force (exerted by the hill) points up. The magnitude of the net force, therefore, is: Fc = mg - N

A block of mass m rests on the floor of an elevator that is moving upward at constant speed. What is the relationship between the force due to gravity and the normal force on the block? a) N > mg b) N = mg c) N < mg (but not zero) d) N = 0 chapter 4

b) N = mg The block is moving at constant speed, so it must have no net force on it since a=0. The forces on it are N (up) and mg (down), so N = mg, just like the block at rest on a table.

A truck, initially at rest, rolls down a frictionless hill and attains a speed of 20m/s at the bottom. To achieve a speed of 40 m/s at the bottom, how many times higher must the hill be?

initial energy: Ei = PEg = mgH final energy: Ef = KE = 1/2 mv 2 Conservation of Energy:Ei = mgH = Ef = 1/2 mv 2 therefore: gH = 1/2 v 2 So if v doubles, H quadruples!

A block is sliding up an inclined ramp. There is no friction between the block and the ramp. Which best depicts the free-body diagram of the block? chapter 4

it would be N upwards and W downwards Velocity never produces a force

A block rests on the horizontal bed of a truck. The truck is moving to the right at a constant speed. Which best depicts the free-body diagram of the block? chapter 4

it would just be N upwards and W downwards No acceleration, so friction force not necessary!

You are driving along and slam on the brakes- your tires skid to a halt and in this time you cover a distance of 72m. • What kind of friction caused you to stop? • Given µs = 0.9 and µk = 0.8 between rubber and pavement- can you find how fast you were driving?

kinetic friction 33.6 m/s

Two blocks of mass m1 and m2 (m1 > m2) slide on a friction less floor and have the same kinetic energy when they hit a long rough stretch (m > 0), which slows them down to a stop. Which one goes farther?

m2 both blocks must have the same work done to them by friction. The friction force is less for m2 so stopping distance must be greater.

what is uniform circular motion?

motion in a circle of CONSTANT radius at CONSTANT speed

are W and N force examples of newtons third law pair? chapter 4

no, must be an interaction between two separate objects

Moving in a circle at a constant speed, the velocity vector is constantly changing due to changes in direction. A change in velocity means there is acceleration. The centripetal acceleration is always ________________ to motion of the object

perpendicular changes direction of object

Three balls of equal mass start from rest and roll down different ramps. All ramps have the same height. Which ball has the greater speed at the bottom of its ramp?

same speed for all balls, All of the balls have the same initial gravitational PE, since they are all at the same height (PE = mgh). Thus, when they get to the bottom, they allhave the same final KE, and hence the same speed (KE = 1/2 mv 2)

what is greater static or kinetic friction?

static friction "It is harder to get something moving than it is to keep it moving." μs > μk

What is the maximum speed with which a 1200-kg car can round a turn of radius 90.0 m on a flat road if the coefficient of friction between tires and road is 0.65? Is this result independent of the mass of the car? what kind of friction?

yes it is independent of mass Vmax = 23.9 m/s The friction is static* if the friction of kinetic the car would skid

A skier moves down a 10º slope at constant speed. What can you say about the coefficient of friction, µk? Assume the speed is low enough that air resistance can be ignored.

µk=0.17

A ball with an initial height of h = 2.3 m is shot directly upward with a velocity of v0 = 4.0 m/s.What is the final velocity of the ball just before it hits the ground?

½ m vo2 + m g yo = ½ m vf2½ vo2 + g yo = ½ v f2½ (4.0) 2 + 9.8 (2.3) = ½ v f230.54 = ½ v f2v f = 7.81 m/s

Two blocks of ice, one four times as heavy as the other, are at rest on a frozen lake. A person pushes each block the same distance d. Ignore friction and assume that an equal force F is exerted on each block. 1.) Which of the following statements is true about the kinetic energy of the heavier block after the push? A. It is smaller than the kinetic energy of the lighter block. B. It is equal to the kinetic energy of the lighter block. C. It is larger than the kinetic energy of the lighter block. D. It cannot be determined without knowing the force and the mass of each block 2.) Compared to the speed of the heavier block, what is the speed of the light block after both blocks move the same distance d? 3.) Now assume that both blocks have the same speed after being pushed with the same force F. What can be said about the distances the two blocks are pushed?

1.) B. It is equal to the kinetic energy of the lighter block. The work-energy theorem states that the change in kinetic energy of an object equals the net work done on that object. The only force doing work on the blocks is the force from the person, which is the same in both cases. Since the initial kinetic energy of each block is zero, both blocks have the same final kinetic energy. 2.) Twice as fast Since the kinetic energy of the lighter block is equal to the kinetic energy of the heavier block, the lighter block must be moving faster than the heavier block. 3.) The heavy block must be pushed 4 times farther than the light block. Because the heavier block has four times the mass of the lighter block, when the two blocks travel with the same speed, the heavier block will have four times as much kinetic energy. The work-energy theorem implies that four times more work must be done on the heavier block than on the lighter block. Since the same force is applied to both blocks, the heavier block must be pushed through four times the distance as the lighter block.

Spring Potential Energy

1/2kx^2, where k is the spring's constant; a measure of the stiffness of the spring

A block is at rest on a horizontal tabletop and is not touching anything except the tabletop. The force of friction on the block in this situation has magnitude A. 0. B. 2 N. C. µsn. D. µkn.

A. 0

conservative force

A force, such as gravity, that performs work over a distance that is independent of the path taken.

If the distance to the Moon were doubled, then the force of attraction between Earth and the Moon would be A. one quarter B. one half C. the same D. two times

A. one quarter

What is nonuniform circular motion?

An object is moving in a circular path but at varying speeds It must have a tangential component to its acceleration as well as the radial one since acceleration is change of velocity and thus direction.

A block is at rest on a horizontal tabletop and is not touching anything except the tabletop. If you exert a continuous 2-N horizontal push on the block, the block does not move; in this situation the force of friction on the block is A. 0. B. 2 N. C. µsn. D. µkn.

B. 2 N

If I double the mass of both m and M and also halve the distance r between them, by what factor does the force between the masses change? A. 8 B. 1/2 C.16 D.4

C.16

Is it possible for the kinetic energy of an object to be negative? (Chapter 6)

Can't be negative because mass can't be negative

Total Mechanical Energy

E = U (Potential Energy) + K (Kinetic Energy)

A block is at rest on a horizontal tabletop and is not touching anything except the tabletop. If you exert a continuous 3-N horizontal push on the block, the block accelerates in the direction of the push; in this situation the force of friction on the accelerating block is A. 0. B. 2 N. C. µsn. D. µkn.

D. µkn.

When non conservative force such as friction does work on a system, it changes the the total mechanical energy of the system.

Delta E, Is not equal to 0

what is the formula for gravitational force?

F = GMm/r^2

what are some examples of applied force? chapter 4

Fpush, Fpull, etc

what is the formula for net centripetal force (Fcent)?

Fcent = mac thus Fcent = mv^2/r

what causes the bucket of water being spun to change direction?

Fext and acceleration (centripetal force)

A box sliding on a frictionless flat surface runs into a fixed spring, which compresses a distance x to stop the box. If the initial speed of the box were doubled, how much would the spring compress in this case?

Four time as much, Use energy conservation:initial energy: Ei = KE = 1/2 mv 2 final energy: Ef = PEs = 1/2 kx 2Conservation of Energy:Ei = 1/2 mv 2 = Ef = 1/2 kx 2therefore: mv 2 = kx 2 So if v doubles, x doubles!

Work is measured in: (Chapter 6)

Joules (J)

formula for kinetic energy (Chapter 6)

KE=1/2mv^2

You are driving along and slam on the brakes- your tires skid to a halt and in this time you cover a distance of 72m. What kind of friction caused you to stop?

Kinetic Friction

what units are used to measure forces?

Newtons (N)

A ball tied to a string is being whirled around in a circle.What can you say about the work done by tension? (Chapter 6)

No work is done because the force acts in a perpendicular direction to the displacement.

In a baseball game, the catcher stops a 90-mph pitch. What can you say about the work done by the catcher on the ball? (Chapter 6)

The force exerted by the catcher is opposite in direction to the displacement of the ball, so the work is negative.

what is kinetic friction?

The friction of a moving object fk = μk n Kinetic friction will always have a consistent value based on the normal force and the coefficient of friction.

Work-Energy Principle (Chapter 6)

The net work done on an object is equal to the change in the object's kinetic energy.

You see a leaf falling to the ground with constant speed. When you first notice it,the leaf has initial total energy PEi + KEi. You watch the leaf until just before it hits the ground, at which point it has final total energy PEf + KEf. How do these total energies compare? a) PEi + KEi > PEf + KEf b) PEi + KEi = PEf + KEf c) PEi + KEi < PEf + KEf d) impossible to tell from the information provided

a) PEi + KEi > PEf + KE As the leaf falls, air resistance exerts a force on it opposite to its direction of motion. This force does negative work, which prevents the leaf from accelerating. This frictional force is a non-conservative force, so the leaf loses energy as it falls, and its final total energy is less than its initial total energy.

Consider two identical blocks, one resting on a flat surface (case A), and the other resting on an incline (case B). For which case is the normal force greater? a) case A b) case B c) both the same (N = mg) d) both the same (0 < N < mg)

a) case A In Case A, we know that N = W. In Case B, due to the angle of the incline, N < W. In fact, we can see that N = W cos(q).

You are a passenger in a car, not wearing a seat belt. The car makes a sharp left turn. From your perspective in the car, what do you feel is happening to you? a) you are thrown to the right b) you feel no particular change c) you are thrown to the left d) you are thrown to the ceiling

a) you are thrown to the right The passenger has the tendency to continue moving in a straight line. From your perspective in the car, it feels like you are being thrown to the right, hitting the passenger door.

what is the formula for centripetal acceleration?

ac = v2/r

If a net external force acts on an object, it _____________ in that direction. chapter 4

accelerates

You're on a Ferris wheel moving in a vertical circle. When the Ferris wheel is at rest, the normal force N exerted by your seat is equal to your weight mg. How does N change at the top of the Ferris wheel when you are in motion? a) N remains equal to mg b) N is smaller than mg c) N is larger than mg d) None of the above

b) N is smaller than mg You are in circular motion, so there has to be a centripetal force pointing inward. At the top, the only two forces are mg (down) and N (up), so N must be smaller than mg. If N was equal to mg, the net motion would not be moving in a circle. If N was greater than mg, the motion would be up or away from the circle

A mass attached to a vertical spring causes the spring to stretch and the mass to move downwards. What can you say about the spring's potential energy (PEel) and the gravitational potential energy (PEg ) of the mass? a) both PEel and PEg decrease b) PEel increases and PEg decreases c) both PEel and PEg increase d) PEel decreases and PEg increases

b) PEel increases and PEg decreases, The spring is stretched, so its elastic PE increases, since PEel = 1/2kx2. The mass moves down to a lower position, so its gravitational PE decreases, since PEg = mgh.

You swing a ball at the end of string in a vertical circle. Since the ball is in circular motion there has to be a centripetal force. At the top of the ball's path, what is Fcent equal to? a) Fnet = T - mg b) Fnet = T + N - mg c) Fnet = T + mg d) Fnet = T e) Fnet = mg

c) Fnet = T + mg Fnet points toward the center of the circle, i.e. downward in this case. The weight vector points down and the tension (exerted by the string) also points down. The magnitude of the net force, therefore, is: Fnet = T + mg

Astronauts in the space shuttle float because: a) They are so far from Earth that Earth's gravity doesn't act any more. b) Gravity's force pulling them inward is cancelled by the centripetal force pushing them outward. c) While gravity is trying to pull them inward, they are trying to continue on a straight-line path. d) Their weight is reduced in space so the force of gravity is much weaker.

c) While gravity is trying to pull them inward, they are trying to continue on a straight-line path. Astronauts in the space shuttle float because they are in "free fall" around Earth, just like a satellite or the Moon. Again, it is gravity that provides the centripetal force that keeps them in circular motion.

Third law pairs must be _______ and _________ forces between two objects. chapter 4

equal and opposite

Is this a correct statement of Newton's First Law (true or false) "Any object will seek a state of rest unless a force acts on it" chapter 4

false

non-conservative forces

forces such as friction that cause energy to transfer from an object and be dissipated into the surroundings

Instantaneous velocity is always ____________ to circle

tangent

kinetic energy is: (Chapter 6)

the energy an object has due to its motion

what is static friction?

the friction of a stationary object fs ≤ μs n Static Friction will resist motion and can be any value up to the maximum value

Force perpendicular to motion (Chapter 6)

the work from that force is zero

Force opposing the direction of the motion (Chapter 6)

theta = 180°, work is negative

Potential energy matters on: (Chapter 6)

where we choose our y = 0, only changes in the potential energy can be measured