coursera questions 1-60

Question 1

While searching for your keys, you place your cup of coffee on the roof of your parked car. Unfortunately, you forget about the coffee and climb into the car without it. As you start driving the car forward, you hear the coffee hit the ground behind the car. Why didn’t the coffee stay on the roof of the car?

Answer 1

The coffee’s inertia kept it essentially motionless as the car accelerated
forward and left the coffee behind.

Question 2

An African swallow is flying east at 60 mph (about 100 km/h) and a European swallow is flying west at 60 mph (about 100 km/h). These two swallows have _______________.

Answer 2

different speeds and different velocities.

Question 3

You kick a soccer ball toward the goal. When the fast-moving ball is midway to the goal and nothing is touching it, why is the ball moving toward the goal? [Ignore any effects due to the air or gravity.]

Answer 3

Inertia keeps the ball in motion.

Question 4

Which of the following is experiencing zero net force?

Answer 4

A water-skier who is being pulled forward by a speedboat and is moving in a straight-line path at a steady speed.

Question 5

You are riding your bicycle forward on a level road, traveling in a straight line at a steady pace. An animal suddenly runs in front of you, so you apply the brakes quickly and stop just in time to avoid hitting the animal. While the brakes are on and you are experiencing a large net force, what is the direction of velocity and acceleration, respectively?

Answer 5

Your velocity is forward, but your acceleration is backward.

Question 6

You are coasting forward at constant velocity on your inline skates. Suddenly, another skater pushes you so that the net force you are experiencing points toward your left. While your net force points toward the left, your acceleration _______________.

Answer 6

points toward the left (in the direction of your net force).

Question 7

Two skaters are coasting forward across the ice. The skater in red has a greater mass than the skater in blue. You begin pushing the two skaters forward with equal forces. How do they move while you are pushing them? [The correct answer must always be true, no matter how fast the skaters were moving before you began pushing them.]

Answer 7

The skater in red experiences less acceleration than the skater in blue.

Question 8

In what circumstance can you be accelerating forward and still be moving at a constant velocity?

Answer 8

A. In no circumstance. If you are accelerating, you velocity is changing with time.

Question 9

You are dragging a heavy chair across the floor and that chair is moving toward the east at constant velocity. The net force on the chair __________________.

Answer 9

is zero

Question 10

A set of dishes sits motionless on a slippery silk tablecloth. If you pull the tablecloth sideways quickly, it will slide out from under the dishes and leave the dishes almost unaffected. Why won’t that same result occur if you pull the tablecloth sideways slowly?

Answer 10

The moving tablecloth exerts small forces on the dishes and, given enough time, those forces will overwhelm inertia and cause the dishes to move with the tablecloth.

Question 11

Which force is your weight?

Answer 11

The force that causes you to accelerate downward when you are high above the surface of a trampoline.

Question 12

You visit a bowling alley and examine the bowling balls that are available for use. They all look identical, but some are heavier (have greater weights) than others. How can you identify the heaviest ball? [Neglect any effects due to air]

Answer 12

Hold each ball motionless in your hand and choose the one that requires the largest upward force to keep it from falling.

Question 13

You are traveling on an intergalactic cruise spaceship in deep, gravity-free space. You find that the ship has a bowling alley! Once again, there are many identical-looking balls available for use. Without gravity, however, they all have the same weight: zero. How can you identify the ball that will be heaviest when your cruise ship lands on a planet and gravity is present? [Neglect any effects due to air]

Answer 13

Shake each ball rapidly back and forth, and choose the ball that accelerates the least in response to the same force as on Earth.

Question 14

You’ve learned to juggle 4 balls at once here on Earth. During a visit to the moon, where the acceleration due to gravity is about 1/6th its Earth value, you decide to try juggling those same 4 balls. You find that, on the moon, each ball has ________________.

Answer 14

less weight and falls more slowly than on Earth. It undergoes the same acceleration as on Earth when exposed to the same force as on Earth.

Question 15

While vacationing on a tropical island, you find the courage to step off a high cliff and fall for 4 seconds before entering the water below. Exactly 2 seconds into your fall, you glance at the cliff face and see a secret treasure embedded in the rock. When you recover from your plunge, you return to the cliff top and find that treasure ______________________.

Answer 15

1/4 the distance down from the cliff top to the water

Question 16

You are playing basketball and take a shot toward the basket. When the ball is midway to the basket and nothing is touching the ball, what is the direction of the net force on the basketball? [Neglect any effects due to air]

Answer 16

downward

Question 17

You throw a handful of different coins up and forward and watch them arc through space. They leave your hand at the same moment and with the same starting velocity. Neglecting any effects due to air, where and when do those coins hit the level ground in front of your feet?

Answer 17

All the coins hit the ground at the same time and at the same distance from your feet.

Question 18

You throw a priceless porcelain vase straight up and watch as it rises to peak height and then drops safely back into your hands. Fortunately, the vase’s owner wasn’t watching. What were the vase’s velocity and acceleration at the moment it reached peak height?

Answer 18

The vase’s velocity was zero. The vase’s acceleration was the acceleration due to gravity, which is not zero.

Question 19

When an archer sends an arrow toward a target, the archer must aim the arrow above the target’s bullseye (its center) in order for the arrow to hit that bullseye. If the archer uses a stronger bow and therefore a faster-moving arrow, how will that change how the archer aims the arrow in order to hit the same target’s bullseye? [Neglect any effects due to air]

Answer 19

The archer must still aim above the target’s bullseye, but less far above the bullseye than with the slower-moving arrow.

Question 20

As you collect plastic bottles for recycling, one of the bottles rolls horizontally off the kitchen counter and bounces on the floor about 1 foot (0.3 meters) outward from the base of
the counter. Why didn’t the bottle drop straight down and hit the floor exactly at the base of the counter?

Answer 20

The bottle coasted horizontally outward as it fell vertically.

Question 21

You are in an ordinary room (both its floor and ceiling are horizontal). You throw a ball directly upward and it bounces off the ceiling. While the ball is touching the ceiling, in which direction is the ceiling’s support force on the ball?

Answer 21

The ceiling’s support force on the ball is directed downward.

Question 22

You are playing volleyball and your teammate has just hit the ball forward – toward your opponents. To increase the ball’s forward speed, you push it with a forward force of 200 newton (45 pounds-force). What force, if any, does the ball exert on you?

Answer 22

A backward force of 200 newtons.

Question 23

When you stand and remain motionless on a bathroom scale, what force is the scale exerting on your feet?

Answer 23

An upward support force equal in amount to your weight.

Question 24

As a ball bounces on the floor, the floor exerts an upward support force on the ball. Can the amount of that upward support force on the ball be different from the ball’s weight?

Answer 24

Yes. It can be greater than the ball’s weight and it can be less than the
ball’s weight.

Question 25

You are using a string to lift a heavy picnic basket up to your treehouse. Alas, the string isn’t strong enough for the job. The picnic basket becomes motionless, even though you are moving the portion of string you are holding upward, and the string breaks. Breaking the string required energy and that energy was provided by

Answer 25

you

Question 26

You are shopping in a store and want to go upward from the second floor to the third floor. You can make that trip using an escalator, an elevator, or a staircase. Which method of going from the second floor to the third floor will increase your gravitational potential energy the most?

Answer 26

All three methods will increase your gravitational potential energy by the same amount.

Question 27

A downhill skier is descending a snow-covered mountain. The skier steps off of a level region of the mountain and onto a steep slope. The skier begins to accelerate rapidly downhill on the slope. What force is causing the skier to accelerate downhill?

Answer 27

The downhill ramp force that is the sum of the skier’s weight and the
support force exerted on the skier by the snow-covered slope.

Question 28

You have shopping cart full of groceries and that cart is on a ramp. You are exerting an uphill force on the cart, so that the net force on the cart is zero. What energy transfer is occurring?

Answer 28

If the cart is moving uphill, you are transferring energy to the cart. If
the cart is moving downhill, the cart is transferring energy to you.

Question 29

You are trying to lift a heavy file cabinet into the back of a truck. The file cabinet weighs 200 pounds (about 900 newtons) and you must raise it 2 feet (about 0.6 meters) upward. The file cabinet has wheels, so it rolls freely. You create a ramp using rigid boards that are 8 feet long and successfully push the wheeled file cabinet up the ramp and into the truck. What force did you exert on the file cabinet to keep it moving up the ramp at constant velocity? [Assume the ramp was smooth, straight, and exactly 8 feet long, and neglect any imperfections, such as friction or air resistance.]

Answer 29

You exerted a force of 50 pounds directed uphill along the ramp.

Question 30

You talking
realize that you are
which object to hit. The tree will not move at all if you hit the tree, but the garbage can will
move if you hit the garbage can. How will your choice of object affect the energy you transfer to that object when you hit it?

Answer 30

You will transfer energy if you hit the garbage can, but you will not transfer energy if you hit the tree.

Question 31

A toy top is a disk-shaped object with a sharp point and a thin stem projecting from its bottom and top, respectively. When you twist the stem hard, the top begins to spin rapidly. When you then set the top’s point on the ground and let go of it, it continues to spin about a vertical axis for a very long time. What keeps the top spinning?

Answer 31

The top has rotational inertia.

Question 32

To start a motionless toy top spinning, you twist it. What determines the direction in which the top spins?

Answer 32

The torque you exert on the toy top has a direction and the top undergoes angular acceleration in the direction of the torque you exert on it.

Question 33

You are traveling through deep space in a large spaceship and everything in the ship is weightless. The ship is experiencing zero net force and it coasts forward. However, in preparation for docking at a space station, the ship is rotating slowly. You notice that one location in the coasting ship moves at constant velocity, even as the rest of the ship rotates about that location. What is this special location in the ship?

Answer 33

The ship’s center of mass.

Question 34

A tall luxury hotel has a rotating restaurant at its top. The disk-shaped floor of the restaurant rotates slowly about the center of the restaurant and completes one full rotation every 30 minutes. When the restaurant opens each day, the manager turns on the motors that make the restaurant spin, but it takes several minutes for the restaurant to begin spinning at its full angular velocity. Why doesn’t the restaurant reach full speed immediately?

Answer 34

The restaurant’s angular acceleration is proportional to the net torque exerted on it. The motors produce a net torque on the restaurant and it immediately undergoes angular acceleration. But it takes time for the angular-accelerating restaurant to reach its full angular velocity.

Question 35

A modern bicycle has two pedals mounted on a rotating device known as a crank. Pushing down on one pedal with your foot produces a torque on the crank, about its pivot, except in which situation(s)?

Answer 35

When the pedal is vertically above or below the pivot, your force on the pedal is directed along the lever arm from the pivot to your force. A force that is parallel to the lever arm produces zero torque.

Question 36

Arm-wrestling is a simple game that two people can play. The players sit across from one another at a table, place their right elbows together on the tabletop and clasp their right hands together. When the competition starts, each person tries to twist the pair of arms counterclockwise from that person’s perspective until those arms touch the table. It’s a rotational problem, with the elbows acting as the pivot and the two players trying to rotate the pair of arms in opposite directions. Suppose you are arm-wrestling with a friend and you are winning. Compare the torque you are exerting on your friend to the torque that your friend is exerting on you.

Answer 36

Those two torques are equal in amount but opposite in direction.

Question 37

You are arm-wrestling another friend and find that you are almost perfectly matched. Your pair of arms is vertical and motionless, even though you are both trying hard to win. To begin winning, you want that pair of arms to rotate counterclockwise from your perspective. What must you do to make that happen?

Answer 37

The counterclockwise torque you exert on the pair of arms must be greater in amount than the clockwise torque your friend is exerting on that pair.

Question 38

You are making pizza and are spinning a ball of pizza dough in midair to make a larger and larger disk. As the diameter of the disk increases, you find it more difficult to change the disk’s angular velocity. Why?

Answer 38

The disk’s rotational mass increases with its diameter, although its mass remains unchanged.

Question 39

Your car has a flat tire and you are using an automobile jack to lift the corner of the car so that you can change the tire. The jack involves a lever and you lift the corner of the car upward by pushing the handle of the lever downward. You notice that as the handle moves downward 10 inches, the corner of the car moves upward only 0.5 inches. Assuming that the jack is not wasting any energy, compare the downward force you exert on the jack handle to the upward force that the jack exerts on the car.

Answer 39

The jack’s upward force on the car is 20 times as large as your downward force on the jack handle.

Question 40

Tower cranes are frequently seen in cities, where they are used to construct tall buildings. In a tower crane, a huge metal beam sits atop a vertical metal tower. The beam extends outward from the tower in two directions and it pivots about the top of the tower. A lifting cable hangs from one end of the beam and heavy weights hang from the other end of the beam. Since the lifting cable end of the beam is the only end that seems to do anything, what useful purpose does the weight-end of the beam serve?

Answer 40

The weight-end of the beam ensures that the beam is approximately balanced about its pivot and experiences approximately zero torque due to gravity.

Question 41

You are walking at constant velocity on a sidewalk that slopes gently uphill. What force(s) is the sidewalk exerting on you?

Answer 41

A frictional force directed uphill, parallel to the sidewalk’s surface, and a support force directed perpendicular to the sidewalk’s surface.

Question 42

For millennia, people have ground grain into flour by placing a thin layer of grain between two surfaces that slide across one another. Those two surfaces are usually stacked on top of one another and the surface above the grain is that of a very heavy object, such as a millstone. When motionless, the heavy millstone will exert large support forces on the grain and thereby crush that grain to some extent, but it won’t make fine flour. Spinning the millstone while keeping the surface below the grain motionless, however, will grind the grain as fine as you like. What role does the millstone’s weight play in this grinding process?

Answer 42

The grinding process uses both support forces to crush the grain and sliding frictional forces to wear the grain. Since frictional forces are approximately proportional to support forces, the millstone’s weight enhances both the crushing and the wearing processes.

Question 43

Walking on ice can be treacherous, but some techniques are safer than others. In general, you are less likely to slip and falling if you lower your foot vertically into place on the ice as you take each step than you are if you slide your foot horizontally into place on the ice as you take each step. Why is the vertical landing method of walking on ice usually safer than the horizontal sliding method?

Answer 43

The static frictional forces you can obtain from the ice using the vertical landing method are larger, and more effective at preventing sideways slips, than the sliding friction forces you are likely to obtain from the ice using the horizontal sliding method.

Question 44

Most automobiles have mechanical brakes on all four of their wheels. Each of these brakes consists of two surfaces—one surface that rotates with the wheel and one surface that doesn’t rotate. When you put your foot on the brake pedal in such an automobile, those two surfaces begin to slide across one another. The harder you press on the brake pedal, the more tightly those surfaces are pressed against one another. Why do the brakes permit the two surfaces to slide across one another, rather than locking those two surfaces together so that they don’t slide across one another?

Answer 44

The purpose of the brakes is to waste the moving automobile’s kinetic energy, using sliding friction, and thereby slowing the automobile safely. Locking the brakes would result in static friction in the brakes and prevent the brakes themselves from wasting more than a tiny fraction of the automobile’s kinetic energy.

Question 45

The harder you press on an automobile’s brake pedal, the greater the support forces that the two surfaces in each brake exert on one another. Why does this increase in support forces in the brakes result in more rapid deceleration (acceleration opposite its velocity) of the automobile?

Answer 45

The sliding frictional forces the two brake surfaces exert on one another are approximately proportional to their support forces on one another. As the support forces increase, the sliding frictional forces also increase; they waste the automobile’s kinetic energy faster, so that it slows more rapidly.

Question 46

The front wheel of your bicycle spins freely on its axle and rotates at almost constant angular velocity if nothing outside the bicycle exerts a torque on it. Suppose the front wheel is motionless as you stand next to your bicycle. You get on the bicycle and pedal forward. As the bicycle begins to move forward, why does its front wheel begin to rotate?

Answer 46

The ground exerts a backward static frictional force on the bottom of the front wheel to prevent that wheel from sliding across the ground. That frictional force, exerted at a lever arm from the wheel’s center of rotation, produces the torque that causes the wheel to begin rotating.

Question 47

When you drive a car on a level (horizontal) road that is slippery with ice, you usually have no problems except when you try to speed up, slow down, or turn. Why does the icy road make those three actions hazardous? [neglect any effects due to air]

Answer 47

Each action involves a horizontal acceleration and requires a horizontal force. The only forces that level pavement can exert on the car are frictional forces and the slippery ice reduces those frictional forces.

Question 48

Pedaling your bicycle provides power to its rear wheel and propels your bicycle forward. What force(s) is principally responsible for the bicycle’s forward acceleration as you pedal your bicycle forward from rest on a level (horizontal) road?

Answer 48

The pavement exerts a forward frictional force on the bottom of the rear wheel.

Question 49

If you throw a ball directly upward at 10 meters-per-second (22 mph), it will rise upward about 5 meters before coming momentarily to a stop. If you throw that ball directly upward at 20 meters-per-second (44 mph), how far will it rise upward before coming momentarily to a stop?

Answer 49

about 20 meters

Question 50

Earth’s oceans exhibit tides, so that the average water level near most shores rises and falls at roughly six-hour intervals. A somewhat simplistic explanation for the tides is that the moon’s and sun’s gravity slightly distort Earth’s oceans, creating high and low regions, and the
Earth’s rotation causes those high and low regions to shift on the Earth’s surface. There are a few tidal generating plants on Earth that use energy in the tides to generate electricity. Where does nearly all of the energy in the tides originally come from?

Answer 50

The kinetic energy in the Earth rotational motion.

Question 51

As it moves through the sky, what physical quantities is the meteor carrying with it? [Ignore any effects due to air or Earth’s gravity]

Answer 51

Energy, momentum directed toward city hall, but no force.

Question 52

your pitch-black apartment when you collide with the wall. You come to a complete stop. Fortunately, your interior decorator mounted a thick woolen tapestry (wall-hanging) on the concrete wall and that soft tapestry saves you from injury. Compare the momentum you transferred while coming to a stop on the tapestry-covered wall to the momentum you would have transferred if you had come to a stop on the bare concrete wall.

Answer 52

You would have transferred the same momentum in either case, but in stopping on the tapestry-covered wall you transferred that momentum with a smaller force over a longer period of time.

Question 53

A car traveling at 60 mph (100 km/h) veers off the road and hits a tree. The car immediately comes to a complete stop. Fortunately, the airbag inflates and the driver comes to a stop in the airbag instead of coming to a stop on the steering wheel. Hitting the airbag rather than the steering wheel saves the driver’s life because the driver

Answer 53

transfers all of her momentum to whatever stops her, but that transfer is slower and involves a smaller force when she hits the airbag.

Question 54

A diver stands upright at the edge of the 10 meter platform at the Olympics. The diver jumps off the platform, folds into a ball shape, completes 3.5 somersaults, unfold out of the ball shape, and plunges head-first into the water. Compare the diver’s angular momentum about the diver’s center of mass at three different moments while that diver is not touching anything: (a) before folding into a ball shape, (b) while ball-shaped, and (c) after unfolding out of the ball shape [Note that the diver’s weight, which acts at the diver’s center of gravity, produces zero torque on the diver about the
diver’s center of mass. Ignore any effects due to the air.]

Answer 54

The diver’s angular momentum is the same at all three moments.

Question 55

The chef at a pizza restaurant tosses a spinning disk of pizza dough into the air. As the disk stretches outward in midair and its diameter increases, what happens to the disk’s angular momentum and angular velocity about the disk’s center of mass? [Note that the disk’s weight, which acts at the disk’s center of gravity, produces zero torque on the disk about the disk’s center of mass. Ignore any effects due to the air.]

Answer 55

The disk’s angular momentum is constant, but the disk’s angular velocity decreases.

Question 56

the moving scenery as the carousel spins about its center of rotation. The ride comes to an end and the carousel gradually slows to a stop. Why does it take so long for the carousel to stop rotating?

Answer 56

The spinning carousel carries a large amount of angular momentum and the angular impulse needed to remove that angular momentum with a reasonable torque requires a long time.

Question 57

A “lazy susan” is a disk-shaped rotating platform that a restaurant places at the center of a large dining table. Dishes of food are placed on the lazy susan and diners can rotate the lazy susan by hand to bring various dishes closer to them. A large torque exerted for a
You are riding on a large carousel at an amusement park and you are enjoying
short time makes the motionless platform begin rotating rapidly, but that dangerous technique risks tipping over some of the food dishes. How can you make the same motionless platform and dishes begin rotating just as rapidly, but with a smaller, safer torque?

Answer 57

Do the same angular impulse as the dangerous technique, but by exerting a smaller torque for a longer time.

Question 58

To win a big prize at the fair or festival, all you have to do is toss a basketball into a bucket located about 10 feet (3 meters) in front of you and have the basketball remain in the bucket. The rigid bucket cannot move and it opens toward you. However, the bucket is tilted upward just enough that the basketball will remain in it if someone places the basketball in the bucket by hand. You try a dozen times to get the basketball to stay in the bucket, but it keeps bouncing back out of the bucket. Why is it so difficult for the basketball to come to rest in the bucket?

Answer 58

To stop moving, the basketball must transfer both energy and momentum to the bucket and, while it transfers momentum easily to the bucket, it transfers almost zero energy to the bucket.

Question 59

You have just added a massive stone sculpture to your modern art collection. Unfortunately, the people who delivered the sculpture accidentally set it on its side. What barbarians! To tip the sculpture onto its proper base, you transfer as much momentum as you can to the highest point on the sculpture. You accomplish this transfer (successfully, I might add) by running full speed toward the sculpture and

Answer 59

hitting the highest point on the sculpture with your feet as you jump against it so that you end up reversing your velocity.

Question 60

Your dynamic sculpture combines magnets, springs, and elastic bands with a variety of moving parts, including levers, pulleys, and pendulums. When someone jostles
those parts, a complicated series of motions occurs and predicting how things will proceed seems nearly impossible. You point out, however, that there is a simple rule governing the motion of each part at any moment in time. That rule is that each part

Answer 60

accelerates in the direction that reduces its total potential energy as quickly as possible.