Module #10

Question 1

Friction

Answer 1

A force that opposes motion, resulting from the contact of two surfaces

Question 2

kinetic friction

Answer 2

Friction that opposes motion once the motion has already started

Question 3

Static Friction

Answer 3

Friction that opposes the initiation of motion

Question 4

State Newton’s Three laws of motion

Answer 4

Newton’s first law- An object in motion will tend to stay in motion until acted upon by an outside force.
Newton’s Second Law- When an object is acted upon by on or more outside forces, the total force is equal to the mass of the object times the resulting acceleration
Newton’s Third Law- For every action, there is an equal and opposite reaction

Question 5

In space, there is almost no air, so there is virtually no friction. If an astronaut throws a ball in space with an initial velocity of 3.0 meters per second to the west, what will the ball’s velocity be in a year? Assume there are no nearby planets.

Answer 5

3.0 meters per second to the west

Question 6

A boy is running north with a beanbag in his hands. He passes a tree and at the moment he is beside the tree, he drops the beanbag. Will the beanbag land next to the tree? If not, will it be north or south of the tree?

Answer 6

It will not fall next to the tree, but north of it.

Question 7

A boy is running northward and his friend stands next to a tree holding a beanbag. As the boy passes, he barely taps the beanbag, causing it to fall out of his friends hands. Will the beanbag land next to the tree? If not, will it land north or south of the tree?

Answer 7

The beanbag will land next to the tree.

Question 8

A busy shopper is driving down the road. Many boxes lie piled on the backseat of the car- evidence of shopping activity. Suddenly, the shopper must hit the brakes to avoid a collision. Will the boxes be slammed farther back into the back seat, or will they slam into the front seat where the driver can feel them?

Answer 8

The boxes will slam into the front seat.

Question 9

When roads get wet, the can get slick. Obviously, then, friction between a car’s tires and the road decreases when the road is wet. Why?

Answer 9

The water fills in the groves in the road, reducing how close the tire molecules can get to the road molecules.

Question 10

In order to slide a refrigerator across the floor, a man must exert an enormous amount of force. Once it is moving, however, the man need not exert nearly as much force to keep it moving. Why?

Answer 10

The static frictional force is greater that the kinetic frictional force.

Question 11

A child is pushing her toy across the room with a constant velocity to the east. If the static friction between this toy and the floor is 15 Newtons, while the kinetic friction is 10 Newtons, what force is the child exerting?

Answer 11

10 Newtons to the east

Question 12

In order to get a 15-kilogram object moving to the west, a force more that 25 Newtons must be exerted. Once it is moving, however, a force of only 20 Newtons accelerates the object at 0.1 meters per second2 to the west. What is the force that static friction can exert on the object? What is the force of kinetic friction?

Answer 12

The static frictional force is 25 Newtons east. The kinetic frictional force is 18.5 Newtons east

Question 13

A father is trying to teach his child to ice skate. As the child stands still, the father pushes him forward with an acceleration of 2.0 meters per second2 north. If the child’s mass is 20 kilograms, what is the force with which the father is pushing? (Since they are on ice, assume you can avoid friction.)

Answer 13

40 Newtons north

Question 14

Static friction can exert a force of up to 700 Newtons on a 500-kilogram box of bricks. The kinetic frictional force is only 220 Newtons. How many Newtons of force must a worker exert to get the box moving? What force must the worker exert to accelerate the box at 0.1 meters per second2 to the south?

Answer 14

The worker must apply more than 700 Newtons of force to get the box moving. The total force must be 270 Newtons south.

Question 15

In order to shove a rock out of the way, the gardener gets it moving by exerting just slightly more than 100 Newton’s of force. To keep it moving at a constant velocity eastward, however, the gardener needs only to exert a 45 – Newton’s of force to the east. What are the static and kinetic friction all forces between the rock and the ground?

Answer 15

The static frictional force is 100 Newtons. The kinetic friction force, then, must be 45 Newtons to the west. .

Question 16

Two men are trying to push a 710 kg rock. The first exerts a force of 156 Newtons east and the second exerts a force of 220 Newtons east. The rock accelerates at 0.20 m/s2 to the east. What is the kinetic frictional force between the rock and the ground?

Answer 16

234 Newtons west

Question 17

A child pushes against a large dog house, trying to move it. The doghouse remains stubbornly unmoved. What exerts the equal and opposite force which Newton’s third law of motion says must happen in response to Childs push? What is the force exerted on it?

Answer 17

The equal and opposite force is exerted by the doghouse on the child.

Question 18

In a baseball game, a player catches a fast-moving ball. The ball stops in the players hand. What evidence tells you that the player exerted a force on the ball? What exerts the equal and opposite force required by Newton’s third law? What evidence does the player have for this force?

Answer 18

The player exerts a force on the ball because the balls velocity changed. This means there was an acceleration, which means a force was exerted on the ball. The equal and opposite force is exerted by the ball on the player and is evidenced by the pain that the player feels when he catches the ball.

Question 19

A man leans up against a wall with a force of 20 Newtons to the east. What is the force exerted by the wall on the man?

Answer 19

20 Newtons west

Question 20

Inertia

Answer 20

The tendency of an object to resist changes in its velocity