1a. define: inertia
the tendency of an object to resist changes in its velocity
1b. define: friction
a force that opposes motion, resulting from the contact of two surfaces
1c. Define: kinetic friction
friction that opposes motion once the motion has already started
1d. define: static friction
friction that opposes the initiation of motion
- State newtons three laws of motion.
Newtons First Law- an object in motion (or at rest) tens to stay in motion (or at rest) until it is acted upon by an outside force
Newtons Second Law- when an object is acted on by one or more outside forces, the total force is equal to the mass of the object times the resulting acceleration
Newtons Third Law- for every action, there is an equal and opposite reaction
- 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.
3.0 meters per second to the west
- 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?
the beanbag will not fall next to the tree, instead it will fall north of the tree
- Suppose the situation in question #4 now changed. The boy is running, but now his friend stands beside the tree with the 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 be north or south of the tree?
the beanbag will land next to the tree
- A busy shopper is driving down the road. Many boxes lie piled up on the back seat 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?
the boxes will slam into the front seat
- When roads get wet, they can get slick. Obviously, then, the friction between a cars tires and the road decreases when the road is wet. Why?
the water fills in the grooves in the road, reducing how close the tire molecules can get to the road molecules
- 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?
the static frictional force is greater than the kinetic frictional force
- 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?
10 Newtons to the east
- A father is trying to teach his child to ice skate. As the child stands still, the father pushes him forward with an accleeration of 2.0 meters per second squared 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 ignore friction.)
40 Newtons north
- In order to get a 15-kilogram object moving to the west, a force of more than 25 Newtons must be exerted. Once it is moving, however, a force of only 20 Newtons acclerates the object at 0.1 meters per second squared to the west. What is the force that static friction can exert on the object? What is the force of kinetic friction?
the static frictional force is 25 Newtons east; the kinetic frictional force is 18.5 Newtons east
- 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 second squared to the south?
Part one: the worker must apply more than 700 Newtons of force to get the box moving. Part two: the workers force is 270 Newtons south.
- In order to shove a rock out of the way, a gardener gets it moving by exerting just slightly more than 100 newtons of force. To keep it moving at a constant velocity eastward, however, the gardener needs only ro exert a 45-newton force to the east. What are the static and kinetic frictional forces between the rock and the ground?
the static frictional force is 100 Newtons, the kinetic frictional force, then, must be 45 Newtons to the west
- Two men are trying to push a 710-kilogram 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 meters per second squared to the east. What is the kinetic frictional force between the rock and the ground.
234 newtons west
- A child pushes against a large doghouse, trying to move it. The doghouse remains stubbornly unmoved. What exerts the equal and opposite force which Newtons Third Law of Motion says must happen in response to the child’s push? What is that force exerted on?
the equal and opposite force is exerted by the doghouse on the child
- In a baseball game, a player cathces a fast-moving ball. The ball stops in the player’s hand. What evidence tells you that the player exerted a force on the ball? What exerts the equal and opposite force required by Newtons Third Law? What evidence does the player have for this force?
The player exerts a force on the ball because the ball’s 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 the the player feels when he catches the ball.
- 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?
20 Newtons west
