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Define inertia.
The tendency of an object to resist changes in its velocity
Define friction.
A force that opposes motion, resulting from the contact of two surfaces
Define kinetic friction.
Friction that opposes motion once the motion has already started
Define static friction.
Friction that opposes the initiation of motion
State Newton's three law's of motion.
1)Newton's First Law-An object in motion (or at rest) will tend to stay in motion (or at rest) until it is acted upon by an outside force.
2)Newton's 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.
3)Newton's 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.
(Newton's First law of Motion tells us that an object will not change velocity until acted on by an outside force. Often this force is friction. In this problem, once the ball is thrown, no forces (not even friction) are operating on the ball. Thus, even in a year, its velocity will still be) 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. (This is once again an application of Newton's First Law. While it is in the boy's hand, the beanbag has a velocity going north. Thus, as it falls, it will travel north. When it lands, then, it will be north of the tree. In fact, ignoring air resistence, when it hits the ground, it will be right next to wherever the boy is at that instant, because it will be traveling north with the boy's velocity.)
A busy shopper is driving down the road. Many boxes lie piled 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. (The boxes have the same velocity as the car. When the car stops, they continue to move with the same velocity. This makes them move forward relative to the car, slamming them into the front seat.)
Suppose the boy who was running toward the tree is running again. 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 friend's 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. (In this case, the beanbag has no initial velocity. It is at rest with the boy standing next to the tree. When the running boy taps the beanbag lightly, it simply falls to the ground.)
When roads get wet, they can get slick. Obviously, then, the friction between teh car's tires and the road decreases when the road is wet. Why?
Water fills the grooves in the road, reducing how close the tire molecules can get to the road molecules. (Remember, friction is caused by molecules on each surface attracting one another, and the strength of the attraction depends on how close they can get to each other. When the road gets wet, the grooves in the road get filled with water. This makes it harder for the bumps on the tires to fit into them, which makes it hard for the molecules to get close to one another. This can become an even bigger problem when a film of water gets trapped under the tires, causing the tires to lose contact with the road. Essentially, they are travelling on the water, not the road. This situation is called "hydroplaning," and it causes the tire molecules to be so far from the road molecules that very little friction exists.)
In order to slide a refridgerator 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. (When the refridgerator is not moving, the man must overcome static friction to get it moving. Once it is moving, the man only needs to overcome 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 the floor is 15 Newtons, while the kinetic friction is 10 Newtons, what force is the child exerting?
10 Newtons to the east, (Since the object is moving with a constant velocity, we know its acceleration is zero. Since the total force exerted on an object is equal to the object's mass times its acceleration (Newton's Second Law), then the total force on the object is zero as well. This means that the child exerts enough force to counteract kinetic friction, but no more. We must be talking about kinetic friction, because the toy is already moving. Thus, the child exerts a force of 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 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 ignore friction.)
Since we know the child's mass and acceleration, we can calculate the total force acting on the child.
F=ma
Total force = (mass) x (acceleration)
Total force = (20 kg) x (2.0 meters per sec2) = 40 Newtons
(Since we are ignoring friction, the only force involved is the force that the father exerts. Thus, the total force is equal to the father's force. Since the child is accelerating north, the father must be pushing with a force of 40 Newton's 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 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?
Since it takes more than 25 Newtons to get the object moving, the static frictional force is 25 Newtons east. Once it is moving, however, it accelerates at 0.1 meters per second2. This means the total force on the object is:
Total force = (mass) x (acceleration)
Total force = (15 kg) x (0.10 meters per sec2) = 1.5 Newtons
This force is the combination of the applied force (20 Newtons) and the kinetic frictional force (we know to use the kinetic frictional force because it is moving). Since the kinetic frictional force opposes motion, it is opposite of the applied force. Thus, the total force is the applied force minus the frictional force.
20 Newtons - kinetic frictional force = 1.5 Newtons
Thus, in order for the total force to be 1.5 Newtons, the kinetic frictional force must be 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 second2 to the south?
If the static frictional force is 700 Newtons, the worker must apply more than 700 Newtons of force to get the box moving. To accelerate the box once it is moving, the total force must be:
Total force = (mass) x (acceleration)
Total force = (500 kg) x (0.10 meters per sec2) = 50 Newtons
This total force is made up of the worker's force minus the kinetic frictional force. We were told the kinetic frictional force is 220 Newtons, so we can say:
Worker's force - 220 Newtons = 50 Newtons
The worker's force, then, must be 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 to exert a 45-Newton force to the east. What are the static and kinetic frictional forces between the rock and the ground?
Static friction keeps objects from moving. If the gardener had to exert slightly more than 100 Newtons of force to get the rock moving, the static frictional force is 100 Newtons. Once it got moving, the gardener keeps it moving at a constant velocity eastward. This tells us that the acceleration is zero, which means the total force on the rock is zero. Thus, the gardener applies enough force to overcome the kinetic frictional force, but no more. The kinetic frictional force,then, must be 45 Newtons to the west.
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 meters per second2 to the east. What is the kinetic frictional force between the rock and the ground?
The total force on the rock can be calculated from the mass and acceleration:
Total force = (mass) x (acceleration)
Total force = (710 kg) x (0.20 meters per sec2) = 142 Newtons
Now what is this force made of? Well, one man is pushing east with 156 Newtons, and the other is pushing east with 220 Newtons. Since those forces are in the same direction, they add. Friction is there as well, however, and it opposes the motion. Thus, it subtracts.
156 Newtons + 220 Newtons - kinetic frictional force = 142 Newtons
When we add 156 and 220, we get 376.
376 - kinetic frictional force = 142 Newtons
So, the kinetic frictional force is equal to whatever number leaves 142 when subtracted from 376. That's 234. Thus, the kinetic frictional force must be 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 Newton's 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 catches 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 Newton's 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 that 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?
The wall exerts a force of 20 Newtons west, because it is equal and opposite of the man's force.
