Module 10 Flashcards
understand Module 10
Inertia
The tendency of an object to resist changes in its velocity
Friction
A force that opposes motion, resulting from the contact of two surfaces
Kinetic friction
Friction that opposes motion once the motion has already started
Static friction
Friction that opposes the initiation of motion
State Newton’s first law of motion.
An object in motion (or at rest) will tend to stay in motion (or at rest) until itis acted upon by an outside force.
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
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. When the boy drops the beanbag, it will still have 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 resistance, 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.
Suppose the situation with the boy with the bean baag is 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 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.
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.
When roads get wet, they can get slick. Obviously, then, the friction between a car’s tires and the
road decreases when the road is wet. Why?
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. Thus, the water fills in the grooves in the
road, reducing how close the tire molecules can get to the road molecules. 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 traveling 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 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. When the refrigerator 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 this toy and the floor is 15 Newtons, while the kinetic friction is 10 Newtons, what force is
the child exerting?
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.
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.
