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Physics Review


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volleyball player has just served the ball. As the ball travels forward horizontally through the air and neglecting air resistance, the volleyball is experiencing
zero forward force; The ball is coasting forward because of its own inertia, not because anything is pushing it forward.
You have many balls to choose from, all with the same mass and size, but with different hardnesses and bouncinesses. To be most effective at dislodging the stuck Frisbee, the ball you use should be
The highest peak force will be exerted by a ball that transfers as much momentum as possible as quickly as possible. A lively ball will transfer more than all of its momentum to the frisbee and a hard ball will transfer whatever momentum it can transfer rather quickly. By combining hard and lively, you get the maximum effect -- a large peak force that can dislodge the frisbee.
Two identical toy rockets are launched simultaneously. One is launched with a rigid surface beneath its rocket engine and the other is launched with nothing but open air beneath its rocket engine. As the two rockets begin to rise upward at launch
they both accelerate upward at the same rate. It is the act of throwing the fuel downward that propels the rocket upward. What that fuel subsequently hits makes no difference to the rocket.
You are riding up a hill in a bus and you are at constant velocity. The force that the bus is exerting on you points
directly upward. Since you are traveling at constant velocity, you are experiencing zero net force. The two individual forces you are experiencing are your weight downward and a force from the bus directly upward.
A stiff metal bottle containing helium floats at the surface of pond. If you add additional helium to that bottle, leaving its temperature and volume unchanged, the bottle will float
lower in the water. If you add helium atoms to the bottle, you'll have added weight to the bottle and it will have to displace more water to support itself.
You have just refilled your aquarium with fresh water, but now you must add salt because you have salt-water fish. A piece of driftwood is floating on the surface. As you add salt and increase the water's density, the piece of driftwood
The driftwood no longer has to displace as much water in order for the buoyant force on it to balance its weight. So it floats higher in the salt water than it did in the fresh water.
As water flows steadily down the pipe from a tall water tower toward the city plumbing system, the water experiences
an increase in pressure but its total energy per liter remains constant. The descending water is transforming its gravitational potential enegy into pressure potential energy.
A child's sipping cup has a round bottom and automatically returns to upright whenever the child tips it. As the cup tips, you can be sure that its center of gravity
rises and its total potential energy increases. The cup is in stable equilibrium because any tip increases its center of gravity and therefore increases its gravitational potential energy. Since gravitational potential is its only potential energy, the sipping cup accelerates in the direction that reduces its gravitational potential energy as quickly as possible: back toward upright.
When a bowling ball hits a bowling pin (neglecting friction or air resistance), the quantities of motion that cannot change are
the total energy and the total momentum. Only total energy and total momentum are conserved quantities. If you start to limit your focus to things like kinetic energy or elastic potential energy, those quantities aren't conserved.
You fill a drinking straw with water and cover one end with your finger. If you hold the straw vertical with the finger-covered end on top, the water will remain in the straw. The pressure inside the straw just below your finger is then
less than atmospheric pressure. The water initially began to fall out of the straw but as it did, the pressure near your finger began to drop below atmospheric. A pressure difference developed: the air at the open bottom of the straw exerted atmospheric pressure on the lower surface of the water and the air in the sealed top of the straw exerted less than atmospheric pressure on the top surface of the water. When the water eventually settled in equilibrium, the pressure imbalance was exactly enough to support the water's weight.
You toss a gold coin straight up and catch it upon its return. Neglecting any effects due to the air, the coin accelerates
downward steadily on its way up and on its way down. The coin is a falling object and the only force it experiences is its weight downward. It accelerates downward the whole time.
While the limousine is stopped at a red light, you're floating motionless on the water. When the light turns green and the limousine begins to accelerate forward, you find yourself drifting toward the back of the pool because
your inertia keeps you motionless as the limousine moves forward around you. The limousine essentially drives out from under you.
You have two different balloons that are inflated with air, a rubber balloon and a Mylar balloon. The rubber one can expand if its pressure increases, but the Mylar one cannot increase its volume. If you leave them both in the hot sun, the net force on t
become more upward but the net force on the Mylar balloon will not change. The upward buoyant force on the rubber balloon increases as it occupies more volume, but the buoyant force on the rigid Mylar balloon never changes.
You are standing in line for the Stones concert, holding a heavy cooler motionless in your arms. As you continue to support this cooler, you are doing
zero work on it and it is doing zero work on you. You and the cooler aren't moving, so you can't be doing work on one another.
Combining business and pleasure, you are taking a balloon ride to the office. You hold your briefcase firmly in your hand as the balloon rises upward at constant velocity. During this ascent, you are doing
work on the briefcase and the balloon is doing work on you. You're pushing the briefcase upward and it's moving upward, so you're doing work on it. And the balloon is pushing you upward as you move upward, so it's doing work on you.
You trip while carrying eggs and one flies across the room and comes to a stop on your pillow without breaking. Had the egg come to a stop on your desk instead, it would have broken. By landing on the pillow, the egg transferred
all of its momentum over a longer time. The impulse the egg does on whatever it lands on is the same, but by prolonging the impulse, the pillow decreases the force involved.
You are pushing a stroller up a hill. There are two roads rising steadily to the hilltop, but one road is exactly twice as long as the other road. By choosing the longer road, rather than the shorter road, you
halve the force required to keep the stroller moving at constant velocity, while leaving the product of that force times the length of the road unchanged. You do the same work pushing the stroller up the hill, but you exert half the force for twice the distance.
You are helping take down the set for the Stones concert. As you drag a huge speaker across the stage at constant velocity, the net force on the speaker is
zero, even though the stage is exerting a backward frictional force on it. To move at constant velocity, the speaker must be experiencing zero net force and therefore not accelerating.
You are practicing tennis serves against a brick wall. Each time the tennis ball hits the rigid wall, the ball and wall transfer
momentum but not energy. The ball can transfer momentum to the wall simply by pushing on it for a time. But it can only transfer energy to a wall if that wall moves in the direction of its push.
An owl perches atop a tall flagpole that's fixed in concrete at its bottom. When the owl suddenly leaps into flight, the top of the flagpole is left swaying back and forth. The pole continues to sway for a long time because it has difficulty getting rid
energy. The flagpole can easily exchange momentum with its base, simply by exerting forces on it for time. But as long as the base doesn't move, the flagpole can't do work on it and can't get rid of energy.
As a pendulum swings back and forth, its total energy
remains constant, but its momentum varies. The pendulum keeps its energy because it can't do work on its support.
A dog is running away from you with a stick in her mouth. You reach out and pull the stick toward you with a force of 10 N. The stick responds by pulling you toward it with a force
If you pull the stick with a force of 10 N, the stick pulls you in the opposite direction with a force of 10 N.
A basketball player bounces the ball off the rigid backboard and it drops through the net. The ball moves slower after the bounce than before the bounce because the ball has
wasted some of its energy as thermal energy. The ball doesn't bounce perfectly, so it loses ordered energy. That loss reduces its speed following the rebound.
A child is swinging back and forth on a playground swing. At the moment she reaches the lowest point in her forward swing, she is accelerating
upward. She is traveling in a arc, so at the bottom of that arc she is still accelerating toward the center of the arc.
Two children slide into the air simultaneously at end of a waterslide. They are both traveling exactly horizontally at first, but one child is moving twice as fast as the other child. They fall toward the swimming below and
both hit the water simultaneously, although the faster child lands at twice the horizontal distance from the waterslide. Their horizontal motions don't affect their fall times and both hit the water simultaneously. But the faster child travels farther outward before hitting the water.
Which of the following is NOT accelerating?
Constant velocity means not accelerating.
You are standing at the end of a springboard at a swimming pool. You are bouncing up and down gently, though your feet never leave the board. As you bounce, your upward acceleration is greatest when you
reach the lowest altitude of each bounce and your upward speed is greatest when you coast through equilibrium on your way upward. The farther downward you deflect the springboard, the greater the upward restoring force it exerts on you and the greater your upward acceleration. You pick up speed as you accelerate upward toward equilibrium, then coast through equilibrium at your peak upwared speed, and finally begin to slow down as you rise beyond equilibrium.
You work as an elevator operator in a posh apartment building. You notice that you feel extremely heavy whenever the elevator starts moving
upward or stops moving downward. Any upward acceleration makes you feel heavy.
There is a fire near the top of a tall building and the firefighters are having trouble getting water to that fire. At present, their water will spray upward no higher than 50 m and will stop flowing from their hose once the end of that hose is above the
spray upward 100 m and will continuing flowing from the hose up to the 20th floor. By doubling the water's total energy, they allow that water to rise twice as high, whether it does so inside a hose or as an open spray.
You have two mallets of equal weight. The head of one mallet is made of firm rubber, while the head of the other mallet is made of hardened steel. You swing the two mallets at equal speeds and each one strikes a brick. The steel mallet is more likely to
momentum to the brick in a shorter time than does the rubber mallet. Faster momentum transfers involve larger impact forces. The steel mallet transfers its momentum faster and exerts a greater force for a shorter time than the rubber mallet.
You are a sculptor and have been asked to design a revolving door for the local art museum. As you work with the heavy bronze, you observe that moving a 5 kilogram ornament from the door's pivot to its outside edge increases the door's
rotational mass and makes the door harder to start turning. The door's mass doesn't change, but moving that mass toward the outside of the door increases its rotational mass (the measure of its rotational inertia) and makes it harder to cause the door to undergo angular acceleration.
You're cleaning a high window on your home by spraying water at that window from far below. As the water rises upward from your hose to the window, the water's velocity is
upward but its acceleration is downward. The water is in free fall, accelerating downward even as it is rising upward.
When a bowling ball first leaves your hand, it is skidding along the bowling lane without spinning. The lane's surface exerts a frictional force on the ball that
slows the ball down and also produces a torque that starts the ball spinning. The force of sliding friction pushes the bottom of the ball backward, causing it to accelerate backward and producing a torque that causes the ball to undergo angular acceleration.
As the bus you are riding in makes a sharp turn toward the left, you find yourself pressed against the person to your right. The net force you are experiencing as the bus turns left points
toward your left. You are accelerating toward your left, so the net force on you must be toward your left.
While waiting for a friend, you try to balance a ballpoint pen on its point. You occasionally get it to remain vertical and motionless for a second or so, but it inevitably falls over. When the pen is vertical and remains motionless for a second, it is
in an unstable equilibrium with a maximum total potential energy. The upright pen experiences zero net force and torque, but as soon as something disturbs it away from that equilibrium, its total potential energy decreases and its accelerates in the direction that decreases that total potential energy as quickly as possible.

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