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Gen Physics 1 Exam 2
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Gravity
Terms in this set (113)
You slam on the brakes of your car in a panic, and skid a certain distance on a straight, level road. If you had been traveling twice as fast, what distance would the car have skidded, under the same conditions
It would have skidded 4 times farther
Why the raindrops fall a constant speed during the later stages of their descent?
Air resistance just balances the force of gravity
A golf club hits a golf ball with a force of 2400 N. The golf ball hits the club with a force?
Exactly 2400 N.
A ball drops some distance and loses 30 J of gravitational potential energy. Do not ignore air resistance. How much kinetic energy did the ball gain?
Less than 30 J
Because Earth's orbit is slightly elliptical, Earth actually gets closer to the Sun during part of the year. When Earth is closer to the Sun its orbital speed
Greater than when Earth is farthest away from the Sun.
A lightweight object and a very heavy object are sliding with equal speeds along a level frictionless surface. They both slide up the same frictionless hill. Which rises to a greater height?
They both slid to the same height.
You are standing in a moving bus at constant speed, facing forward. You suddenly move forward as the bus comes to an immediate stop. What force caused you to move forward?
No forces were responsible for your fall.
An object of mass m sits on a flat table. The earth pulls on this object with force mg, which we will call the action force. What is the reaction force?
The object pulling upward on the Earth with force mg.
Two men, Joel and Jerry, push against a wall. Jerry stops after 10 min, while Joel is able to push for 5 min longer. Compare the work they do.
Neither of them do any work
Your bat hits the ball pitched to you with a 1500 N instantaneous force. The ball hits the bat with an instantaneous force, whose magnitude is
Exactly equal to 1500 N
A ball drops some distance and gains 30 J of Kinetic energy. Do not ignore air resistance. How much gravitational potential energy did the ball lose?
More than 30 J
As a car drives with its tires rolling freely without any slippage, the type of friction acting between the tires and the road is
Static Friction
Because Earth's orbit is slightly elliptical, Earth actually gets farther away from the Sun during part of the year. When Earth is farther from the Sun its orbital speed is
Less than when Earth is closest to the Sun.
Swimmers at a water park have a choice of two frictionless water slide. Although both slides drop over the same height, h, slide 1 is straight while slide 2 is curved, dropping quickly at first and then leveling out. How does the speed v1 of a swimmer reaching the end of slide 1 compares with v2, the speed of a swimmer reaching the end of slide 2?
v1=v2
A ping pong ball moving east at a speed of 4 m/s, collides with a stationary bowling ball. The Ping-Pong ball bounces back to the west, and the bowling ball moves very slowly to the east. Which object experiences the greater magnitude impulse during the collision?
Neither, both experienced the same magnitude impulse
A lightweight object and a very heavy object are sliding with equal speed along a level frictionless surface. They both slide up the same frictionless hill. Which rises to a greater height?
They both slide to the same height.
When a parachutist jumps from an airplane, he eventually reaches a constant speed, called the terminal velocity. This means that.
The force of air resistance is equal to the weight of the parachutist.
A truck is traveling horizontally to the right. When the truck starts to slow down, the crate on the frictionless truck bed starts to slide. In what direction could the net force be on the crate?
The crate does not accelerate up or down, so the net force cannot be vertical. The truck bed is frictionless and the crate is not in contact with any other surface, so there are no horizontal forces. Therefore, no net force acts on the crate. As the truck slows down, the crate continues to move forward at constant speed.
You are trying to push your stalled car. Although you apply a horizontal force of 400 N to the car, it doesn't budge, and neither do you. Which forces must also have a magnitude of 400 N?
You exert a force of 400 N on the car; by Newton's third law the force exerted by the car on you also has a magnitude of 400 N. Since the car doesn't move and the only horizontal forces acting on the car are your pushing and the force of friction on the car from the road, Newton's second law requires these forces to have equal magnitudes (400 N) in the opposite direction. Since the road exerts a force of 400 N on the car by friction, Newton's third law requires that the friction force on the road from the car must also be 400 N.The car is exerting a 400-N horizontal force on you, and since you are not accelerating, and the only horizontal forces acting on you are the force from the car and the frictional force from the ground, Newton's second law requires that the ground must be exerting an equal and opposite horizontal force. Therefore, the magnitude of the friction force exerted on you by the road is 400 N.
The force of gravity on a 2-kg rock is twice as great as that on a 1-kg rock. Why then doesn't the heavier rock fall faster?
The acceleration of both rocks is found by dividing their weight (the force of gravity on them) by their mass. The 2-kg rock has a force of gravity on it that is twice as great as the force of gravity on the 1-kg rock, but also twice as great a mass as the 1-kg rock, so the acceleration is the same for both.
If a man is able to a move truck it's because
The ground exerts a greater friction force on Matt than it does on the truck.
If man is able to paddle a boat its because
The force the water exerts on the paddle.
Suppose an object is accelerated by a force of 100 N. Suddenly a second force of 100 N in the opposite direction is exerted on the object, so that the forces the cancel. The object
Continues at the velocity it had before the second force was applied.
A golf ball is hit with a golf club. While the ball flies through the air, which forces act on the ball? Neglect air resistance.
The force of gravity acting on the ball.
In outer space, a bowling ball and a Ping-Pong ball attract each other due to gravitational forces. How do the magnitudes of these attractive forces compare?
The forces are equal.
When a skier skis down a hill, the normal force exerted on the skier by the hill is
Less than the weight of the skier.
You kick a smooth flat stone out on a frozen pond. The stone slides, slows down, and eventually stops. You conclude that:
A net force acted on it all along.
In outer space, gravitational forces exerted by a bowling ball and a Ping-Pong ball on each other are equal and opposite. How do their accelerations compare?
Acclerations are opposite, but bigger for the Ping-Pong ball
You put your book on the bus seat next to you. When the bus suddenly, the book slides forward off the seat. Why?
No net force acted on it.
A hockey puck slides on ice at a constant velocity. What is the net force acting on the puck?
Zero
A small car collides with a large truck. Which experiences the greater impact force?
Both the same
In the collision between the car and the truck, which has the greater acceleration?
The car
Consider a cart on a horizontal frictionless table. Once the cart has been given a push and released, what will happen to the cart?
Continue with constant velocity
We just decided that the cart continues with constant velocity. What would have to e done in order to have the cart continue with constant acceleration?
Push the cart continuously
Ryan walked to a cliff and dropped a stone. Neglecting any effects due to the air as it falls vertically, which one of the following is the reaction force to the Earth's gravity on the stone?
The gravitational force of the stone on the Earth.
You are pushing a heavy box across a rough floor. When you are initially pushing the box and it is accelerating?
The force you exert on the box is equal to the force of the box pushing back on you.
A 50-N crate sits on a horizontal floor where the coefficient of static friction between the crate and the floor is .50. A 20-N force is applied to the crate acting to the right. What is the resulting static friction force acting on the crate?
20 N to the left
While driving fast around a sharp right turn, you find yourself pressing against the car door. What is happening?
The door is exerting a rightward force on you.
Which of the following point toward the center of the circle in uniform circular motion?
Acceleration, net force.
A car drives at steady speed around a perfectly circular track
Both the acceleration and net force on the car point inward.
A child whirls a ball in a vertical circle. Assuming the speed of the ball is constant (an approximation), when would the tension in the cord connected to the ball be greatest?
At the top of the circle
The Moon does not crash into the Earth because
It is freely falling, but it has a high tangential velocity.
A car accelerates from rest to 30 km/h. Later, on a highway, it accelerates from 30 km/h to 60 km/h. Which takes more energy, going from 0 to 30, or from 30 to 60?
30 to 60 km/h
A ball is thrown straight up. At what point does the ball have the most energy? Ignore air resistance.
Everywhere; the energy of the ball is the same at all of these points.
When the speed of your car is doubled, by what factor does its kinetic energy increase?
4
A car traveling at a velocity v can stop in a minimum distance d. What would be the car's minimum stopping distance if it were traveling at a velocity of 2v?
4d
A bowling ball is dropped from a height h onto the center of a trampoline, which launches the ball back up into the air. How high will the ball rise?
No more than h—probably a little less.
Two paths lead to the top of a big hill. One is steep and direct, while the other is twice as long but less steep. How much more potential energy would you gain if you take the longer path?
As long as we ignore friction, the potential energy gain to the top of the hill is path independent due to the conservative gravity force.
In a baseball game, the catcher stops a 90 mph pitch. What can you say about the Work done by the catcher on the ball?
Catcher has done positive work
Car #1 has twice the mass of car #2, but they both have the same kinetic energy. How do their speeds compare?
2v1=v2
A truck, initially at rest, rolls down a frictionless hill and attains a speed of 20 m/s at the bottom. To achieve a speed of 40 m/s at the bottom, how many times higher must the hill be?
Four times the height
You throw a ball straight up into the air. In addition to gravity, the ball feels a force due to air resistance. Compared to the time it takes the ball to go up, the time it takes to come back down is:
Greater
A man pushes a block up an incline at a constant speed. As the block moves up the incline.
The kinetic energy depends on the speed and not the position of the block. Since the block moves with constant speed, the kinetic energy remains constant. As the block moves up the incline its elevation increases, so its potential energy also increases.
You push a heavy crate down a ramp at a constant velocity. Only four forces act on the crate. Which force does the greatest magnitude of work on the crate?
The speed of the crate is constant, so the net (total) work done on the crate is zero.
The normal force is perpendicular to the direction of motion, so it does no work. Your applied force and a component of gravity are in the direction of motion, so both do
positive work. The force of friction opposes the direction of motion and does negative
work. For the total work to be zero, the work done by friction must equal the sum of the
work done by gravity and by you.
A block is pulled at constant speed along a rough level surface by a
rope that makes an angle θ with respect to the horizontal. The applied force along the rope is F. The force of kinetic friction between the block and the surface is f. Which one of the following actions will increase the frictional force on the block?
Increasing the weight of the block
Which one of the following has the largest terminal
velocity?
a shot put ball
A light ping-pong ball and a heavy rubber ball of exactly the same size are each launched at the same angle and initial velocity, but the rubber ball goes
much farther than the ping-pong ball. Which one of the following statements best explains this result?
The ping-pong ball has less mass, so the same drag force slows the ping-pong ball down more.
You're on a Ferris wheel moving in a vertical circle. When the Ferris wheel is at rest, the normal force N exerted by your seat is equal to your weight mg. How does N change at the top of the Ferris wheel when you are in motion?
N is smaller than mg
You are a passenger in a car, not wearing a seat belt. The car makes a sharp left turn. From your perspective in the car, what do you feel is happening to you?
You are thrown to the right
During that sharp left turn, you found yourself hitting the passenger door. What is the correct description of what is actually happening?
The door is exerting a leftward force on you.
You drive your dad's car too fast around a curve and the car starts to skid. What is the correct description of this situation?
Friction between tires and road is not strong enough to keep car in a circle
Two equal-mass rocks tied to strings are whirled in horizontal circles. The radius of circle 2 is twice that of circle 1. If the period of motion is the same for both rocks, what is the tension in cord 2 compared to cord 1?
T2=2T
Which is stronger, Earth's pull on the moon, or the moon's pull on earth?
They pull on each other equally.
If the distance to the Moon were doubled, then the force of attraction between Earth and the Moon would be:
One quarter
You weigh yourself on a scale inside an airplane that is flying with constant speed at an altitude of 20,000 feet. How does your measured weight in the airplane compare with your weight as measured on the surface of the Earth?
Less than
Two satellites A and B of the same mass are going around Earth in concentric orbits. The distance of satellite B from Earth's center is twice that of satellite A. What is the ratio of the centripetal force acting on B compared to that acting on A?
1/4
Newtonian Mechanics
Velocity of an object can change when the object is acted on by one or more force(pushes or pulls) from other objects. Newtonian mechanics relates accelerations and forces.
Force
Pushes or Pulls
Net Force
The vector sum of all the forces acting on the body.
Newtons First Law
An object will remain at rest until acted upon a force. An object will stay in motion unless acted upon by a force.
News Second Law
Fnet=ma, Fnetx=max, Fnety=may, Fnetz=maz
Newtons Third Law
For every action there is an equal and opposite re-action.
Inertial Reference Frames
Reference frames in which Newtonian mechanics hold.
Noninertial Reference Frames
Reference frames in which Newtonian mechanics don't hold.
Mass
The characteristic of that body that relates the body's acceleration to the net force causing the acceleration.
Free body diagram
The diagram shows what forces are acting on a body.
Gravitational Force
Fg=mg. A pull by another body, usually it's earth.
Weight
W=mg. W of a body is the magnitude of the upward force needed to balance the gravitational force on the body.
Normal Force
The force on a body from a surface against which the body presses.
Frictional Force
The force on a body when the body slides or attempts to slide along a force. On a frictionless surface, the frictional force is negligible.
Tension
Describes the pulling force exerted by each end of a string, cable, chain, or similar one-dimensional continuous object, or by each end of a rod, truss member, or similar three dimensional object. For a massless cord the pulls at each end of the cord have the same T.
Friction
When a force tends to slide a body along a surface, a frictional force is parallel to the surface and directed so as to oppose the sliding.
Static Friction (Fs)
If the body does not slide. then the frictional force is static.
Kinetic Friction (Fk)
If there is sliding, the frictional force is kinetic.
Drag Force
When there is relative motion between air and a body.
Uniform Circular Motion
If a particle moves in a circle or a circular arc of radius R.
Effective cross-sectional area
the area of a cross section taken perpendicular to the relative velocity.
Kinetic Energy
The Kinetic energy K associated with the motion of a particle of mass m and speed v, where v is well below the speed of light. K=.5(mv^2)
A force is required to keep an object moving in a straight line
False
An object receiving two equal and opposite vertical forces must be at rest
False
An objects Acceleration is proportional to the forces acting on it.
True
A book is lying at rest on a table. The book will remain there at rest because
There is no net force on the book.
Forces always come in pairs
True
A force F acts on mass M for a time interval T, giving it a final speed v. If the same force acts for the same time on a different mass 2m, where would be the final speed of the bigger mass.
1/2 v
When you climb up a rope, the first thing you do is pull down on the rope. How do you manage to go up the rope by doing that?
The rope actually pulls on you
You tie a rope to a tree and you pull on the rope with a force of 100 N. what is the tension in the rope?
100 N
Two Tug-of-war opponents each pull with a force of 100 N on opposite ends of a rope. What is the tension in the rope?
100 N
What would happen if you were to accelerate up in an elevator?
The scale's force would get bigger.
What would happen if you were to accelerate down in an elevator?
The scale's force would get smaller.
What can you say about the force of gravity Fg acting on a stone and a feather?
Fg is greater on the stone.
An astronaut on Earth kicks a bowling ball and hurts his foot. A year later, the same astronaut kicks a bowling ball on the moon with the same force. His foot hurts
The same
Your little sister wants you to give her a ride on her sled. On level ground, what is the easiest way to accomplish this?
Pulling her from the front.
A box sits on a flat board. You lift one end of the board, making an angle withthe foor. As you increase the angle, the box will eventually begin to slide down why
Component of the gravity force parallel to the plane incresed. Normal Force exerted by the board decreased.
Consider two people, 1 meter apart from each other. The gravitational force exerted on person a by person b is
very small
Consider two people, 1 meter apart from each other. The gravitational force exerted on person b by person a is
smaller than the force on person a by person b
If you weight yourself at the equator of Earth, would you get a bigger, smaller, or similar value if you weigh yourself at one of the poles
smaller value
What is cosmology?
Science of the universe
A box is being pulled across a rough floor at a constant speed. What can you say about the work done by friction?
Friction does negative work.
You lift a book with your hand in such a way that it moves up a constant speed. While it is moving, what is the total work done with the book?
Zero
Mike applied 10 N of force over 3 m in 10 secs. Joe applied the same force over the same distance in 1 minute. Who did more work?
Both did the same work
Mike performed 5 J of work in 10 secs. Joe did 3 J of work in 5 secs. Who produced the greater power.
JOe produced more power.
Engine 1 produces twice the power of engine 2. Can we conclude that engine 1 does twice as much work as engine 2?
No
A box sliding on a frictionless flat surface runs into a fixed spring, which compresses a distance x to stop the box. If the initial speed of the box were doubled, how much would the spring compress in this case?
Twice as much
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