PHYS exam 2 ch. 7 - ch. 17
If one person is pushing to the right on a 300-kg cart with a force of 200 N, and another person is pushing to the left on the same cart with a force of 100 N, what is the acceleration of the cart?
0.33 m/s/s a=F/m = 100/300
What is the magnitude of the gravitational force between Earth and a 1-kg body?
10 N
A gun with a muzzle velocity of 100 m/s is fired horizontally from a tower. Neglecting air resistance, how far downrange will the bullet be 1 second later?
100
On a playground slide, a child has potential energy that decreases by 1000 J while her kinetic energy increases by 900 J. What other form of energy is involved, and how much?
100 J of potential energy that doesn't go into increasing her kinetic energy goes into thermal.
A weight watcher who normally weighs 400 N stands on top of a very tall ladder so she is one Earth radius above the Earth's surface. How much would she weigh there?
100 N
Do 100 J of work in 50 s and your power output is
2 W.
Calculate the work done when a force of 1.1N moves a book 2.5m .
2.8J Work = Force x distance = 1.1*2.5
The work that is done when twice the load is lifted twice the distance is
4x as much
A stone is thrown horizontally from the top of a cliff. One second after it has left your hand, its vertical distance below the top of the cliff is
5 m
What is the force of gravity on a 500-newton woman standing on the Earth's surface
500 N
Since the Moon is gravitationally attracted to Earth, why doesn't it simply crash into Earth?
The Moon's tangential velocity is what keeps the Moon coasting around the Earth rather than crashing into it. If its tangential velocity were reduced to zero, then it would fall straight into the Earth!
Calculate the force of gravity between a newborn baby (mass = 3.1kg ) and the planet Mars (mass = 6.4×10^23kg) when Mars is at its closest to Earth (distance = 5.6×10^10m).
F= (Gm1m2)/d^2 4.2×10^−8 N
A circularly moving object requires a centripetal force. What supplies this force for satellites that orbit Earth?
Gravity
Suppose you roll a ball off a tabletop. Will the time to hit the floor depend on the speed of the ball? (Will a fast ball take a longer time to hit the floor?) Defend your answer.
No, vertical motion is independent of horizontal motion.
What prevents satellites such as the space shuttle from falling?
Nothing; they are continually falling as they orbit the Earth
Calculate the power expended when a 30-N force pushes a cart 4.0m in a time of 0.46s .
P = 260W Power = Work/time = (30N*4.0m)/.46s
Calculate the change in potential energy of 8.4×106 kg of water dropping 50 m over Niagara Falls.
PE = 4200 MJ PE = mgh
How many joules of potential energy does a 1.4-kg book gain when it is elevated 4.2m? When it is elevated 8.4m?
PE = 59 J PE = 120 J PE = mgh
An apple falls because of the gravitational attraction to Earth. How does the gravitational attraction of Earth to the apple compare? (Apply Newon's 3rd law)
The Earth's gravitational force is pulling on the apple and the apple is pushing on the Earth's gravitational force.
Why do the astronauts in the Space Shuttle float around?
The Space Shuttle is in free fall, so the shuttle and the astronauts inside are continuously falling towards the Earth. They thus experience apparent weightlessness.
What is the ultimate source of energy for fossil fuels, dams, and windmills?
The Sun
Object A has 3x as the mass of object B. Identical forces are exerted on the two objects. Which statement is true?
The acceleration of object B is three times that of object A. a = F/m
The second floor of a house is 6 m above the street level. How much work is required to lift a 260-kg refrigerator to the second-story level?
W = 1.6x10^4 J
Is gravitational force acting on a person who falls off a cliff?
Yes
Is gravitational force acting on an astronaut inside an orbiting space shuttle?
Yes
If your friend pushes a lawnmower 4 times as far as you do while exerting only half the force, which one of you does more work? How much more?
Your friend. Twice as much.
What do we call the gravitational force between Earth and your body?
Your weight
When the distance between two stars decreases by half, the force between them
increases to four times as much.
The horizontal and vertical components of velocity for a projectile are
independent of each other
How does the force of gravity between two bodies change when the [distance] between them is doubled?
it decreases by a quarter.
Predict how the time of flight would change if you increased the angle of the projectile, keeping the initial speed constant.
it increases
An object that has kinetic energy must be
moving
One's weightlessness in space has most to do with
no support force
If you push for a half hour or a whole hour against a stationary wall
no work on the wall is done in either case.
Imagine that Earth had an identical twin planet, "Farth", which is 2x as far away from the sun as Earth is. Compare to the force of gravity that Exerts on the Sun, how strong is the force of gravity that Farth exerts on the Sun
one-fourth as strong
The gravitational field on the surface of the earth is stronger than that on the surface of the moon. If a rock is transported from the moon to the earth, which properties of the rock will change?
only its weight, never the mass
Which is most responsible for the ocean tides?
the moon
When one does twice the work in twice the time, the power expended is
the same
After a rock thrown straight up reaches the top of its path and then falls a short distance, its acceleration is (neglect air resistance)
the same as it was at the top of its path
Which ball, dropped/thrown horizontally, (if either) has the greatest speed at the moment of impact?
thrown horizontally The two balls have the same vertical velocity when they land, but the thrown ball has an additional horizontal velocity component. Since speed is defined as the magnitude of the resultant velocity vector, the thrown ball is moving faster when it lands.
If you push an object twice as far while applying the same force, you do
twice as much work.
If an object is raised twice as high, its potential energy will be
twice as much.
Emily holds a banana of mass m over the edge of a bridge of height h. She drops the banana and it falls to the river below. Use conservation of energy to find the speed of the banana just before hitting the water.
v = square root of 2gh **look in ch. 7 notes
Running a car's air conditioner usually increases fuel consumption. But, at certain speeds, a car with its windows open and with the air conditioner turned off can consume more fuel. Explain.
A car with windows open experiences more air drag, which causes more fuel to be burned in maintaining motion. This may more than offset the savings from turning off the air conditioner.
If the Moon pulls Earth as strongly as Earth pulls the Moon, why doesn't Earth rotate around the Moon, or why don't both rotate around a point midway between them?
Because the moon has a smaller mass than Earth and they have to rotate at a central point.
In the absence of air resistance, why does the horizontal component of a projectile's motion not change, while the vertical component does?
Because there is no acting force on the horizontal component.
The crew of a cargo plane wishes to drop a crate of supplies on a target below. To hit the target, when should the crew drop the crate? Ignore air resistance.
Before the plane is directly over the target At the moment it is released, the crate shares the plane's horizontal velocity. In the absence of air resistance, the crate would remain directly below the plane as it fell.
A very massive object A and a less massive object B move toward each other under the influence of gravitation. Which force, if either, is greater?
Both forces are the same.
What happens to the force of attraction between two planets when the [distance] between them is doubled?
Decreases to one quarter (distance = decrease)
An object is placed exactly halfway between the Earth and moon. The object will fall toward the
Earth
To combat wasteful habits, we often speak of "conserving energy," by which we mean turning off lights and hot water when they are not being used and keeping thermostats at a moderate level. In this chapter, we also speak of "energy conservation." Distinguish between these two usages.
Energy conservation in physics term means that energy is never created nor destroyed. It is simply transformed/transferred into another form of energy
Calculate the force of gravity between a newborn baby of mass 2.9kg and the obstetrician of mass 100 kg, who is 0.56m from the baby. Which exerts more gravitational force on the baby, Mars or the obstetrician?
F = 6.2×10^−8 N The obstetrician
A physics instructor demonstrates energy conservation by releasing a heavy pendulum bob, as shown in the sketch, allowing it to swing to and fro. What would happen if, in his exuberance, he gave the bob a slight shove as it left his nose? Explain.
If the ball is given an initial KE, it will return to its starting points with that KE and hit the instructor.
Why does the gravity at the Space Shuttle compare with the gravity on Earth the way it does?
In orbit, the Space Shuttle is about the same distance from the center of the Earth as it was when it was on the surface of the Earth
Predict how the [VERTICAL COMPONENT] of the velocity will change with time after the projectile is fired.
It first decreases to zero and then increases in the opposite direction.
A stone is thrown upward at an angle. What happens to the horizontal component of its velocity as it rises? As it falls?
It remains the same if there's no force acting on it.
Predict how the [HORIZONTAL COMPONENT] of the velocity will change with time after the projectile is fired.
It stays constant.
In the absence of air resistance, a ball thrown vertically upward with a certain initial KE will return to its original level with the same KE. When air resistance is a factor affecting the ball, will it return to its original level with the same, less, or more KE? Does your answer contradict the law of energy conservati
It will have less KE and no because energy will be transferred elsewhere i.e. thermal energy
A car is raised a certain distance in a service-station lift and therefore has potential energy relative to the floor. If it were raised twice as high, how much potential energy would it have relative to the floor?
It would have twice as much potential energy
What would be the path of the Moon if somehow all gravitational forces on it vanished to zero?
It would move in a straight line path with its speed remaining constant
Calculate the kinetic energy of a 2.0-kg toy cart that moves at 3.8m/s .
KE = 14J KE = 1/2mv^2
Belly-flop Bernie dives from atop a tall flagpole into a swimming pool below. His potential energy at the top is 9000J (relative to the surface of the pool). What is his kinetic energy when his potential energy reduces to 1000 J?
KE = 8000 J
At what point in its motion is the KE of a pendulum bob at a maximum? At what point is its PE at a maximum? When its KE is at half its maximum value, how much PE does it have relative to its PE at the center of the swing?
KE max = at the bottom or where it moves the fastest PE max = at the ends when KE or V = 0 When KE is half way it is equal to PE.
Consider a ball thrown straight up in the air. At what position is its kinetic energy at a maximum? Where is its gravitational potential energy at a maximum?
KE max = point of release PE max = at the top of ball's trajectory
Somewhere between Earth and the Mon, gravity from these two bodies on a space pod would cancel. Is this location nearer Earth or the Moon?
Moon because it has a smaller mass and lesser pull at equal distance
A heavy crate accidentally falls from a high-flying airplane just as it flies directly above a shiny red Porsche smartly parked in a car lot. Relative to the Porsche, where will the crate crash?
The crate will fall past the Porsche.
When a driver applies brakes to keep a car going downhill at constant speed and constant kinetic energy, the potential energy of the car decreases. Where does this energy go? Where does most of it go with a hybrid vehicle?
The energy will change into thermal energy from the friction of the brakes and the street. In a hybrid vehicle, energy will be reserved to charge the battery of a car.
Why does the vertical component of velocity for a projectile change with time, whereas the horizontal component of velocity doesn't?
The force of gravity is acting on the vertical component of an object. There's no acting force on the horizontal component
Why would a bowling ball and a small marble fall down to the surface of the Moon at the same rate?
The force of gravity is proportional to the object's mass The object's acceleration is equal to the force of gravity divided by its mass, and the force of gravity is proportional to the mass; therefore, the bowling ball and the marble fall with the same acceleration.
What happens to the force of attraction between two planets when the [masses] of both are doubled?
The force quadruples (mass = increase)
How does the gravity in the Space Shuttle compare with the gravity on Earth's surface?
The gravity in the Space Shuttle is appx. equal to the gravity on the surface of the Earth.
Two cars are raised to the same elevation on service-station lifts. If one car is twice as massive as the other, how do their gains of potential energies compare?
The heavier (more massive) one will have greater gravitational potential energy.
Why is it easier to stop a lightly loaded truck than a heavier one that has equal speed?
The lightly loaded truck has less kinetic energy and will require less work
A satellite can orbit at 5 km above the Moon, but not at 5 km above Earth. Why?
The moon does not have an atmosphere to slow the satellite down.
In what sense does the Moon "fall"?
The moon falls away from the straight line it would follow if there were no forces acting on it.
An apple hanging from a limb has potential energy because of its height. If it falls, what becomes of this energy just before it hits the ground? When it hits the ground?
The potential energy turns into kinetic energy as it falls and then thermal/sound energy when it hits the ground.
If you stood on a planet having a mass four times that of Earth, and a radius two times that of Earth, how much would you weigh on the planet?
The same Weight is inversely proportional to the square of the planet's radius. Therefore, even though the planet is 4x as massive, the fact that the planet has twice the radius exactly offsets the effect of the higher mass.
A parachutist is falling toward the ground. The downward force of gravity is exactly equal to the upward force of air resistance. Which statement is true?
The velocity of the parachutist is not changing with time. (Since the net force on the parachutist is not zero, the parachutist is not accelerating)
Two people who weigh the same climb a flight of stairs. The first person climbs the stairs in 30 s, and the second person climbs them in 40 s. Which person does more work? Which uses more power?
The work amount would be the same, but the one who climbs 30s will use more power because the time is shorter.
With what force does a rock that weighs 10 N strike the ground if dropped from a rest position 10 m high? In fact, the question cannot be answered unless you know more. Why?
The work that the rock does on the ground is equal to its PE before being dropped, mgh = 100 joules. The force of impact, however, depends on the distance that the rock penetrates into the ground. If we do not know this distance we cannot calculate the force. (If we knew the time during which the impulse occurs we could calculate the force from the impulse-momentum relationship—but not knowing the distance or time of the rock's penetration into the ground, we cannot calculate the force.)
Which requires more work - lifting a 50-kg sack a vertical distance of 2 m or lifting a 25-kg sack a vertical distance of 4 m?
They both have the same amount of work.
You're in an airplane that flies horizontally with speed 1000km/h (280m/s ) when an engine falls off. Neglecting air resistance, assume it takes 31s for the engine to hit the ground. a) Find height b) Find horizontal distance that the aircraft engine falls c) If the airplane somehow continues to fly as if nothing had happened, where is the engine relative to the airplane at the moment the engine hits the ground?
a) H = 4.8 m b) d = 8.7 km c) S = 4.8 m Prob. 10.2 copy
The amount of gravitational force that acts on the space shuttle while in orbit is
almost as much as the shuttle's weight on the Earth's surface.
No force due to Earth's gravity is acting on the astronauts inside the orbiting space shuttle. This statement is
always false
A bullet fired horizontally from a rifle begins to fall
as soon as it leaves the barrel
Earth satellites are typically more than 100 km high so as to be above the Earth's
atmosphere
Energy cannot be
created nor destroyed
Quantities that represent mass
grams and kilograms
A "weightless" astronaut in an orbiting shuttle is
like the shuttle, pulled by Earth's gravitation.
An object may have potential energy because of its
location
The constant G in Newton's equation
makes the units of measurement consistent
A kilogram is a measure of an object's
mass
Each of us weighs a tiny bit less inside the ground floor of a skyscraper than we do on the ground away from the skyscraper. The reason for this is the
mass of the building attracts us upward slightly
An object is lifted from the surface of a planet to an altitude equal to the radius of the planet (such that is distance from the center of the Earth is now doubled). As a result, which of the following changes in the properties of the object takes place?
mass remains the same; weight decreases
The reason the moon does not crash into the Earth is that the
moon has a sufficient tangential speed.
Quantities that represent weight
pounds (lb), kilonewtons (kN), newtons (N)
The force of gravity acts on all apples on an apple tree. Some apples are twice as far from the ground as others. These twice-as-high apples, for the same mass, have
practically the same weight
Suppose the Sun suddenly shrunk, reducing its radius by half (but its mass remaining the same). The force of gravity exerted on the Earth by the Sun would
remain the same Since the distance between the Sun's center and the Earth's center doesn't change when the Sun shrinks, the force of gravity doesn't change
Roll a bowling ball off the edge of a table. As it falls, its horizontal component of velocity
remains constant.
Prob. 10.4
rework
