PHYSICS EXAM 2
Angular Momentum = mvr =(50)(3)(2)= 300 m^2/s
50-kg person sits 3.0 m from the center of a horizontally rotating platform with a tangential speed of 2.0 m/s. What is the angular momentum of the person?
(mv)before=(mv)after
A 2.0-kg blob of play dough moving at 5 m/s slams into a 2.0-kg blob of play dough at rest. Find the speed of the two stuck-together blobs immediately after colliding.
W= mgh =(0.0025)(9.8)(382)= 9.3345 J
A 2.5 g penny is dropped from the top of the Empire State Building, falling 381 m to the sidewalk below. If the penny experiences almost no air resistance as it falls (very unrealistic!), find its kinetic energy just before striking the sidewalk.
=(6)(1.4)= 8.4 m/s
A 6.0-kg fish swimming 1.4 m/s swallows an absentminded 1.0-kg fish swimming toward it at a speed that brings both fish to a halt immediately after lunch. Find the speed of the approaching smaller fish before lunch.
you
A ball is thrown horizontally from a cliff at a speed of 10 m/s. You predict that its speed 1 ss later will be slightly greater than 14 m/s . Your friend says it will be 10 m/s. Who is correct?
F=mv/t F=(80)(28)/(0.1)= 22,400 N
A car carrying a 80-kg crash-test dummy drives into a wall at 28 m/s and is brought to rest in 0.1 s. What is the average force exerted by the seat belt on the dummy?
Immediately before hitting the ground the peach's energy is kinetic energy; when it hits the ground, its energy becomes thermal energy.
A peach 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?
75 degrees
A projectile is launched upward at an angle of 15∘ above the horizontal and strikes the ground a certain distance downrange. What other launching angle at the same speed produces the same distance?
The velocity of the airtanker will increase. (In order to keep momentum constant, with the decrease of mass, there is an increase of velocity.)
An airtanker fighting a forest fire drops flame retardant when flying at a constant horizontal velocity. What will happen to the velocity of the airtanker?
increasing the force or increasing the time interval.
An impulse can be increased by
Both gained the same potential energy
An old man and his grandchild, both of the same mass, go up the stairs to their first-floor apartment. The child gets to the apartment well before his grandad. Which one gained more potential energy?
Greatest nearest Earth; least furthest from Earth
At what part of an elliptical orbit does an Earth satellite have the greatest speed? The least speed?
The difference in lunar pulls is greater than the difference in solar pulls.
Both the Moon and the Sun produce our ocean tides, but the tides are primarily due to the Moon. Why?
KE= 1/2mv^2 =(1/2)(108)(12)^2= 7800 J
Calculate the kinetic energy of a 108-kg scooter moving at 12 m/s.
Centripetal Force= (mv^2)/r =((2.4)(1.5 m/s)^2)/2.4= 3.375 N
Calculate the tension in a horizontal string that whirls a 2.4 kg toy in a circle of radius 1.6 m when it moves at 1.5 m/s on an icy surface.
Torque= Lever Arm x Force = (56)(0.25)= 14 N*m
Calculate the torque produced by a 56 N perpendicular force at the end of a 0.25 m-long wrench.
3 rotations per second
Calculate the torque produced by a 56-NN perpendicular force at the end of a 0.25-mm-long wrench.
Torque= Lever Arm x Force =(58)(0.22)= 13 N*m
Calculate the torque produced by a 58-N perpendicular force at the end of a 0.22-m-long wrench.
W=Fd W= (12)(1.5)= 18 J
Calculate the work done when a force of 12 N moves a book 1.5 m. (1 N⋅m = 1 J)
her hand is made to bounce from the bricks (A large impulse in a short time produces a considerable force.)
Cassy can get more force on the bricks she breaks with a blow of her bare hand when _______.
It takes four times the work and four times the stopping distance.
Compared with some original speed, how much work must the brakes supply to stop a car that is moving twice as fast? How will the stopping distance compare?
v= root of 2gh
Emily drops a banana of mass m over the edge of a bridge of height h and it falls to the river below. From the initial PE of the banana converting to KE, find the speed of the banana just before hitting the water.
destroyed
Energy cannot be _______.
11.2 km/s
Escape speed from Earth is any speed equal to or greater than __________.
impulse on the ball (The greater the impulse exerted on something, the greater will be its change in momentum.)
Follow through is important in golf because it results in a greater _______.
Speed, gravitational force, and distance from Earth
For an Earth satellite in an elliptical orbit, list all the values that change.
the Sun
Fossil fuels, hydroelectric power, and wind power ultimately get their energy from _______.
It falls away from the straight path that it would follow if there were no forces acting on it.
How does the Moon "fall" without getting closer to us?
It decreases.
How does the force needed to turn the wrench change if you increase the lever arm?
The force quadruples.
How does the force of gravity between two bodies change when the distance between them is halved?
It decreases.
How does the lever arm change if you decrease the angle of the force?
It decreases.
How does the lever arm change when you decrease the distance to the nut?
The rotational inertia of the rod with the mass closer to the top is greater than the rotational inertia of the rod with the mass closer to the bottom.
How does the rotational inertia of the rod with the mass toward the bottom compare with the rotational inertia of the mass toward the top?
Impulse = change in momentum: Ft=Δmv =(40)(3.2)= 128 N*s
How much impulse stops a 40-kg carton sliding at 3.2 m/s when it meets a rough surface?
The same (Double the force times half of the distance gives the same work.)
How much work is done if double the force is applied for half the displacement?
W= Force*distance W= (270)(10)(6)= 16200 J
How much work is required to lift a 270-kg appliance to the second-story level of a house 6.0 mm above street level?
It would increase.
If Earth shrank with no change in its mass, what would happen to your weight at the surface?
60%
If an input of 100 J in a pulley system increases the potential energy of a load by 60 J, what is the efficiency of the system?
If the kinetic energy is zero, that means that all particles are at rest or have a velocity of zero. And since momentum is mass multiplied by velocity, then momentum must also equal zero.
If an object's KE is zero, what is its momentum?
If the ice caps were to melt, I think this would slightly slow down the rotation of the Earth. This is because the mass would "scatter" instead of being localized in the poles.
If the polar ice caps of Earth were to melt, the oceans would be deeper. Strictly speaking, what effect would this have on Earth's rotation?
W=Fd W= (100-70)(10)= 300 J
If you push a crate horizontally with 100 NN across a 10-mm factory floor and friction between the crate and the floor is a steady 70 NN, how much kinetic energy is gained by the crate?
Airbags reduce the chances of injuries and accidents increasing the time of the collision, or how long it takes for the passenger to stop moving. Because of this, this reduces the passenger's change in momentum causing a reduction in the chances of injury.
In terms of impulse and momentum, why do air bags in cars reduce the chances of injury in accidents?
Momentum is conserved in elastic and inelastic collisions.
In which type or kind of collision is momentum conserved: elastic or inelastic?
The tub of the machine exerts centripetal inward force on clothes.
Is it an inward force or an outward force that is exerted on the clothes during the spin cycle of a washing machine?
It is easier for the performer to balance a long rod held vertically with people at the other end because the rotational inertia is greater.
Is it easier for a circus performer to balance a long rod held vertically with people hanging off the other end, or the same long rod without the people at the other end, and why?
It is easier when the mass is farther from your hand.
Is it easier to balance a long rod with a mass attached to it when the mass is closer to your hand or when the mass is farther away?
Both
Is the sum of kinetic and potential energies a constant for satellites in circular orbits, in elliptical orbits, or in both?
It's at its lowest point
Kinetic Energy is greatest when...
Mass x Velocity
Momentum equation:
kinetic energy
Momentum should be the same as _______ ________ when ranking.
the Moon and the Sun
Ocean tides on Earth are mainly affected by ___________.
the weakest
Of the four fundamental forces presently known, the force of gravity is ________________.
It's at its highest point
Potential energy is greatest when...
It stays constant.
Predict how the horizontal 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.
Predict how the vertical component of the velocity will change with time after the projectile is fired.
PE =mgh= (3.0 kg) (10 m/s2) (2.0 m) = 60 J
Select the correct equations that show that when a 3.0-kg book is lifted 2.0 m its increase in gravitational potential energy is 60 J. (Don't forget g, which can be expressed in units N/kg, equivalent to m/s2.)
The moon's velocity and distance from Earth allow it to make a perfect balance between fall and escape. In case the velocity of rotation of the moon was a little bit faster, it would have escaped the Earth's Gravity. On the other hand, if it's a little bit slower, it would have fallen on Earth.
Since the Moon is gravitationally attracted to Earth, why doesn't it simply crash into Earth?
True
T/F: Momentum is a vector quantity
product of force and time
The quantity that is called impulse can be measured by the _______.
greater than toward the center
The tangential speed on the outer edge of a rotating carousel is _______.
four times as much
The work that is done when twice the load is lifted twice the distance is _______.
be four times larger
To double the rotational speed the centripetal force must _______.
Exert a large force over the longest time because momentum is proportional to both force and time.
To increase the momentum of an object, should you exert a large force, or extend that force for as long a time as possible--or both? Explain.
The force is perpendicular to the wrench and the force is farthest away from the nut
Torque is greatest when...
The force would remain the same.
What happens to the force of attraction between two planets if their masses are doubled and the distance between them is also doubled?
I=Ft =(10)(2)= 20 N*s
What impulse occurs when an average force of 10 N is exerted on a cart for 2.0 s?
It is the perpendicular distance from the rotational axis to the line along which the force acts.
What is meant by the "lever arm" of a torque?
Impulse= Ft = mv I= (6)(2.2)= 13.2 N*s
What is the impulse on a 6.0-kgkg ball rolling at 2.2 m/sm/s when it bumps into a basket and stops?
p=mv p=(60)(5)= 300 (kg*m)/s
What is the momentum of a 60-kg carton that slides at 5.0 m/s across an icy surface?
p=mv = (9)(2.5)=25 (kg*m)/s
What is the momentum of a 9.0-kg bowling ball rolling at 2.8 m/s?
-10 kJ
What will be the kinetic energy of a pile driver ram starting from rest that undergoes a 10-kJkJ decrease in potential energy?
It decreases while rising, but it increases while falling.
When a ball is thrown upward at an angle, what happens to the vertical component of its velocity as it rises? As it falls? Suppose that air resistance can be neglected.
remains the same (The total momentum of the system is conserved.)
When a big fish swims into an oncoming smaller fish and swallows it, the momentum of the two-fish system _______.
equal and opposite to the momentum of the fired cannonball (For the total momentum of the system to be conserved, the momentum of the cannon had to be equal and opposite to the momentum of the fired cannonball.)
When a cannonball is fired from a cannon, the momentum of the recoiling cannon is momentarily _______.
vectorially add up to equal the initial momentum of the firecracker (The total momentum of the system is conserved.)
When a falling firecracker explodes, the momenta of its pieces _______.
Because the rifle has a longer barrel and therefore the bullet has to be fired for a longer distance, the bullet requires more work to be acted upon it. This causes for a greater kinetic energy, so the bullet emerges with a great velocity.
When a rifle with a longer barrel is fired, the force of expanding gases acts on the bullet for a longer distance. What effect does this have on the velocity of the emerging bullet? (Do you see why long-range cannons have such long barrels?)
quadruples the tension in the string
When a rock tied to a string is whirled in a horizontal circle, doubling the speed _______.
the same
When one does twice the work in twice the time, the power expended is _______.
doubles
When the rotational speed of a rotating system doubles, its angular momentum _______.
doubles
When the speed of an object doubles, its momentum...
50 percent
When the useful energy output of a simple machine is 100 J, and the total energy input is 200 J, the efficiency is _______.
by four times
When traveling twice as fast your kinetic energy is increased _______.
whether the collision is elastic or inelastic (Momentum is conserved in collisions.)
When two vehicles collide, momentum is conserved _______.
four times
When you lift twice the load twice as high, in half the time, the increase in potential energy is _______.
You weigh more in Death Valley because you are closer to the center of Earth.
Where do you weigh more--at the bottom of Death Valley or atop one of the peaks of the Sierra Nevada? Defend your answer.
The running mouse.
Which has a greater momentum, a heavy truck at rest or a mouse running along the street?
Both take the same 1000 J.
Which requires more work-lifting a 50 kg sack vertically 2 m or lifting a 25 kg sack vertically 4 m?
Tangential speed increases with distance. Rotational speed is constant.
Which varies with radial distance on a rotating turntable, tangential speed or rotational speed?
The Sun exerts a stronger force on the side of Earth nearest the Sun, and the Moon exerts a stronger force on the side nearest the Moon.
Why do both the Sun and the Moon exert a greater gravitational force on one side of Earth than the other side?
Bending your legs shortens them, thus reducing rotational inertia.
Why does bending your legs when running enable you to swing your legs to and from more rapidly?
Rotational inertia depends on whether the mass is farther or closer to the point of rotation. The farther the mass is, the higher the rotational inertia.
Why does the rotational inertia of the rod with the attached mass closer to your hand compare the way it does with the rotational inertial of the rod with the attached mass farther away?
The force is at a right angle to the velocity.
Why doesn't the force of gravity change the speed of a satellite in circular orbit?
The escape velocity of the black hole is greater than the speed of light.
Why is a black hole invisible?
he escape velocity of the black hole is greater than the speed of light.
Why is a black hole invisible?
Because the force of gravity is always perpendicular to the satellite velocity
Why is the tangential speed of a satellite not affected by gravity?
Because a higher torque is applied
Why it is easier to remove an old bolt if we add an extension to the wrench handle?
a.) W= fd = (130)(15)= 1950 J b.) (130-70)(15)= 900 J c.) 1950-900= 1,050 J
a.) How much work is done when you push a crate horizontally with 130 N across a 15 m factory floor? b.) If the force of friction on the crate is a steady 70 N, find the KE gained by the crate. c.) Find the "missing energy".
