Energy: Work and Power Review

A grocery clerk lifts a bag of groceries that weighs 130 N. If the bag is lifted .60 m, how much work does the clerk do?

(130N)(.6m)= 78J

After protecting the goal, a 90 kg goalie celebrates by leaping 110 cm off the ground. How much work was done in the celebration?

(90kg)(10 m/s2)(1.10m)- 990 J

How much work do you need to do to lower a 4.6 kg box from a 3.2 m high shelf?

-144 J

Ali uses a pulley system to raise a 30.0‐kg carton a vertical distance of 15.3 m. He exerts force of 211 N and pulls the rope 28.0 m. a. What is the AMA of this pulley system?

1.39 AMA= weight/applied force AMA= 30 kg(9.8 m/s2)(211 N) AMA= 249N/211N= 1.39

A 3500 W motor is used to lift a 750 kg elephant. How high can it lift the elephant in 30.0 seconds?

14.3 m

James drops a 4-kg rock over the edge of a bridge. At one point the rock is moving with a speed of 8.5 m/s. What is the kinetic energy of the rock at this point?

144.5 J

A 21 kg rock sits on a cliff that is 77 m high. How much gravitational potential energy does the rock possess?

15.800 J

A flower pot with a mass of .15 kg is sitting on a shelf that is 20 m above the ground. A cat jumps up on the shelf and knocks the flower pot off the shelf. what will be the kinetic energy of the flower pot halfway down?

15J

A force of 5.0 N acts on a block of wood at an angle of 60 degrees above the horizontal as the block moves a distance of 6.4 meters to the right. Calculate the work done on the block by the force.

16 J

The kinetic energy of a moving object is 4 J. If the velocity of the object is doubled, its new kinetic energy is ______

16 J In this case, velocity doubled, so that is 2X greater. Since velocity is squared in the equation (2X)^2 = 4X greater and 4X 4 J = 16 J.

On Earth what is the mass of an object if there is a gain of 377.84 J of gravitational potential energy when it is lifted 18.35 m?

2.1 kg

What is the mass of a cow moving at a velocity of 0.5 m/s with 25 J of kinetic energy?

200 kg

Brad Pitt, who has a mass of 100 kg, goes up a flight of 24 steps, each of which is 0.18 m high. It takes Brad 20 seconds to climb these steps. How much power does he use?

211.7 W

What is the gravitational acceleration on Planet X if there is an increase in gravitational potential energy equal to 351.14 J when a 8.96 kg object is lifted 17.33 m?

351.14/(8.96)(17.33)=a a= 2.26 m/s2

How much work is done when a 12 N force pushes an object 3 m?

36J

On Earth a 7.47 kg object is lifted 19.3 m. What is the value for the gravitational potential energy?

412.88 J

Brad Pitt, who has a mass of 100 kg, goes up a flight of 24 steps, each of which is 0.18 m high. How much work does Brad do?

4233.6 J

A car that is moving at 60 mph (miles per hour) has _____ kinetic energy than a car moving at 30 mph.

4x more

How fast is a 3.7 kg object moving if it has 51.6 J of kinetic energy?

51.6/(.5) (3.7)= v2 square root 27.89= 5.28 m/s

A 3500 W motor is used to lift a 750 kg elephant. How long does it take to lift the elephant 30.0 meters?

63 s

Ali uses a pulley system to raise a 30.0‐kg carton a vertical distance of 15.3 m. He exerts a force of 211 N and pulls the rope 28.0 m. What is the efficiency of the system?

75.9% % Efficiency= AMA/IMA 1.39/1.83 (100)= .76(100)

A frictionless inclined plane is 8 m long and rests on a wall that is 2 m high. How much force is needed to push a block of ice weighing 300 N up the plane?

75N

Object A has a mass of 5 kg. Object B has a mass of 10 kg. If they are both dropped off a tower and fall the

B will have 2x the KE of A

What is the purpose of simple machines

Change the direction of the force so that work is easier

The Newton•meter is a unit of power.

F

A 60-kg boy runs up a 2.0 meter staircase in 1.5 seconds. His power is approximately 80 Watt. T/F

F The work would be (m•g)•d or approximately 1200 J. The power is work divided by time - 1200 J/1.5 s = 800 W.

If the block of ice has a weight of 500N, how work would be done to lift it to the top of the ramp from the ground?

F(in) (6m)= 500N (3m) F(in) (6m)= 1500J F(in)= 250J

A frictionless inclined plane is 6 m long and rests on a wall that is 2 m high. How much force is needed to push a block of ice weighing 300 N up the plane?

F(in) * d(in) = f(out) * d(out) 100N

How many joules of work are done on an object when a force of 10 N pushes it a distance of 10 meters?

F= 20N d= 10 m w= FD (20N)(10m)= 200J

A 45 kg mass has an acceleration of 3 m/s2. The mass accelerates for 5 meters. What is the work to accelerate the object?

F= ma (45kg)(3 m/s2)= 135 N 135 N(5 m)= 675 J

The kinetic energy of an object is dependent upon the weight and the speed of an object. T/F

FALSE (sort of) - Kinetic energy depends upon mass and speed. Two objects of the same mass could have different weights if in a different gravitational field; so it is not appropriate to say that kinetic energy depends upon weight.

Faster moving objects always have a greater kinetic energy. T/F

FALSE - Faster moving objects would have more kinetic energy than other objects of the same mass. However, another object could have less speed and make up for this lack of speed in terms of a greater mass.

An eraser is tied to a string; a person holds the string and applies a tension force as the eraser is moved in a circle at constant speed. This is an example of work being done. T/F

FALSE - For uniform circular motion, the force acts perpendicular to the direction of the motion and so the force never does any work upon the object.

A falling object always gains kinetic energy as it falls. T/F

FALSE - If an object is falling at a constant velocity (i.e., the air resistance force equals the downward force of gravity), then there is not an increase in kinetic energy. It is true however that free-falling objects always increase their kinetic energy as they fall.

If an object is on the ground, then it does not have any kinetic energy. T/F

FALSE - If an object is on the ground, then it does not have potential energy (relative to the ground).

Kinetic energy is the form of mechanical energy which depends upon the position of an object. T/F

FALSE - Kinetic energy depends upon the speed of the object; potential energy depends upon the position of the object.

More massive objects always have a greater kinetic energy. T/F

FALSE - More massive objects would have more kinetic energy than other objects with the same speed. However, another object could have less mass and make up for this lack of mass in terms of a greater speed.

Superman applies a force on a truck to prevent it from moving down a hill. This is an example of work being done.

FALSE - Since Superman does not cause a displacement, no work is done; he is merely holding the car to prevent its descent down the hill.

A 1-kg mass at a height of 1 meter has a potential energy of 1 Joule. T/F

FALSE - The final potential energy is calculated as PE = m•g•h = (1 kg)•(~10 m/s/s)•(1 m) = ~10 J.

A 1-kg object is accelerated from rest to a speed of 2.0 m/s. This object gains 4.0 Joules of kinetic energy. T/F

FALSE - The kinetic energy increases from 0 J to 2 J (0.5•1•22); that's an increase by 2 J.

An upward force is applied to a bucket as it is carried 20 m across the yard. This is an example of work being done.

FALSE - The upward force does not cause the horizontal displacement so this is a NON-example of work.

Powerful people or powerful machines are simply people or machines which always do a lot of work. T/F

FALSE - This is not always the case. A machine can do a lot of work but if it fails to do it rapidly, then it is not necessarily powerful. In fact two machines can do the same task (and therefore the same work), yet they can have drastically di

If person A and person B do the same job but person B does it faster, then person A does more work but person B has more power. T/F

FALSE - Vice versa. If two people do the same job, then they're doing the same amount of work. The person who does it fastest generates more power.

Work is a time-based quantity; it is dependent upon how fast a force displaces an object.

FALSE - Work is not dependent on how rapidly the force displaces an object; power is time-based and calculated by force multiplied by speed.

Ali uses a pulley system to raise a 30.0‐kg carton a vertical distance of 15.3 m. He exerts force of 211 N and pulls the rope 28.0 m What is the IMA of this pulley system?

IMA= d(pulley rope)/d(height) 28 m/15.3= 1.83

If the roller coaster car in the above problem were moving with twice the speed, then what would be its new kinetic energy?

If the speed is doubled, then the KE is quadrupled. Thus, KE = 4 * (1.04653 x 105 J) = 4.19 x 105 Joules. or KE = 0.5*m*v2 KE = 0.5*625 kg*(36.6 m/s)2 KE = 4.19 x 105 Joules

What is the unit for kinetic energy?

Joule

Determine the kinetic energy of a 625-kg roller coaster car that is moving with a speed of 18.3 m/s.

KE = (0.5) * (625 kg) * (18.3 m/s)2 KE = 1.05 x105 Joules

What is the formula for Kinetic energy

KE = 0.5 • m • v2

A 900-kg compact car moving at 60 mi/hr has approximately 320 000 Joules of kinetic energy. Estimate its new kinetic energy if it is moving at 30 mi/hr.

KE = 80 000 J The KE is directly related to the square of the speed. If the speed is reduced by a factor of 2 (as in from 60 mi/hr to 30 mi/hr) then the KE will be reduced by a factor of 4. Thus, the new KE is (320 000 J)/4 or 80 000 J.

What is the unit of work?

Newton meter or a Joule

A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed.

No, this is not an example of work. Yes, there is a force--the force of the arm holding the tray above his head. Yes, there is a displacement--across the room. However, the Force and displacement do not occur in the same direction, therefore, the force did not cause the displacement. So NO work.

Dr. Tillotson pushes on the wall in her classroom for 3 hours and becomes exhausted. In physics terms, is this an example of work?

No, this is not an example. No matter how hard or how long Dr. Tillotson pushes on the wall, the wall does not move. If the wall does not move, no work was done.

A flower pot with a mass of .15 kg is sitting on a shelf that is 20 m above the ground. A cat jumps up on the shelf and knocks the flower pot off the shelf. what will be the kinetic energy of the flower pot right before it hits the ground?

Set KE=PE and slove for PE KE= .15kg (10 m/s2) (20m) KE= PE=30J All of the 30J of PE has converted to KE right before it hits the ground

A vertical force can never cause a horizontal displacement T/F

T

An object that has motion - whether it is vertical or horizontal motion - has kinetic energy T/F

T

Work is a form of energy. T/F

T

a vertical force does not do work on a horizontally displaced object. T/F

T

What is the formula for Total Energy

TE= KE + PE

A force is exerted on an object to move it at a constant speed. The power delivered by this force is the magnitude of the force multiplied by the speed of the object. T/F

TRUE

Power is a time-based quantity. T/F

TRUE

Power refers to how fast work is done upon an object. T/F

TRUE -

Units of work would be equivalent to a Newton times a meter.

TRUE - A N•m is equal to a Joule.

A kg•m2/s2 would be a unit of work.

TRUE - A kg•m2/s2 is a mass unit times a speed squared unit, making it a kinetic energy unit and equivalent to a Joule.

Gravitational potential energy is lost as objects free-fall to the ground. T/F

TRUE - As objects free-fall, the height (h) decreases; subsequently, the PE decreases.

If an object is at rest, then it does not have any kinetic energy. T/F

TRUE - Kinetic energy depends upon speed. If there is no speed (the object is at rest), then there is no kinetic energy.

An object has a kinetic energy of 40 J. If its mass were twice as much, then its kinetic energy would be 80 J. T/F

TRUE - Kinetic energy is directly related to the mass of an object.

A 300-Newton force is applied to a skier to drag her up a ski hill at a constant speed of 1.5 m/s. The power delivered by the toe rope is 450 Watts. T/F

TRUE - Since force and speed are given, use Power = F•v. The calculation yields 450 W.

The higher that an object is, the more potential energy which it will have. T/F

TRUE - The equation states that PEgrav = m•g•h; PE is directly related to height.

If the mass of an elevated object is doubled, then its gravitational potential energy will be doubled as well. T/F

TRUE - The equation states that PEgrav = m•g•h; if the h is doubled, then the PE will be doubled as well.

A force is applied by a chain to a roller coaster car to carry it up the hill of the first drop of the Shockwave ride. This is an example of work being done.

TRUE - There is a component of force in the direction of displacement and so this is an example of work.

The force of friction acts upon a softball player as she makes a headfirst dive into third base. This is an example of work being done.

TRUE - There is a force and a displacement; the force acts in the opposite direction as the displacement and so this force does negative work.

Why do cheerleaders and volleyball players bend their knees?

To "load" their muscles with Potential Energy, which they released as they straightened their knees and jumped--thus doing work.

What is the symbol for gravitational potential energy

Ug

What is the gravitational potential energy for a 4 kg object that is lifted 5 m?

Ug= (4kg)(10m/s2)(5)= 200 J

What is the formula for potential energy

Ug= mgh

What is the formula for work?

W = Fd

A crane lifts a box of physics books onto the top of a ramp. The box of books has a mass of 75 kg and the top of the ramp is 54 m above the ground. a. What is the work needed to reach the top of the ramp. b. TE c. PE and KE values (there are 4 steps on the ramp)

W= mgd (75kg)(10 m/s2)( 54m) = 40,5000J PE= mgh (75kg)(10m/2)(54m)= 40,5000 J Take 40,500 divide by 3 steps= 13,500 1st position PE= 40,5000 KE= 0J 2nd position PE= 27,000 KE= 13,500 3rd Position PE= 13,5000 KE= 27,000 4th position on the ground PE= 0 KE= 40,500

Is this a measurement of work? foot pound

Yes

A book falls off a table and free falls to the ground. Is this an example of work?

Yes, this is an example of work. The force in this case is gravity and the distance is the distance from the rable to the gound.

What is work?

a force acts upon an object to cause a displacement of the object

kinetic energy of an object is _____ proportional to the square of its speed. a. directly b. indirectly

directly

Which has more potential energy? mouse on your roof elephant on your roof your dog on your roof

elephant on your roof

A Watt is the standard metric unit of work.

false, Watt is the standard metric unit of power; J

a twofold increase in speed, the kinetic energy will increase by a factor of ___

four

The distance the force acts is the ____ that the object is lifted

height

the energy from an object's motion

kinetic energy

A 1475 kg car is traveling at a speed of 44.0 m/s. How much kinetic energy does the car possess?

m = 1475kg v = 44.0m/s KE = 0.5*1475kg*(44.0m/s)^2 KE = 1427800 J

If a 42 kg student walks up a flight of stairs 12m high in 8 seconds, what is her power output?

m= 42 kg d= 12m t= 8s power=w/t mgd/t= power Must find work first W= FD (42kg)(10 m/s2) (12,)/8s Power=

Near the end of the Shockwave ride, a braking system exerts an applied force upon the coaster car to bring it to a stop. Positive or negative work

negative work

When the force acts in the direction opposite the objects motion in order to slow it down this is what type of work?

negative work

a threefold increase in speed, the kinetic energy will increase by a factor of ___.

nine

Does a parked car have kinetic energy?

no

Rusty Nales uses a hammer to exert an applied force upon a stubborn nail to drive it into the wall. Positive or negative work

positive work

Is kinetic energy scalar or vector?

scalar

What has the least kinetic energy? shoe on your closet floor goldfish mom dragging her screaming child to school

shoe on your closet floor

The kinetic energy is dependent upon the square of the ___.

speed

The kinetic energy of an object depends on the object's: speed and mass speed mass

speed and mass

How does mass relate to potential energy? the more massive the object, the less potential energy it has the more massive the object, the more potential energy it has mass and potential energy are not related

the more massive the object, the more potential energy it has

What are 3 types of kinetic energy?

vibrational (the energy due to vibrational motion), rotational (the energy due to rotational motion), and translational (the energy due to motion from one location to another)