# Energy & Work in Physics Flashcards

Energy & Work in Physics Flashcards
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A 5 kg ball is dropped from the top of a building. In the absence of air resistance, describe how the kinetic energy of the object changes as it falls.

The further the ball falls, the faster it will go because of acceleration due to gravity. This means the ball's kinetic energy will continue to increase, and will not be conserved.

Got it
Find the kinetic energy of a 95 kg skier gliding down a slope at a velocity of 5 m/s.

KE = 1/2 * mv2 = 1/2 * 95 kg * (5 m/s)2 = 1,187.5 J

Got it
Kinetic Energy

The energy of a moving object

KE = 1/2 * mass * velocity2

Got it

Mechanical advantage = slope of inclined plane (the hypotenuse) / height from the ground

MA = s / h

Got it
Simple Machine

A device that changes the direction and/or magnitude of an applied force, making it easier to do work

Examples: pulley, lever, and inclined plane

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A 10 kg exercise ball is pushed off the edge of a table and falls 5 m. Calculate the amount of work performed on the ball by gravity.

Work = force * distance

Force = mass * acceleration = 10 kg * 9.8 m/s2 = 98 N

W = 98 N * 5 m = 490 J

Got it
Energy

A quantity that measures the ability of something to do work

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Calculate the amount of work a father performs on 5 N wagon carrying a 200 N child over a distance of 100 m.

Work = force * distance = (200 + 5) * 100 m = 20,500 J

Got it
Work

The amount an object moves in one direction due to a constant force

Calculated by multiplying force * distance

Unit: joules (J)

Got it

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## Flashcard Content Overview

In our everyday lives, we talk about work, energy, and power in different ways. In physics, however, these words have very specific definitions. These flashcards will review these definitions and describe the different calculations that can be done to describe how objects do work, use energy, and have power.

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Back
Work

The amount an object moves in one direction due to a constant force

Calculated by multiplying force * distance

Unit: joules (J)

Calculate the amount of work a father performs on 5 N wagon carrying a 200 N child over a distance of 100 m.

Work = force * distance = (200 + 5) * 100 m = 20,500 J

Energy

A quantity that measures the ability of something to do work

A 10 kg exercise ball is pushed off the edge of a table and falls 5 m. Calculate the amount of work performed on the ball by gravity.

Work = force * distance

Force = mass * acceleration = 10 kg * 9.8 m/s2 = 98 N

W = 98 N * 5 m = 490 J

Simple Machine

A device that changes the direction and/or magnitude of an applied force, making it easier to do work

Examples: pulley, lever, and inclined plane

Mechanical advantage = slope of inclined plane (the hypotenuse) / height from the ground

MA = s / h

Kinetic Energy

The energy of a moving object

KE = 1/2 * mass * velocity2

Find the kinetic energy of a 95 kg skier gliding down a slope at a velocity of 5 m/s.

KE = 1/2 * mv2 = 1/2 * 95 kg * (5 m/s)2 = 1,187.5 J

A 5 kg ball is dropped from the top of a building. In the absence of air resistance, describe how the kinetic energy of the object changes as it falls.

The further the ball falls, the faster it will go because of acceleration due to gravity. This means the ball's kinetic energy will continue to increase, and will not be conserved.

Gravitational Potential Energy

The energy stored in an object due to its position

PE = mass * acceleration of gravity * height

Find the potential energy of a 500 g water balloon held by someone on top of a roof that is 20 m off the ground.

PE = mgh = 0.500 kg * 9.8 m/s2 * 20 m = 90 J

Find the energy stored in a spring that has been compressed 5 cm and has a spring constant of 400 N/m.

Potential energy = 1/2 * spring constant * distance2 = 1/2 * 400 * (0.05)2 = 0.5 J

A cyclist is pedaling at a speed of 15 m/s when they come to a hill. Calculate how high up the hill the cyclist can get to without pedaling. Assume no energy is lost due to friction or heat.

mgh = 1/2 * mv2

m * 9.8 * h = 1/2 * m * (15)2

h = 112.5m / 9.8m = 11.5 m

A 5 g bouncy ball has a kinetic energy of 0.35 J when it is 5 m off the ground. Determine the ball's speed the moment before it bounces. Assume no energy is lost to friction/heat.

GPE = 0.005 kg * 9.8 m/s2 * 5 m = 0.245 J

ME = KE + GPE = 0.245 J + 0.35 J = 0.595 J

KE when GPE is 0 = 0.595 J = 1/2 * 0.005 * v2

v = 15.4 m/s

A 15 kg cannon ball was fired at 50 m/s at an angle of 45° above the horizontal. Assuming no air resistance, what is the object's horizontal kinetic energy at the top of its path?

x component of speed = 50 * cos(45°) = 35.4 m/s

KE = 1/2 * 15 kg * (35.4 m/s)2 = 9,399 J

A box is sliding down a ramp. At the top of the ramp, the gravitational PE is measured to be 100 J. At the bottom of the ramp, the KE is measured to be 95 J. Explain the energy loss.

As the box slides down, friction converts some of the energy into heat (waste).

A 10 kg ball is dropped from rest at a height of 15 m. Determine the amount of mechanical energy the ball has at 10 m above the ground. Assume there is no air resistance.

ME = KE + PE

At rest, KE = 0 and GPE = mgh = 10 * 9.8 * 15 = 1,470 J

ME = 0 + 1,470 J = 1,470 J

If no friction, ME is conserved and = 1,470 J at all heights

Calculate the amount of work performed by a 100 watt light bulb that burns for 30 minutes.

Power = work done / time interval

1 watt = 1 J/s

30 min * 60 s / 1 min = 180 s

P = 100 watts / 180 s = 0.56 J

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