Resource Lesson
Work and Energy
Printer Friendly Version
According to
Newton's Third Law
, a
force
can be defined as the
interaction
between two objects: when object A exerts a force on object B, then object B exerts an equal but opposite force on object A. These forces are measured in a unit known as a
newton
.
The definition of this unit is based on
Newton's Second Law
which states that the
acceleration
an object experiences is directly proportional to the net force (sum of the forces) acting on the object and is inversely proportional to the object's mass; that is,
net F = ma
Refer to the following information for the next three questions.
Suppose a 5-kg mass, initially at rest, is accelerated to a final velocity of 8 m/sec in 4 seconds across a frictionless surface.
What acceleration does it experience?
How large a force was acting on the object to accomplish this task?
Suppose the mass is now pulled across a second surface which is no longer smooth. On this rough surface the object encounters a constant frictional "retarding force" of 7 newtons as it is moves forward. How large would the new pulling force have to be to achieve the same acceleration?
When a force is exerted on an object and moves it through a distance we say that
work
is done on the object.
How much work is done on our 5-kg mass when the object is pulled 16 meters across the original frictionless surface by a 10-N force?
How much work is done on our 5-kg mass when the object was pulled 16 meters across the rough surface by a 17-N force?
How much work is done on our 5-kg mass by the 7-N frictional force when the object was pulled 16 meters across the rough surface?
If friction is said to do "negative work" because it decreases an object's velocity (that is, it opposes an object's forward motion), then how much "net work" was done in pulling the 5-kg mass 16 meters across the rough surface?
The
work-energy theorem
states that the net work done on an object equals the change in its kinetic energy. That is, when an external force moves an object through a distance it does work on the object which is evidenced by a change in its velocity or its kinetic energy, KE = ½mv
2
.
W
done
= deltaKE
Recall from our lesson on mechanical energy that energy is measured in a unit called a joule which equals a kg m
2
/sec
2
. This same unit is used to measure the work done on an object when a force moves it through a distance: nt m = (kg m/sec
2
) m = kg m
2
/sec
2
.
In our example, how much kinetic energy did the 5-kg mass gain as it was accelerated from 0 to 8 m/sec in 4 seconds?
Could this acceleration have occurred in 16 meters?
Another graph of F vs d- questions about work, change in KE, final velocity, acceleration
If an object is moving at a constant velocity then there must be at least two forces acting on it - one force causing a "forward" acceleration and another force causing an "opposing" acceleration. These forces must balance to a net force of zero resulting in a corresponding acceleration of zero. Note that each force is doing work on the object but their "work" cancels to no net change in the object's kinetic energy.
When a path-independent force, or conservative force (for example, the pull of gravity), acts on an object, the change in the object's kinetic energy is related to a change in its potential energy.
PE
lost
= KE
gained
Related Documents
Lab:
Labs -
A Battering Ram
Labs -
A Photoelectric Effect Analogy
Labs -
Air Track Collisions
Labs -
Ballistic Pendulum
Labs -
Ballistic Pendulum: Muzzle Velocity
Labs -
Bouncing Steel Spheres
Labs -
Coefficient of Friction
Labs -
Coefficient of Friction
Labs -
Coefficient of Kinetic Friction (pulley, incline, block)
Labs -
Collision Pendulum: Muzzle Velocity
Labs -
Conservation of Energy and Vertical Circles
Labs -
Conservation of Momentum in Two-Dimensions
Labs -
Falling Coffee Filters
Labs -
Force Table - Force Vectors in Equilibrium
Labs -
Inelastic Collision - Velocity of a Softball
Labs -
Inertial Mass
Labs -
LabPro: Newton's 2nd Law
Labs -
Loop-the-Loop
Labs -
Mass of a Rolling Cart
Labs -
Moment of Inertia of a Bicycle Wheel
Labs -
Ramps: Sliding vs Rolling
Labs -
Relationship Between Tension in a String and Wave Speed
Labs -
Relationship Between Tension in a String and Wave Speed Along the String
Labs -
Roller Coaster, Projectile Motion, and Energy
Labs -
Rotational Inertia
Labs -
Rube Goldberg Challenge
Labs -
Spring Carts
Labs -
Static Equilibrium Lab
Labs -
Static Springs: Hooke's Law
Labs -
Static Springs: Hooke's Law
Labs -
Static Springs: LabPro Data for Hooke's Law
Labs -
Target Lab: Ball Bearing Rolling Down an Inclined Plane
Labs -
Terminal Velocity
Labs -
Video LAB: A Gravitron
Labs -
Video LAB: Ball Re-Bounding From a Wall
Labs -
Video Lab: Blowdart Colliding with Cart
Labs -
Video LAB: Circular Motion
Labs -
Video Lab: Falling Coffee Filters
Labs -
Video Lab: M&M Collides with Pop Can
Labs -
Video Lab: Marble Collides with Ballistic Pendulum
Resource Lesson:
RL -
Advanced Gravitational Forces
RL -
Air Resistance
RL -
Air Resistance: Terminal Velocity
RL -
APC: Work Notation
RL -
Conservation of Energy and Springs
RL -
Energy Conservation in Simple Pendulums
RL -
Forces Acting at an Angle
RL -
Freebody Diagrams
RL -
Gravitational Energy Wells
RL -
Inclined Planes
RL -
Inertial vs Gravitational Mass
RL -
Mechanical Energy
RL -
Momentum and Energy
RL -
Newton's Laws of Motion
RL -
Non-constant Resistance Forces
RL -
Potential Energy Functions
RL -
Principal of Least Action
RL -
Properties of Friction
RL -
Rotational Dynamics: Pivoting Rods
RL -
Rotational Kinetic Energy
RL -
Springs and Blocks
RL -
Springs: Hooke's Law
RL -
Static Equilibrium
RL -
Symmetries in Physics
RL -
Systems of Bodies
RL -
Tension Cases: Four Special Situations
RL -
The Law of Universal Gravitation
RL -
Universal Gravitation and Satellites
RL -
Universal Gravitation and Weight
RL -
What is Mass?
RL -
Work
Worksheet:
APP -
Big Fist
APP -
Family Reunion
APP -
The Antelope
APP -
The Box Seat
APP -
The Jogger
APP -
The Pepsi Challenge
APP -
The Pet Rock
APP -
The Pool Game
CP -
Action-Reaction #1
CP -
Action-Reaction #2
CP -
Conservation of Energy
CP -
Equilibrium on an Inclined Plane
CP -
Falling and Air Resistance
CP -
Force and Acceleration
CP -
Force and Weight
CP -
Force Vectors and the Parallelogram Rule
CP -
Freebody Diagrams
CP -
Gravitational Interactions
CP -
Incline Places: Force Vector Resultants
CP -
Incline Planes - Force Vector Components
CP -
Inertia
CP -
Mobiles: Rotational Equilibrium
CP -
Momentum and Energy
CP -
Momentum and Kinetic Energy
CP -
Net Force
CP -
Newton's Law of Motion: Friction
CP -
Power Production
CP -
Satellites: Circular and Elliptical
CP -
Static Equilibrium
CP -
Tensions and Equilibrium
CP -
Work and Energy
NT -
Acceleration
NT -
Air Resistance #1
NT -
An Apple on a Table
NT -
Apex #1
NT -
Apex #2
NT -
Cliffs
NT -
Elliptical Orbits
NT -
Escape Velocity
NT -
Falling Rock
NT -
Falling Spheres
NT -
Friction
NT -
Frictionless Pulley
NT -
Gravitation #1
NT -
Gravitation #2
NT -
Head-on Collisions #1
NT -
Head-on Collisions #2
NT -
Ice Boat
NT -
Ramps
NT -
Rotating Disk
NT -
Sailboats #1
NT -
Sailboats #2
NT -
Satellite Positions
NT -
Scale Reading
NT -
Settling
NT -
Skidding Distances
NT -
Spiral Tube
NT -
Tensile Strength
NT -
Terminal Velocity
NT -
Tug of War #1
NT -
Tug of War #2
NT -
Two-block Systems
WS -
Advanced Properties of Freely Falling Bodies #1
WS -
Advanced Properties of Freely Falling Bodies #2
WS -
Advanced Properties of Freely Falling Bodies #3
WS -
Calculating Force Components
WS -
Charged Projectiles in Uniform Electric Fields
WS -
Combining Kinematics and Dynamics
WS -
Distinguishing 2nd and 3rd Law Forces
WS -
Energy Methods: More Practice with Projectiles
WS -
Energy Methods: Projectiles
WS -
Energy/Work Vocabulary
WS -
Force vs Displacement Graphs
WS -
Freebody Diagrams #1
WS -
Freebody Diagrams #2
WS -
Freebody Diagrams #3
WS -
Freebody Diagrams #4
WS -
Introduction to Springs
WS -
Kinematics Along With Work/Energy
WS -
Lab Discussion: Gravitational Field Strength and the Acceleration Due to Gravity
WS -
Lab Discussion: Inertial and Gravitational Mass
WS -
net F = ma
WS -
Potential Energy Functions
WS -
Practice: Momentum and Energy #1
WS -
Practice: Momentum and Energy #2
WS -
Practice: Vertical Circular Motion
WS -
Ropes and Pulleys in Static Equilibrium
WS -
Rotational Kinetic Energy
WS -
Standard Model: Particles and Forces
WS -
Static Springs: The Basics
WS -
Vocabulary for Newton's Laws
WS -
Work and Energy Practice: An Assortment of Situations
WS -
Work and Energy Practice: Forces at Angles
TB -
Systems of Bodies (including pulleys)
TB -
Work, Power, Kinetic Energy
PhysicsLAB
Copyright © 1997-2024
Catharine H. Colwell
All rights reserved.
Application Programmer
Mark Acton