Relationship between Work and Mechanical Energy
by Ron Kurtus (revised 19 January 2015)
Work is the measurement of the force on an object that overcomes a resistive force (such as friction or gravity) times the distance the object is moved. If there is no distance, there is no work, no matter what the effort.
In certain situations there is a relationship between that work and mechanical energy (as opposed to heat or radiant energy).
When you accelerate an object, you are doing work against inertia, such that the work equals the change in kinetic energy of the object.
Change in KE means you have accelerated the object.
When you accelerate an object, you are changing its velocity and thus its KE. By accelerating the object over a period of time, you are moving the object some distance, while changing its veleocity. Thus, you arwe doing work against inertia
you are doing work against inertia, such that the work equals the change in kinetic energy of the object.
When you are doing work against continuous resistive forces, such as gravity or spring tension, work done equals the change in potential energy of the object.
Questions you may have include:
- What is the equation for work?
- How is work the change in kinetic energy?
- When is work the change in potential energy?
This lesson will answer those questions. Useful tool: Units Conversion
Work as force times distance
The definition of mechanical work (as opposed to thermodynamic work) is that it equals a force against some resistance times the distance traveled while that force is being applied.
Note: Unfortunately, many Physics textbooks carelessly omit the idea that there is the resistive force force of inertia, as well as other possible resistances.
The equation for work is:
W = Fd
- W is the work in joules (J or kg-m²/s²)
- F is the force applied to an object, overcoming any resistance, in newtons (N or kg-m/s²)
- d is the distance or displacement the object moves in the direction of the force in meters (m)
- Fd is F times d
Thus, if you would apply a force of F = 3 newtons (3 N) to overcome inertia and perhaps friction and move an object a distance of d = 5 meters (5 m), the work done would be W = Fd = 15 joules (15 J).
Work as change in kinetic energy
When you accelerate an object, you are changing its velocity.
When you accelerate an object, you are doing work against inertia plus any resistive forces—such as gravity or friction—over the distance that the object is accelerated.
This means that the object's velocity—and thus its kinetic energy—changes over the distance moved.
The kinetic energy of an object is:
KE = mv2/2
- KE is the kinetic energy in J or kg-m2/s2
- m is the mass of the object in kg
- v is the velocity in m/s
The work done in changing the velocity against some resistance is then equal to the change in kinetic energy of the object:
ΔKE = mvf2/2 − mvi2/2
W = ΔKE
- Δ means change (Greek letter capital delta)
- vf is the final velocity of the object
- vi is the initial velocity
Thus, in certain situations, work is the change in kinetic energy.
(See Proof that Work Can be the Change in Kinetic Energy for more information.)
Work as the change on potential energy
Some resistive forces, such as gravity and spring tension, act continuously on an object, such that the object has a potential of moving from its present position. This is called its potential energy (PE).
Moving the object from one PE to a higher one requires work, which can be measured by the change in potential energy (ΔPE).
Example with gravity
For example, the PE of an object due to the force of gravity is:
PE = mghi
- m is the mass of the object
- g is the acceleration due to gravity
- hi is the initial height above the ground
If you lifted the object to another height (hf), the new potential energy would be:
PE = mghf
The amount of work done would be
W = Fd
- F = mg
- d = hf − hi
W = mg(hf − hi)
W = mghf − mghi
W = PEf − PEi = ΔPE
W = ΔPE
Thus in certain situations, work is the change in potential energy.
Work as a combination of KE and PE
If you project an object upward, you are doing work both against gravity and inertia. In this situation, the total work done is:
W = ΔPE + ΔKE
When you apply enough force on an object to overcome a resistive force, such that you move that object, you are doing work on that object. There is a relationship between that work and mechanical energy.
When you accelerate the object, you are doing work against inertia, such that the work equals the change in kinetic energy of the object.
When you are doing work against continuous resistive forces, such as gravity or spring tension, you change the potential energy of the object.
Resources and references
Work: the transfer of mechanical energy - From online physics book by Benjamin Crowell
Definition and Mathematics of Work - Physics Classroom
Work (Physics) - Wikipedia
Work, Energy, and Power - HyperPhysics
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