Posted In: Physics

Work, energy and power are important concepts in Physics. Let us take a brief look at them

Work is done when the force applied to an object moves that object. It is calculated by multiplying the force by the amount of movement of the object. The equation can be stated as:

Work done  = Force * Distance

The unit of force is newtons and the unit of distance is meters. Work is measured in joules.

The force on an object of mass m at the surface of the earth is m * g where m is the mass of the object and g is the gravitational pull ( 9.81 ). Thus work done can also be calculated as

Work done = mass * gravity * height

Height is the distance the object is lifted in the vertical direction.
Energy is defined as the capacity to do work. Work is closely related to energy. When work is done energy is transferred from one place to another.

Work done (Joules) = Energy transferred (Joules)

The unit of energy is the same as the unit of work that is Joules.

Energy can be either potential or kinetic energy. The work energy principle states that the work done by all forces acting on an object is equal to the change in the kinetic energy of that object. If the net work done by all forces acting on the object is positive then the kinetic energy of the object increases. If the net work done by all forces acting on the object is negative then the kinetic energy of the object decreases.

Power is the measure of the rate at which work is being done. It can be computed by the equation:

Power = Work / time

The unit of work is Joules and the unit of time is seconds. The unit of power is watts.

Power can be used to determine the rate at which a machine is capable of doing work. Power can also be computed as

Power     = work / time
Since work is equal to force * distance
Power    = force * distance / time
    = force * velocity

Work energy and power problems and solutions
Question: If a force of 200 Newtons, can move a box by 50 meters how much is the work done?
Answer: Work    = Force * Distance
        = 200 * 50
        = 10000 Joules

Question: How much work is done if 68 Watts of power is produced in 18 seconds?
Answer: Power     = work / time
    Therefore work = power * time
            = 68 * 18
            = 1224 Joules

Question: A man exerts a force of 1000 Newtons on a rock. However the rock does not move. How much is the work done?
Answer: Work    = Force * Distance
        = 1000 * 0
        = 0 Joules

Question: A man of mass 80 kgs climbs a slope with a height of 5 meters. It takes him 10 seconds to reach the top. How much is the work done? Also, calculate the man’s power?
    Work done     = Energy transferred
            = mass * gravity * height
            = 80 * 9.8 * 5
            = 3920 Joules

    Power        = work / time
            = 3920 / 10
            = 392 Watts

Question: How far will a 10 N force pull a car if the work done is 20J?
    Work         = force * distance
    distance    = work / force
            = 20 / 10
            = 2 meters

Question: How much is the work done in raising 100 kg of water through a vertical distance of 3 meters.
    Work done     = mass * gravity * height
            = 100 * 9.8 * 3
            = 2940 Joules



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