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A forklift lifts a crate of mass 100 kg at a constant velocity to a height of 8 m over a time of 4 s. The forklift then holds the crate in place for 20 s. Calculate how much power the forklift exerts in lifting the crate? How much power does the forklift exert in holding the crate in place?

  1. We are given:

    • mass of crate: m =100 kg
    • height that crate is raised: h =8 m
    • time taken to raise crate: t r =4 s
    • time that crate is held in place: t s =20 s

    We are required to calculate the power exerted.

  2. We can use:

    P = F Δ x Δ t

    to calculate power. The force required to raise the crate is equal to the weight of the crate.

  3. P = F Δ x Δ t = m · g Δ x Δ t = ( 100 kg ) ( 9 , 8 m · s - 2 ) 8 m 4 s = 1 960 W
  4. While the crate is being held in place, there is no displacement. This means there is no work done on the crate and therefore there is no power exerted.

  5. 1 960 W of power is exerted to raise the crate and no power is exerted to hold the crate in place.

Got questions? Get instant answers now!

Experiment : simple measurements of human power

You can perform various physical activities, for example lifting measured weights or climbing a flight of stairs to estimate your output power, using a stop watch. Note: the human body is not very efficient in these activities, so your actual power will be much greater than estimated here.


  1. [IEB 2005/11 HG] Which of the following is equivalent to the SI unit of power:
    1. V · A
    2. V · A - 1
    3. k g · m · s - 1
    4. kg · m · s - 2
  2. Two students, Bill and Bob, are in the weight lifting room of their local gum. Bill lifts the 50 kg barbell over his head 10 times in one minute while Bob lifts the 50 kg barbell over his head 10 times in 10 seconds. Who does the most work? Who delivers the most power? Explain your answers.
  3. Jack and Jill ran up the hill. Jack is twice as massive as Jill; yet Jill ascended the same distance in half the time. Who did the most work? Who delivered the most power? Explain your answers.
  4. Alex (mass 60 kg) is training for the Comrades Marathon. Part of Alex's training schedule involves push-ups. Alex does his push-ups by applying a force to elevate his center-of-mass by 20 cm. Determine the number of push-ups that Alex must do in order to do 10 J of work. If Alex does all this work in 60 s, then determine Alex's power.
  5. When doing a chin-up, a physics student lifts her 40 kg body a distance of 0.25 m in 2 s. What is the power delivered by the student's biceps?
  6. The unit of power that is used on a monthly electricity account is kilowatt-hours (symbol kWh). This is a unit of energy delivered by the flow of l kW of electricity for 1 hour. Show how many joules of energy you get when you buy 1 kWh of electricity.
  7. An escalator is used to move 20 passengers every minute from the first floor of a shopping mall to the second. The second floor is located 5-meters above the first floor. The average passenger's mass is 70 kg. Determine the power requirement of the escalator in order to move this number of passengers in this amount of time.
  8. Calculate the power required for an electric motor to pump 20kg of water up to ground level from a borehole of depth 10m in half a minute.

These two videos provide a summary of some of the concepts covered in this chapter.

Khan academy video on work and energy - 1

Khan academy video on work and energy - 2

Important equations and quantities

Commonly used units
Quantity Symbol Unit S.I. Units Direction
velocity v m s or m . s - 1
momentum p k g . m s or k g . m . s - 1
energy E J k g . m 2 s 2 or kg . m 2 s - 2
Work W J N . m or k g . m 2 . s - 2
Kinetic Energy E K J N . m or k g . m 2 . s - 2
Potential Energy E P J N . m or k g . m 2 . s - 2
Mechanical Energy U J N . m or k g . m 2 . s - 2
Power P W N . m . s - 1 or k g . m 2 . s - 3

Momentum :

p = m v

Kinetic energy :

E k = 1 2 m v 2
  • Energy is never created nor destroyed, but is merely transformed from one form to another.
  • In the absence of friction, the total mechanical energy of an object is conserved.

When a force moves in the direction along which it acts, work is done.

Work is the process of converting energy.

Energy is the ability to do work.

End of chapter exercises

  1. The force vs. displacement graph shows the amount of force applied to an object by three different people. Abdul applies force to the object for the first 4 m of its displacement, Beth applies force from the 4 m point to the 6 m point, and Charles applies force from the 6 m point to the 8 m point. Calculate the work done by each person on the object? Which of the three does the most work on the object?
  2. How much work does a person do in pushing a shopping trolley with a force of 200 N over a distance of 80 m in the direction of the force?
  3. How much work does the force of gravity do in pulling a 20 kg box down a 45 frictionless inclined plane of length 18 m?
  4. [IEB 2001/11 HG1] Of which one of the following quantities is kg.m 2 .s - 3 the base S.I. unit?
    1. Energy
    2. Force
    3. Power
    4. Momentum
  5. [IEB 2003/11 HG1] A motor is used to raise a mass m through a vertical height h in time t.What is the power of the motor while doing this?
    1. m g h t
    2. m g h t
    3. m g t h
    4. h t m g
  6. [IEB 2002/11 HG1] An electric motor lifts a load of mass M vertically through a height h at a constant speed v. Which of the following expressions can be used to correctly calculate the power transferred by the motor to the load while it is lifted at a constant speed?
    1. M g h
    2. M g h + 1 2 Mv 2
    3. M g v
    4. M g v + 1 2 Mv 3 h
  7. [IEB 2001/11 HG1] An escalator is a moving staircase that is powered by an electric motor. People are lifted up the escalator at a constant speed of v through a vertical height h.What is the energy gained by a person of mass m standing on the escalator when he is lifted from the bottom to the top?
    1. mgh
    2. mgh sin θ
    3. mgh sin θ
    4. 1 2 mv 2
  8. [IEB 2003/11 HG1] In which of the following situations is there no work done on the object?
    1. An apple falls to the ground.
    2. A brick is lifted from the ground to the top of a building.
    3. A car slows down to a stop.
    4. A box moves at constant velocity across a frictionless horizontal surface.

Questions & Answers

are UV rays dangerous?
Khathutshelo Reply
what's this?
i hv a problem with this chapter because i also have problem with Newton's laws of motion
Kearabetswe Reply
what is the wave model of atom ?
Trazy Reply
Can you state the Doppler effect in words
Nombuyiselo Reply
State Coulomb's law in words
the magnitude of the electrostatic force exerted point on each other is directly proportional to the product on magnitude of charges and inversely proportional to the square distance between them
Newton's first law of motion
Nnuso Reply
When an object is at rest or traveling at a constant velocity it will remain at rest unless an unbalanced force acts upon it.
yes that's true
Always know Newton's second law of motion. It appears to be in every question paper
Is the normal force always 0
Mpilo Reply
no. newtons 3rd law states that if something exerts a force into something else it will experience that same magnitude of force but in the opposite direction. so the net force is equal to the force the object applies to the surface but in the opposite direction
define the term rate of reaction in word
David Reply
I think it a chemical process in which substance act mutually on each other.
how many bones are in the human body?
it's the rate at which the reactants are able to change to products
what is wave lengh
mama Reply
is the de broglie wavelength of the particle
calcium and magnesium, which one can displace aluminum from its compound?
Markia Reply
how is aluminum ion formed?
a substance that has the lower electronegativity which will be in this case the Ca -1.0 this is because the Ca would have a lower ionization energy -needs less energy to fill its outer most shell and therefore will cause a displacement of the Al
an Al ion would form when it has lost electrons and will normally be a cation -positively charged (3+)
molecules with the same molecular formula but different chain
Malaza Reply
what will be the answer
chain isomer
what is structural isomer?
Hey if you define an isomer it will come as an organic molecule with the same number of atoms,same molecule formula ,same molecular mass but different STRUCTURAL FORMULA so what we have is only the chain ,positional and functionally isomer.Never heard of STRUCTURAL isomer
what does the newton's law says?
there are 3 newtons laws which are newton's law of universal gravitation, law of cooling, law of motion
Did the feather or leaf hit the ground first?
Shaloom Reply
both at the same time. air friction ignored
2 to 30 minut long distance race 20 km what's the avarage speed for the race
Jamilla Reply
what is meaning of covalent bonds
Lungani Reply
bonds that exists between non metal atoms. each atom contribute electron(s) which will form a bond joining the two atoms. electrons in the bond now belongs to both atoms
how do we name haloalkanes
Thobeka Reply
how do we name haloalkanes
what are examples of haloalkanes
haloalkanes are compounds that consist of the halogen group chlorine bromine flourine and so on like 2-bromobutan

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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