AP Physics 1
Spring 2022
Instructor: G. Peralez
Work and Energy
Preview
In this chapter you will be introduced to the concept of work and the related concepts
of kinetic energy and potential energy. After completion of the chapter you will be able
to calculate the work done by constant forces, the kinetic energy of a moving object, the
gravitational potential energy of an object, and the power developed by a force.
In addition, you will be able to use the principle of conservation of mechanical energy to
solve a variety of problems in a much simpler way than you have done previously.
Important Terms
Work The work done by a CONSTANT force acting on the object is the component of
the force along the displacement of the object times the magnitude of the
displacement.
Kinetic Energy The energy of an object due to its motion.
Gravitational Potential Energy The energy of an object due to its position relative to
the earth.
Total Mechanical Energy The sum of the kinetic and potential energies of an object.
Conservative Force A force which does work on an object which is independent of the
path taken by the object between its starting point and its ending point.
Average Power The work done by a force on an object divided by the time taken to do
the work.
Work
The work done on an object by a force, F, is
W = (F cos Ꮎ)s
Adapted from Physics Student Study Guide 7th Edition, Cutnell and Johnson, 2007
AP Physics 1
Spring 2022
Instructor: G. Peralez
where s is the magnitude of the displacement of the object and Ꮎ is the angle between
the force and the displacement.
The work done by gravity on an object is
Wgravity = mg (ho – hf)
where m is the mass of the object, ho is the initial height of the object, and hf is the final
height of the object.
The work done by nonconservative forces acting on an object is
Wnc = (Ошибка!mvf2 - Ошибка!mvo2) + (mghf – mgho)
Wnc = (KEf – KEo) + (PEf – PEo)
Wnc = ∆KE + ∆PE
Energy
The kinetic energy of an object of mass, m, and speed, v, is
KE = Ошибка!mv2
The gravitational potential energy of an object a height, h, above the reference level is
PE = mgh
The total mechanical energy of an object is the sum of its potential and kinetic energies
Adapted from Physics Student Study Guide 7th Edition, Cutnell and Johnson, 2007
AP Physics 1
Spring 2022
Instructor: G. Peralez
E = KE + PE
The work done by nonconservative forces produces a change in the total mechanical
energy of the object
Wnc = Ef – Eo
Power
The average power developed by a force which has done work, W, in time, t, is
𝑃̅ = Ошибка! = Ошибка!
The average power developed by a force of magnitude F, moving an object with an
average speed, 𝑣̅ is
𝑃̅ = F𝑣̅
Theorems and Principles
Work-Energy Theorem – the total work done by all forces acting on an object is
W = KEf – KEo = Ошибка!mvf2 - Ошибка!mvo2
Principle of Conservation of Mechanical Energy – the total mechanical energy of an
object remains constant as the object moves, provided that no net work is done by
nonconservative forces.
Adapted from Physics Student Study Guide 7th Edition, Cutnell and Johnson, 2007
AP Physics 1
Spring 2022
Instructor: G. Peralez
Ef = Eo
Adapted from Physics Student Study Guide 7th Edition, Cutnell and Johnson, 2007