Download Class notes on Work Potential Energy and

Classwork Unit 3- Work Done by Varying Force, Work-Energy Principle, Gravitational Potential Energy,
Hooke’s Law,
College Physics
Mrs. Ashley
Work
Work done on an object by a constant force (constant in magnitude and direction) if the product of the
magnitude of the displacement time the component of the force parallel to the displacement.
W = Force times displacement times cos θ
W = F d cos θ (parallel to force being applied)
Remember: if  is 1800 then cos  = -1
If  is 900 then cos  = 0
Work is scalar and can be positive or negative. Work is measured in newton meters or joules.
Energy is the capacity for doing work, which is inherent to an object or physical system. Energy is
measured in joules and can neither be created nor destroyed, but is always conserved through a given
system. Energy can take multiple forms, potential or kinetic energy, ambient heat and sound. A system
can lose the capacity to do useful work over time which is covered in the field of thermodynamics.
Kinetic and potential energy:
Kinetic Energy: An object’s kinetic energy is the energy it has due to motion. Kinetic energy is calculated
from the amount of work required to accelerate the body from rest to its current speed. The simple
formula is K.E. = ½ mv2
Notes on Derived formula for Kinetic Energy:
Work-Energy Principle: The net work done on an object is equal to the change in the objects kinetic
energy.
The net work done on an object is equal to the change in the object’s kinetic energy.
Wnet = change in KE
Practice Problem #1: How much net work is required to accelerate a 1000 kg car from 20 m/s to 30
m/s? (distance is not given and how long the acceleration was applied is not given)
Potential Energy: This is the energy that an object has due to its position in a system. The most familiar
form is gravitational potential energy. Gravitational Potential Energy is calculated from: P.E. = mgh
where h is the height above some reference point (yf - yi ).
The higher an object is above the ground, the more potential energy it has. The change in potential
energy is equal to the negative of the work done on the object (without acceleration) if the object is
moved from one point to a second point.
The change in potential energy is related to the work done.
Practice Problem #2: A 1.60 meter tall person lifts a 1.65 kg book from the ground so that it is 2.20 m
above the ground. What is the potential energy of the book relative to the ground and relative to the
top of the person’s head? How is the work done by the person related to the two answers above?
Other forms of Potential Energy: Elastic, chemical, electrical, magnetic.
All physical processes are transfers of energy.
Potential Energy of a Spring is Elastic Potential Energy
Hooke's Law - Fs =-kx x is the distance the spring is stretched and K is the spring stiffness constant.
Elastic potential energy is proportional to the square of the of the amount stretched, because the force
is not constant over the distance. The force is greater the more the spring is stretched.
W = Fx = 1/2 kx2 x here is the final amount stretched.
Elastic PE = 1/2kx2
X can be either the amount stretched or the amount compressed. zero PE is springs natural position.
Practice Problem #3: A vertical spring (ignore mass) whose spring stiffness constant is 950 N/m is
attached to a table and is compressed down 0.150 m, what upward speed can it give to a 0.30 kg ball
when released?
Power is simply the rate at which work is done or energy is used. It is measured in watts. One watt is
equivalent to 1 joule of work done per second.