Download Notes 5.1: Work and Kinetic Energy - Physics Honors I

NOTES 5.1: WORK AND
KINETIC ENERGY
An Adventure
created by Billy J.
Jenkins
Energy:
Energy can be transformed from one type to another and transferred from one
object to another, but the total amount is always the same. Energy is
conserved.
Energy is a scalar quantity (not a vector).
Kinetic Energy (K):
Kinetic energy – the energy associated with the state of motion of an
object. In other words, the kinetic energy of an object is the energy
that the object possesses due to its motion. An object that has motion –
has kinetic energy.
Kinetic energy is defined as the work needed to accelerate a body of a
given mass from rest to its stated velocity.
1
𝐾 = 𝑚𝑣 2
2
S.I. Unit for Kinetic energy is:
CHECKPOINT I:
1. What does it mean to say that Energy is Conserved?
2. What is Kinetic Energy?
3. What is the equation for Kinetic Energy?
4. What is the SI Unit for Energy (Kinetic Energy, Potential Energy, Work, or
Heat)?
Work (W):
• Work W is the energy transferred to or from an object by means of a force
acting on the object
.
W = Fd
• Energy transferred to the object is positive work
• Energy transferred from the object is negative work.
.
• EMPHASIS: Work is transferred energy; doing work is the act of
transferring the energy.
Very Important: Work is a scalar quantity.
Work Done by a Constant Force:
F = the magnitude of force F
d = displacement
∅ = the angle between the directions of the displacement d and the force F
 Two Restrictions to the use of the equation W = Fd and W = Fdcosθ:
1) The force must be a constant force i.e. must not change in magnitude
or direction as the object moves.
2) The object must be particle-like i.e. the object is rigid – that is, all
parts of it move together in the same direction.
•The SI unit for work is the same as the SI unit for energy which is the
Joule since work is the transfer of energy.
CHECK POINT 2:
5. What is Work?
6. What is equation for Work?
7. Work done on an object is what kind of work? Positive or Negative?
8. Work done by an object is what kind of work? Positive or Negative?
(Net Work):
When two or more forces act on an object, the net work done on the
object is the sum of the works done by the individual forces. The net
work can be found in two ways:
1) Find the work done by each force and then sum those works to find
the net work 𝑊𝑛𝑒𝑡 .
Wnet = W1 + W2 + ⋯ + WN
= F1 d1 + F2 d2 + ⋯ + FN dN
2) First, find the net force Fnet of those forces and substitute the found net
force Fnet into:
W = Fdcos𝜙 = Fnet dcos𝜙
- The angle is the angle between the directions of the net force Fnet and
the displacement vector 𝑑 for 𝑊 = Fnet ∙ 𝑑.
CHECK POINT 3:
9. List the two ways that net work can be found?