Download Work and Kinetic Energy Serway (7.1 – 7.3)

Kinetic Energy
Physics 1D03 - Lecture 19
Kinetic Energy
Definition: for a particle moving with speed v, the kinetic
energy is
K = ½ mv2
(a SCALAR quantity)
Then the Work-Energy Theorem says:
The total work done by all external forces acting on a
particle is equal to the increase in its kinetic energy.
W = ΔK
= Kf – Ki
Physics 1D03 - Lecture 19
• Kinetic Energy is measured in joules (1J=1N·m).
• Kinetic energy is a scalar; the work-energy theorem is a
scalar relation.
• This theorem is equivalent to Newton’s Second Law. In
principle, either method can be used for any problem in
particle dynamics.
Physics 1D03 - Lecture 19
How to deal with friction
If there is friction in the system, then:
ΔK=Wf
= -ffd
Since
ΔK = Kf - Ki = -ffd
Therefore Kf = Ki - ffd
Physics 1D03 - Lecture 19
Example 1
A bartender slides a 1-kg glass 3 m along the bar to a
customer. The glass is moving at 4 m/s when the
bartender lets go, and at 2 m/s when the customer
catches it.
Find the work done by friction, and calculate the force of
friction.
Physics 1D03 - Lecture 19
Example 2
A 6.0 kg block initially at rest is pulled to the right for
3.0m with a force of 12N over a surface.
Determine its final velocity if:
a) the surface has no friction
b) the surface has a coefficient of kinetic friction of 0.15
How else could we solve this problem ????
Try it !!!
Physics 1D03 - Lecture 19
Example 3
A block of mass 1.6kg resting on a frictionless surface is
attached to a horizontal spring with a spring constant
k=1.0x103 N/m. The spring is compressed to 2.0cm and
released from rest.
a) Calculate the speed of the block as it passes the x=0
point.
b) Calculate the block’s speed at the x=1.0 cm point.
c) Calculate the block’s speed at the x=0cm point if
there is a constant frictional force of 4.0 N.
Physics 1D03 - Lecture 19