Transcript Welcome to PHY 1151: Principles of Physics I

Chapter 8
Potential Energy and Conservative
Forces
Dr. Jie Zou PHY 1151G
Department of Physics
1
Gravitational Potential Energy

Gravitational potential
energy U: The energy of a
body due to elevated positions
is called gravitational potential
energy.


U = weight  height = mgh
The gravitational potential
energy is relative to the
reference level and depends only
on mg and the height h.
Dr. Jie Zou PHY 1151G
Department of Physics
2
Examples


Example 1: Find the gravitational
potential energy of a 65-kg person
on a 3.0-m-high diving board. Let U
= 0 be at water level.
Example 2: An 82.0-kg mountain
climber is in the final stage of the
ascent of 4301-m-high Pikes Peak.
What is the change in gravitational
potential energy as the climber gains
the last 100.0 m of altitude? Let U =
0 be (a) at sea level and (b) at the
top of the peak.
Dr. Jie Zou PHY 1151G
Department of Physics
3
Conservation of Mechanical
Energy


Mechanical energy E: The
sum of the potential and
kinetic energy of an object.
E = U + K.
Conservation of
mechanical energy: In
systems with conservative
force (such as gravity) only,
the mechanical energy E is
conserved. Ui + Ki = Uf + Kf.
Dr. Jie Zou PHY 1151G
Department of Physics
4
Example 8-6

A player hits a 0.15-kg
baseball over the outfield
fence. The ball leaves the
bat with a speed of 36 m/s,
and a fan in the bleachers
catches it 7.2 m above the
point where it was hit.
Assuming frictional forces
can be ignored, find (a) the
kinetic energy of the ball
when it is caught and (b)
its speed when caught.
Dr. Jie Zou PHY 1151G
Department of Physics
5
Example 8-7

A 55-kg skateboarder
enters a ramp moving
horizontally with a
speed of 6.5 m/s, and
leaves the ramp
moving vertically with
a speed of 4.1 m/s.
Find the height of the
ramp, assuming no
energy loss to
frictional forces.
Dr. Jie Zou PHY 1151G
Department of Physics
6
Homework

Chapter 8, Page 227, Problems: # 7,
13, 14.
Dr. Jie Zou PHY 1151G
Department of Physics
7