Mechanical Energy
Copy these notes into your journal.
Mechanical Energy
• Two types we will study:
 Potential
 Kinetic
 Potential energy is associated with differences in
position or location.
 Examples – A stationary mass above the ground; an electron in
an orbital above the nucleus
 PE = mgh, where m = kg; g = accel. due to gravity; h = meters
 Unit: Joule
 Example problem: A student holds a 1000 g weight 650 mm
above the floor. What is the potential energy of the weight?
PE = mgh = (1.0 kg)(9.8 m/s2)(0.650 m) = 6.37 Joules
Mechanical Energy
 Kinetic Energy is associated with objects in motion
 Examples – A football after being kicked; a car traveling on the
road
 Unit: Joule
 KE = ½ mv2, where m = kg; v = m/s
 Example problem: A ball of 200 g mass is moving at a
rate of 6.5 m/s. What is the kinetic energy?
 KE = ½ mv2 = (½)(0.200 kg)(6.5 m/s)2 = 4.225 J
• Kinetic and potential energy can be exchanged
by the motion of an object.
• If a rock falls off a cliff, it loses potential
energy as it descends (by losing height) but
gains kinetic energy as its velocity increases
(accelerates).
• Energy is conserved. The amount of potential
energy lost in the above example = the
amount of kinetic energy gained at any given
point as the mass falls.
Relationship, Potential & Kinetic Energy: An object with mass m (kg) is
pushed off of a ledge whose height is h (meters) above the ground.
Evaluate the potential and kinetic energy at different points in its path.
m (kg)
h (m)
2/3h
1/2h
1/3h
In your journal, copy and solve these problems and give the
result. Show your mathematical work.
1. A mountain climber has a mass of 70 kg, and is 100 m up the
face of a cliff. What is his potential energy?
2. This same climber dislodges a rock whose mass is 2.5 kg. What
is the rock’s kinetic energy when it hits the ground below?
3. What is the rock’s velocity when it first hits the ground?
4. What is the rock’s velocity when it is 35 m above the ground?
5. What is the rock’s potential energy after it has fallen 45 m?
6. What is the rock’s kinetic energy after it has fallen 45 meters?