Mechanical Energy: Potential & Kinetic. Units of Energy The Joule is the unit of energy. 1 Joule = 1...
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Transcript of Mechanical Energy: Potential & Kinetic. Units of Energy The Joule is the unit of energy. 1 Joule = 1...
Units of Energy The Joule is the unit of energy. 1 Joule = 1 kg x 1 m/s2 x 1 meter 1 J = 1 N x m Joules are part of the metric system as
use the same prefixes as grams, meters, and liters! (kilo-, centi-, milli-)
Example: 1000J = 1 kJ
1. Gravitational Potential Energy PEgrav The potential energy stored by
objects that are above the Earth’s Surface
PEgrav = (m)(Ag)(Δh) m = mass of object (kg) Ag = acceleration due to gravity
on Earth it equals 9.8 m/s2
Δ h = change in height of object (meters) Pegrav INCREASES
when an object is raised to a higher level
Example 1 A 5kg coconut is at the top of a 15m tree. What is
the potential energy of the coconut? Given:
m = 5kg g = 9.8 m/s2
Δh = 15m Unknown: PEgrav Equation: PEgrav = (m)(Ag)(Δh) Substitute: PEgrav = (5kg)x(9.8 m/s2)x(15m) Solve: PEgrav = 735 J
Example 2 A skier with a weight of 882N is at the top of a 130m
Olympic Ski Jump. What is the potential energy of the skier?
Given: Fw = 882N (hint: Fw =mAg) Δh = 130m
Unknown: PEgrav Equation: PEgrav = (Fw)(Δh) Substitute: PEgrav = 882N x 130m Solve: PEgrav = 114,660 J or 114.66kJ
2. Elastic Potential Energy
The potential energy of an object that is stretched or compressed. It springs back to its original shape
after it is stretched or compressed!Examples:
Stretching a rubber band Pulling a cello string Shock absorbers on a bike Air compressed when a basketball
bounces
Kinetic Energy KE Energy of an object due to
motion Depends on mass & velocity
of object
Examples: Rollercoasters going downhill Skateboarding up and down a
half pipe Wind
KE = ½(m)(v2)
Example 1 Determine the kinetic energy of a 625-kg roller coaster
car that is moving with a speed of 18.3 m/s. G: mass is 625kg, velocity is 18.3 m/s
U: KE
E: KE=(1/2)(m)(v2)
S: KE=(1/2)∙(625kg)∙(18.3m/s)2
S: KE=104653.13J=1.05x105J
Example 2 If the roller coaster car in the previous
problem were moving with twice the speed, then what would be its new kinetic energy?G: mass is 635kg, velocity= 2∙18.3m/s=36.6m/sU: New kinetic energyS: KE=(1/2)(m)(v2)S: KE=(1/2)∙(625kg)∙(36.6m/s)2
S: KE=418612.5J= 4.19x105J
Energy Conservation Energy can be converted from one form to
another. Energy cannot be created or destroyed. The gravitational potential energy of an object
is converted to kinetic energy as an object falls.
Mechanical Energy = KE+PE
Conservation of Mechanical Energy Calculation(KE+PE)beginning= (KE+PE)end