Download Mechanical Energy PP

Mechanical Energy
What is it?
What makes it change?
Mechanical Energy is…
• The total amount of kinetic and (gravitational)
potential energy
• Changed when an external force does work on
a “system”
• Conserved when no external forces do work
(fairly good) Examples of Mechanical
energy being conserved…
PEi + KEi = PEf + KEf
•
•
•
•
Swinging pendulums
Roller coasters
Water slides
Sleds on hills
Reality: The Law of the Conservation
of Energy
PEi + KEi + W = PEf + KEf
Remember…
W=…
PE=…
KE=…
W= (F)(Dr) cosq
PE = mgh
KE=1/2
2
mv
PEi + KEi + W = PEf + KEf
A Dodge Viper (1490kg) starts at rest and coasts
down a 400m long incline that is tilted 5° relative to
horizontal. The road then is level.
a) Ignoring friction and drag, what speed will
the car reach by the bottom of the hill?
b) If the coefficient of static friction for the tires on
the road is 0.95, how far will it take the car to
stop?
PEi + KEi + W = PEf + KEf
A 30kg child on a 5 kg sled starts from rest at the
top of a 100m long, 17m tall hill.
a) Compute the speed the child would
reach by the bottom of the hill if it was
frictionless.
b) Real hills, even icy ones have friction.
When the child reaches the bottom,
she is going 10.0m/s. Compute the work
done by friction, and size of the friction
force.
What is the mathematical relationship
between final speed (m/s) and amount
of gravitational potential energy (J)
“lost”? (Note: the initial speed is
zero.)
• Use the Law of the conservation of energy to
predict the shape of the graph you will see when
you perform this experiment.
• Devise a way to perform this experiment using
the set-up shown on the front board.
• Compare your results to your hypothesis.
Account for any differences.