Efficiency
What is efficiency
the ratio of useful work to energy input
What is work?
the change in energy
Which takes more work: running up a flight of stars or walking?
They take the same amount of work
What is an example of a person exerting effort but no work being done?
Answers may vary
Draw the free-body diagram of a football in the air
See drawings
What is the efficiency of a machine with 2000 J input and 500 J output
25%
How much work is done on a 5N book if it is lifted onto a 1.2m shelf?
6 J
Why is a perpetual motion machine a myth?
There is always some loss of energy to the environment due to friction/air resistance
A roller coaster has 60,000 J of potential energy when it is at the top of a hill. How much kinetic energy will it have at the bottom of the hill?
60,000 J
What is the displacement of a pod of dolphins that migrate 1200 km south and then 1200 km back north?
0
How much energy is required to get 100J of useful work out of a machine with 21% efficiency?
476 J
How much does a crate weigh that takes 1500 J of work to lift it onto a 2 m shelf?
750 N
Mechanical energy can be transformed into... (name at least 3)
Light, sound, electrical, chemical, elastic potential, magnetic, etc.
What is the mechanical energy of a roller coaster cart with 12,000 KE and 20,000 PE?
What is the acceleration of a 1200 km car when 24,000 N of force is applied?
20 m/s2
How much useful work can be done with a machine with 15% efficiency if 3600 J of energy is put into it?
540 J
How far does a 12,000 N truck travel when 320,000 J of work is done?
Ignoring friction, how much does the mechanical energy of cart at the top of a hill change compared to the bottom of a hill?
The ME would stay the same
A cart with 20 kg*m/s of momentum is moving towards a cart at rest. The carts collide and then bounce apart. What is the momentum after the collision?
20 kg*m/s
What is the final velocity of a plane that starts from rest and has an acceleration of 12 m/s2 for 5.2 seconds?
62.4 m/s
How high up is a 1200 kg ski lift pulled if a 20% efficient machine uses 3,850,000 J of energy?
64 m
A 1500 kg car is traveling at 12 m/s at the bottom of a hill. Moving up the hill, the velocity stays the same and 250,000 J of work is done on the car. How tall is the hill?
17 m
What does the conservation of energy say?
Energy can never be created or destroyed, it only changes forms.
A person pulls a pendulum back to a certain height. Will it ever return to exactly the same height? Why or why not?
No, some energy will be lost to air resistance/friction, so it will lose energy to the environment and will never return to the same height.
What is the momentum of a 21,000 kg truck traveling at 16 m/s?
336,000 kg*m/s