Download Kinetic and Potential Energy

Kinetic and Potential Energy
Energy is defined as the ability to do work or cause change. Energy can be the ability to do work
(potential energy) or the energy it has due to its motion (kinetic energy).
Kinetic Energy (KE) or the energy of motion (kine = motion in Greek) is determined by the mass
of the object and its velocity (think speed!). Imagine a wrecking ball hitting an object. A larger wrecking
ball will do more damage (work) than a smaller ball will. Or if it is moving faster when it hits something,
it will do more work. The formula to calculate kinetic energy is KE = ½ x mass x velocity2. Mass is
measured on Newtons. Ten Newtons roughly equals 1kg. The label is Joules (J). Let’s consider how
changing mass and velocity affect kinetic energy. If the mass is increased, KE will go up- but by how
much. Since the velocity is squared, the resulting KE (assuming mass does not change) will be
quadrupled. If you are not sure about this, work a couple of problems. First make the mass and velocity
1 and see what you get. Then make the velocity 2 and recalculate. Since mass is not squared, it has less
effect on KE. Doubling the mass will double the KE.
There are different types of potential energy, the ability to do work because of the objects
position. One type is elastic potential energy like a stretched rubber band or a bow would have.
Another (they type we will focus on) is gravitational potential energy (GPE) or the energy of position. To
calculate GPE, take the mass x height x acceleration of gravity (9.8m/s2). The label is also Joule.
Think of a pendulum for a minute.
When pendulum is pulled back (position 1) to
its highest point before it is let go, it will have
all the GPE it will have, but no KE.
So what happens to the GPE when it is let go
(position 2)? Answer this question!
What happens to KE?
What about GPE at position 3?
As it swings to position 4 and 5, what happens to GPE and KE?
Due to forces like air resistance and friction, what would happen to the pendulums swing each time (and
GPE and KE)?
If forces that make it unbalanced could be eliminated, and the only force on it was inertia, the
pendulum would swing forever. Then it would show conservation of energy.
So if the conservation of energy is true, then consider the next diagram and answer the next set of
questions. Assume the GPE is 100J. Explain your choice either by using logic.
Top is 2m
Mass of ball is 2kg
How much KE is at the top of the ramp before the ball is released?
How much GPE will the ball have half way down the ramp?
How much KE will the ball have half way down the ramp?
How much GPE will the ball have when it reaches the bottom?
How much KE will the ball have when it reaches the bottom?