In lesson 2 of this module we learned how to calculate the
mechanical energy
of a system. In this lesson we will concentrate more on how to use mechanical energy to make predictions about how the system will behave.
Once we know the total mechanical energy of a system, there are two possibilities. Either work is done on the system — see lesson 1 of this module—and the total energy changes, or no work is done and the system obeys
the Law of Conservation of Energy.
In the following simulation:
- Set the friction slider to 'none.'
- Select 'Show Energy of 1.'
- Select
1
16
time.'
- Use your mouse to add one of the 100 gram masses to the end of spring one.
- Stretch the spring so that the mass is almost at the bottom of the screen and then release the mass and watch the values on the graph.
Masses and Springs
What happens to the kinetic energy as the mass moves up and down?
Check your answer
What happens to the gravitational potential energy as the mass moves up and down?
Check your answer
What happens to the spring potential energy as the mass moves up and down?
Check your answer
What happens to the total energy as the mass moves up and down?
Check your answer
Example problem 1
A 45 kg child is playing on a swing. At her highest point the swing is 1.75 meters above the ground, and at the lowest point it is 0.45 meters above the ground. How fast is she going at the lowest point?
Hint #1
Hint #2
Hint #3
Solution
Example problem 2
A 2 kg mass is sliding on a table at 3 m/s. It hits the end of a long relaxed spring with spring constant k = 100 N/m. How far will the mass compress the spring before coming to rest?
Hint #1
Hint #2
Hint #3
Solution