1. Goal: To understand Energy
Objectives:
To learn about Spring Forces
To learn about Spring potential energy
To learn about Power

2. Spring Force As you compress a spring it pushes back.
The more you compress it the harder it pushes back.
F = -kx (which means that the overall direction of the positive force is towards x = 0)
Note that x is positive for compression and negative for decompression

3. Example A spring has a spring constant of 250 N/m.
The spring is compressed by a 25 N weight to a distance of 0.1 m.
A) What is the force the spring exerts on the weight?
B) What is the acceleration of the weight at that moment?

4. Spring vs gravity
If you orient a spring vertically gravity will be a part of the picture.
You will now have to work with net force (which means the spring force and the gravity force on any object attached to the spring).
Example:
A spring with spring constant of 120 N/m is hung from a ceiling.
Below the spring a 10 N Weight is tied to the spring.
How far down does the spring need to decompress so that the 10 N weight is perfectly supported (hint, think forces). That is to say where will the weight be motionless (net force…)

5. Spring potential
It takes energy to compress or to decompress a spring.
The spring then stores that energy.
This gives a spring potential energy.
U = ½ k x2

6. Example
A spring with a spring constant of 340 N/m is compressed by 0.2 m.
A) What is the potential energy at that point.
B) What is the Force at that point.

7. Using Energy to Find compression
Suppose that a car had an energy of 1.1 * 105 J.
The car runs into a large spring with a spring constant of 1100 N/m.
A) Find how far the car will compress the spring
B) Find the force on the car at that point

8. Power Power is quite simply the rate at which you do work.
Power = Work / time = Energy / time
Energy = Power * Time
So, the faster you do work, the more power you are providing.
Units: Power = Energy / time = J/s
J/s = Watts

9. Example
You have 5 incandescent light bulbs that you leave on for 10 hours per day.
The power of each bulb is 60 W.
A) How much energy does each bulb use per day?
Note that a Watt * hour = Watt-hour which is a unit of energy.
B) If 1000 Watt-hours of energy costs you about 10 cents what is the daily cost of using these bulbs?

10. Conclusion
We have learn about springs
We explored power