2. Work and Energy

3. 1 st Law of Thermodynamics  Energy cannot be created or destroyed. It can only be converted from one form into another.

4. Potential Energy and Kinetic Energy  When an object is lifted it increases the potential energy of the system.  The potential energy= mass x gravity x height or P.E. = mgh  Once a system starts to drop or move horizontially it picks up kinetic energy.  Kinetic energy is equal to one-half the mass of an object times the velocity squared. KE = 1/2 mv 2

5. Conservation of Energy  Since energy is conserved, when an object falls its potential energy gets converted into kinetic energy.

6. Sample Questions  An object is dropped from a height of 20 m. What is its speed just before it hits the ground?  A rollercoaster car has a speed of 30 m/s at the lowest point in the track. What is the highest point it can reach?

7.  The work needed to compress a spring is equal to the area under its force vs. distance curve.  E.P.E = 1/2 kx 2 Elastic Potential Energy

8. Sample Questions  A spring gun with a spring constant of 250 N/m is compressed 5 cm. How fast will a 0.025 kg dart go when it leaves the gun?  A 5 kg rock is dropped a distance of 1 m onto a spring. It compresses the spring.2 m. What is the spring constant?

9. Work  Work can only be done to a system by an external force; a force from something that is not a part of the system.  So if our system is a box sitting on a table...and I come along and push the box or lift it up, I can increase the energy of the box...I am doing work to the box.  Work = Force x Displacement OR W = Fd

10. Sample Problem  Since work changed the energy of a system: the units of energy must be the same as the units of work.  The units of both work and energy are the Joule.  How much work is done lifting a 10 kg box up 2 m and carrying it 10 m?

11. Work Energy Theorem  Work can also be defined as the change in kinetic energy of an object.  Sample Problem  A 875 kg compact car speeds up from 22 to 44 m/s while passing another car. How much work was done to increase its speed?

12. Power  Power is defined as the rate that work is done: P = W / t  Since work is measured in Joules (J) and time is measured in seconds (s) the unit of power is Joules per second (J/s).  However, in honor of James Watt, the unit of power is also know as a Watt (W).

13. Sample Question  A forklift lifts a 50 kg box 5 m in 12 s at a constant speed. What is the power supplied by the forklift?