Aim: What are Potential and Kinetic Energy?

1. Aim: What are Potential and Kinetic Energy? Do Now: A girl lifts a 10 kg object 1.5 m straight up. How much work has she done? W = Fd W = Fgd W = (10 kg)(9.8 m/s2)(1.5 m) W = 147 J

3. Energy What is energy? The ability to do work Scalar quantity Measured in Joules

4. Work – Energy relationship The work that you put into a system is equal to the energy that system gains

5. Potential Energy • Stored energy • Gravitational Potential Energy – the energy stored due to objects vertical position • Measured from a specific reference point (ground: h = 0 m) • PE = mgh

6. The heavy ram of a pile driver possesses potential energy

7. Back to the Do Now Problem How much potential energy does the object have? PE = mgh PE = (10 kg)(9.8 m/s2)(1.5 m) PE = 147 J Work – Energy Relationship If 147 J of work go into lifting an object, then the object obtains 147 J of potential energy

9. Change in Potential energy (loss or gain) ΔPE = mgΔh The 10 kg object from the do now falls 1 m. How much potential energy has it lost? ΔPE = mgΔh ΔPE = (10 kg)(9.8 m/s2)(1 m) ΔPE = 98 J

10. Kinetic Energy • Energy of Motion • KE = ½mv2

11. Example • What is the kinetic energy of a 1200 kg car moving at 100 m/s? • KE = ½mv2 • KE = ½(1200 kg)(100 m/s)2 • KE = 6 x 106 J

12. What is the velocity of a 75 kg person running with 1,250 J of kinetic energy? KE = ½mv2 1,250 J = (½)(75 kg)v2 1,250 J = (37.5 kg)v2 v2 = 33.3 m2/s2 v = 5.8 m/s

13. 200 J of work are put into pushing a box across the floor. How much kinetic energy has the object gained? 200 J Work – energy relationship