Mr. Dehn's 1st Period Physics Class-Blog

Definition:

The Law of Conservation of Energy is that energy can’t be created or destroyed but it can be transferred from
one object to another.

Mathematical Representation

Abbreviated Version: Ei=Ef

Extended Version:  GPEi+SPEi+KEi=PGEf+SPEf+KEf

Different Units of Energy

SI Units: Joules

America Units: Calories, VTU

          The video that I chose to demonstrate the Law of Conservation of Energy includes a Newton's Cradle. The Newton's cradle demonstrates that energy is transferred from one object to another.  When you grab the ball and make it hit the others, the energy transfers through the balls to the other ball causing it to swing up.  

6. A man (70kg) is jumping off a cliff into the lake. The cliff is 61 meters high. What will be his final velocity?

KNOWN     UNKNOWN                              (mvi^2 / 2) + mghi  = (mvf^2 / 2) + mgh2

M= 70 kg      Vf= ???????                                 (70x0) / 2 + (70 x 9.8 x 61) = (70 x v^2) / 2

H=61m                                                                    41,846 = ((70 x v^2) / 2) 2
G=9.8                                                                       2x41,846 = (70 x v^2)
Vi=0m/s                                                                83,692/70 = (70 x v^2) / 70
                                                                                   1,195.6 = v^2
                                                                                √1,195.6 = v
                                                                                    34.57 = Vf

EXAMPLE:

Tony Hawk  rides his skateboard at a local skate park. He starts from rest at the top of the track as seen in the figure below and begins a descent down the track, always maintaining contact with the surface. The mass of the skateboard is negligible, as is friction except where noted.

(a) What is Tony’s speed when he reaches the bottom of the initial dip, 18.0 m below the starting point?

(b) He then ascends the other side of the dip to the top of a hill, 8.0 m above the ground. What is his speed when he reaches this point?

Solution:

a. From the conservation of energy: Potential energy at the top of the 18 m transforms into the Kinetic energy at the bottom of the dip.

b. From the conservation of energy: Potential energy at the top of the 18 m transforms into the Kinetic and Potential energy at the top of a hill.

Answer  and  

1. In the first picture, potential energy is the highest because its at 0m/s which means it isn't moving, which means that the energy is stored and ready to come out.

2. In the second picture, kinetic energy is high because the skateboarder is moving and the definition of kinetic energy is energy in motion.

3. In the third picture, both kinetic and potential energy are almost the same because he's at the top of the loop and he's kinda slowing down but still has energy stored up which helps him go down the ramp fast.

8. 1. 0m/s=v ( because its not moving)

    2. mvi  / 2 + mgh1 = mgh2 + mvf^2 / 2

              mgh1 = mvf^2 / 2
               gh1 = vf^2 / 2

               (9.8)(8)= vi^/2

              (78.4) 2 = (vf^2 / 2) 2

                156.8 = vf^2
                √156.8 = vf
                12.52 m/s= Vf

     3. mgh1 = mgh2 + mv^2 / 2

             gh1 - gh2 = (v^2) / 2
       (9.8) (8) - (9.8) (3.5) = (v^2) / 2
          78.4 - 61.74 =  (v^2) / 2
        2 (16.66) =  ((v^2) / 2) 2
            √(33.32) = √(v^2)
              5.77 m/s = v