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Conservation of Energy

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Conservation of Energy
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Students will learn how to apply energy conservation in a closed system.

Key Equations

 \sum E_{\text{initial}} = \sum E_\text{final} \; \; \text{The total energy does not change in closed systems}

Guidance
Energy is conserved in a closed system. That is, if you add up all the energy of an object(s) at one time it will equal all the energy of said object(s) at a later time. A closed system is a system where no energy is transferred in or out. The total energy of the universe is a constant (i.e. it does not change). The problems below do not consider the situation of energy transfer (called work). So friction and other sources where energy leaves the system are not present. Thus, one simply adds up all the potential energy and kinetic energy before and sets it equal to the addition of the total potential energy and kinetic energy after .

Example 1

Billy is standing at the bottom of a ramp inclined at 30 degrees. Billy slides a 2 kg puck up the ramp with an initial velocity of 4 m/s. How far up the ramp does the ball travel before it begins to roll back down? Ignore the effects of friction.

Solution

The potential energy of the puck when it stops at the top of it's path will be equal to the kinetic energy that it was initially rolled with. We can use this to determine the how high above the ground the puck will be above the ground when it stops, and then use trigonometry to find out how far up the ramp the puck will be when it stops.

PE_i + KE_i &= PE_f + KE_f && \text{start with conservation of energy}\\0 + \frac{1}{2}mv^2 &= mgh + 0 && \text{take out the energy terms we know will be zero and substitute the equations for potential and kinetic energy.}\\\frac{1}{2}v^2 &= gh && \text{simplify the equation}\\h&=\frac{v^2}{2g} && \text{solve for h}\\h&=\frac{(4\;\text{m/s})^2}{2*9.8\;\text{m/s}^2} && \text{substitute in the known values}\\h&=0.82\;\text{m}\\

Now we can find the distance up the ramp the ball traveled since we know the angle of the ramp and the height of the ball above the ground.

\sin(30)&=\frac{h}{x}\\x&=\frac{h}{\sin(30)}\\x&=\frac{0.82\;\text{m}}{\sin(30)}\\x&=1.6\;\text{m}\\

Watch this Explanation

Simulation

Energy Skate Park (PhET Simulation)

Time for Practice

  1. At 8:00 AM, a bomb exploded in mid-air. Which of the following is true of the pieces of the bomb after explosion? (Select all that apply.)
    1. The vector sum of the momenta of all the pieces is zero.
    2. The total kinetic energy of all the pieces is zero.
    3. The chemical potential energy of the bomb has been converted entirely to the kinetic energy of the pieces.
    4. Energy is lost from the system to sound, heat, and a pressure wave.
  2. You are at rest on your bicycle at the top of a hill that is 20 \;\mathrm{m} tall. You start rolling down the hill. At the bottom of the hill you have a speed of 22 \;\mathrm{m/s} . Your mass is 80 \;\mathrm{kg} . Assuming no energy is gained by or lost to any other source, which of the following must be true?
    1. The wind must be doing work on you.
    2. You must be doing work on the wind.
    3. No work has been done on either you or the wind.
    4. Not enough information to choose from the first three.
  3. A stationary bomb explodes into hundreds of pieces. Which of the following statements best describes the situation?
    1. The kinetic energy of the bomb was converted into heat.
    2. The chemical potential energy stored in the bomb was converted into heat and gravitational potential energy.
    3. The chemical potential energy stored in the bomb was converted into heat and kinetic energy.
    4. The chemical potential energy stored in the bomb was converted into heat, sound, kinetic energy, and gravitational potential energy.
    5. The kinetic and chemical potential energy stored in the bomb was converted into heat, sound, kinetic energy, and gravitational potential energy.
  4. You hike up to the top of Granite Peak in the Trinity Alps to think about physics.
    1. Do you have more potential or kinetic energy at the top of the mountain than you did at the bottom? Explain.
    2. Do you have more, less, or the same amount of energy at the top of the mountain than when you started? (Let’s assume you did not eat anything on the way up.) Explain.
    3. How has the total energy of the Solar System changed due to your hike up the mountain? Explain.
    4. If you push a rock off the top, will it end up with more, less, or the same amount of energy at the bottom? Explain.
    5. For each of the following types of energy, describe whether you gained it, you lost it, or it stayed the same during your hike:
      1. Gravitational potential energy
      2. Energy stored in the atomic nuclei in your body
      3. Heat energy
      4. Chemical potential energy stored in the fat cells in your body
      5. Sound energy from your footsteps
      6. Energy given to you by a wind blowing at your back
  5. A 1200 kg> car traveling with a speed of 29 m/s drives horizontally off of a 90 m cliff.
    1. Sketch the situation.
    2. Calculate the potential energy, the kinetic energy, and the total energy of the car as it leaves the cliff.
    3. Make a graph displaying the kinetic, gravitational potential, and total energy of the car at each 10 m increment of height as it drops
  6. A roller coaster begins at rest 120 \;\mathrm{m} above the ground, as shown. Assume no friction from the wheels and air, and that no energy is lost to heat, sound, and so on. The radius of the loop is 40 \;\mathrm{m} .
    1. Find the speed of the roller coaster at points B, C, D, E, F , and H .
    2. Assume that 25 % of the initial potential energy of the coaster is lost due to heat, sound, and air resistance along its route. How far short of point H will the coaster stop?
  7. A pendulum has a string with length 1.2 m. You hold it at an angle of 22 degrees to the vertical and release it. The pendulum bob has a mass of 2.0 kg.
    1. What is the potential energy of the bob before it is released? ( Hint: use geometry to determine the height when released. )
    2. What is its speed when it passes through the midpoint of its swing?
    3. Now the pendulum is transported to Mars, where the acceleration of gravity g is 2.3 \;\mathrm{m/s}^2 . Answer parts (a) and (b) again, but this time using the acceleration on Mars.

Answers to Selected Problems

  1. discuss in class
  2. discuss in class
  3. d
  4. discuss in class
  5. b. KE = 504,600 \;\mathrm{J}; U_g = 1,058,400 \;\mathrm{J}; E_{total} = 1,563,000 \;\mathrm{J}
  6. a. 34 \;\mathrm{m/s \ at \ B}; 49 \;\mathrm{m/s \ at \ C \ and \ F};  28 \;\mathrm{m/s \ at \ D}, 40 \;\mathrm{m/s \ at \ E}, 0 \;\mathrm{m/s \ at \ H} b. it will make it up to only 90m, so 30m short of point H
  7. a. 1.7 J b. 1.3 m/s c. 0.4 J, 0.63 m/s

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