Physics Honors Unit 7: Work & Energy Summary
Work
- Work describes when a force acts on an object, and the object moves some distance.
- When the force and the displacement are in opposite directions, the work done on the object is negative.
- When calculating work, the parallel component of force is used.
Energy
- Energy is the ability to do work.
Kinetic Energy (KE)
- Kinetic Energy is energy in motion.
- Any object with mass and velocity has KE.
Work-Energy Principle
- Links work and energy: the net work done on an object equals the object’s change in kinetic energy (or change in potential energy, depending on the situation).
Potential Energy (PE)
- An object at some height from a reference point has Potential Energy—the potential to do work based on its position relative to its surroundings.
- Gravitational PE is independent of path—it only relies on the vertical height.
Conservative vs. Nonconservative Forces
- Any force for which work is independent of path is a conservative force.
- Any force for which work depends on the path taken is a nonconservative force.
Elastic Potential Energy
- When a spring is stretched or compressed, it stores elastic potential energy.
Conservation of Mechanical Energy
- When no nonconservative forces are present, mechanical energy is conserved.
- The total mechanical energy before a process is the same as after the process—the total energy stays constant.
- Energy can be transformed from one form into another; oftentimes energy is transferred from one object to another.
- Work is done when energy is transferred.
Law of Conservation of Energy
- Total energy is neither increased nor decreased in any process.
- Energy can be transformed and transferred, but the total amount remains constant.
Power
- Power is the rate at which work is done or energy is transferred.
Efficiency
- Efficiency is the ratio of power output to power input.
- Only a perfect machine could reach an efficiency of 1 or 100%.
Skills to Acquire
- Define all terms related to Work and Energy.
- Explain Unit 7 concepts in your own words.
- Pinpoint locations of maximum and minimum KE and PE on diagrams and analyze them.
- Use Unit 7 formulas to analyze real-life scenarios (roller coasters, cars, falling objects, pushing objects, calculating friction, etc.).
- Work: W = Fdcos\theta
- Kinetic Energy: KE = \frac{1}{2}mv^2
- Potential Energy: PE = mgh
- Elastic Potential Energy: EPE = \frac{1}{2}kx^2
- Net Work and Change in Kinetic Energy: W_{net} = \Delta KE
- External Work and Change in Potential Energy: W_{ext} = \Delta PE
- Work done by gravity and change in Potential Energy: W_g = -\Delta PE
- Work done by non-conservative forces: W_{NC} = \Delta KE + \Delta PE
- Conservation of Mechanical Energy (0 = change in KE + change in PE): 0 = \Delta KE + \Delta PE
- Law of Conservation of Energy (KE1 + PE1 = KE2 + PE2): KE1 + PE1 = KE2 + PE2
- Law of Conservation of Energy with transformed energy: KE1 + PE1 = KE2 + PE2 + E_{transformed}
- Power (Work/time or Energy/time): P = \frac{W}{t} = \frac{E}{t} = Fv
- Horsepower conversion: 1 hp = 746 W