Unit 6: Work, Energy, Power, and Simple Machines
Energy as the bookkeeping trick that beats tracking every force: how it changes form, gets traded for speed, and gets delivered over time.
11 topics · 36 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Work
- How it worksHow does work only count the part of a force that acts along the direction of motion?
- Worked exampleFind the work done pushing a 15 kg box 4 meters with a 30 N horizontal force.
- Common mistakeWhy doesn't carrying a suitcase across a flat floor do any work on it at all?
- Going furtherWhat happens to the work done on an object when the force is applied perpendicular to its motion the whole time?
Your turn1 of 2
Find the work done pushing a box 19.65 meters with a 140 N horizontal force applied in the direction of motion.Kinetic Energy
- How it worksHow does squaring the velocity inside the kinetic energy formula explain why speed matters so much?
- Worked exampleFind the kinetic energy of a 60 kg cyclist moving at 8 m/s.
- Common mistakeWhy isn't kinetic energy a vector that points in the direction of motion?
Your turn1 of 2
Find the kinetic energy of a 107 kg cyclist moving at 16.4 m/s.Gravitational Potential Energy
- How it worksHow does lifting an object against gravity store energy that height alone can measure?
- Worked exampleFind the gravitational potential energy of a 2 kg book on a shelf 1.5 meters high.
- Common mistakeWhy isn't gravitational potential energy a fixed property of an object with no reference point involved?
Your turn1 of 2
Find the gravitational potential energy of a 27.92 kg book on a shelf 13.05 meters high.Elastic Potential Energy in Springs
- How it worksHow does compressing or stretching a spring convert your effort into energy waiting to be released?
- Worked exampleFind the elastic potential energy stored in a spring with a constant of 200 N/m stretched 0.3 meters.
- Going furtherWhat happens to the energy stored in a spring when its stretch distance is doubled?
Your turn1 of 2
Find the elastic potential energy stored in a spring with a constant of 699 N/m stretched 1.31 meters.Conservation of Mechanical Energy
- How it worksHow does height trade smoothly for speed as a rollercoaster car falls, with the total energy never changing?
- Worked exampleFind the speed of a rollercoaster car at the bottom of a 20 meter drop, starting from rest.
- Common mistakeWhy doesn't a swinging pendulum ever rise back higher than the height it was released from?
- Going furtherWhat happens to an energy bar chart for a falling object the moment friction is added to the picture?
- Ties togetherHow does conservation of mechanical energy turn into the work-energy theorem the moment friction enters the picture?
Your turn1 of 2
Find the speed of a rollercoaster car at the bottom of a 84 meter drop, starting from rest and ignoring friction.The Work-Energy Theorem
- How it worksHow does net work translate directly into a change in kinetic energy with nothing left over?
- Worked exampleFind the final speed of a 4 kg object starting at rest after 80 J of net work is done on it.
- Common mistakeWhy doesn't work done against friction ever come back as kinetic energy?
Your turn1 of 2
Find the final speed of a 41.94 kg object starting at rest after 1300 J of net work is done on it.Power
- How it worksHow does power measure how fast energy gets delivered rather than how much total energy is used?
- Worked exampleFind the power output of a motor that does 6000 J of work in 12 seconds.
- Common mistakeWhy isn't power the same thing as the total amount of work or energy involved?
Your turn1 of 2
Find the power output of a motor that does 13944 J of work in 39 seconds.Simple Machines and Mechanical Advantage
- How it worksHow does a lever trade a smaller force over a longer distance for a bigger force over a shorter one?
- Worked exampleFind the mechanical advantage of a lever whose effort arm is 3 times longer than its load arm.
- Common mistakeWhy doesn't a simple machine ever let you get more work out of it than you put in?
Your turn1 of 2
Find the mechanical advantage of a lever whose effort arm is 10.81 times longer than its load arm.Efficiency of Machines
- How it worksHow does efficiency compare the useful energy a machine outputs to the total energy put into it?
- Worked exampleFind the efficiency of a machine that puts out 300 J of useful work for every 500 J of energy input.
- Common mistakeWhy isn't a machine with 100 percent efficiency possible in the real world?
Your turn1 of 2
Find the efficiency of a machine that puts out 336 J of useful work for every 565 J of energy input.Energy Lost to Friction
- How it worksHow does friction convert mechanical energy into thermal energy that spreads into the surroundings?
- Worked exampleFind the energy lost to friction when a 10 kg box slows from 6 m/s to 2 m/s while sliding across a floor.
- Common mistakeWhy doesn't energy disappear when a moving object is brought to rest by friction?
Your turn1 of 2
Find the energy lost to friction when a 56 kg box slows from 23 m/s to 8 m/s while sliding across a floor.Energy Transformations in Everyday Devices
- How it worksHow does a swinging pendulum clock keep converting energy back and forth between height and speed to keep ticking?
- Common mistakeWhy doesn't a device that transforms energy from one form to another ever create new energy in the process?
- Going furtherWhat happens to the chain of energy transformations inside a toaster the moment it's unplugged partway through toasting?
Your turn1 of 2
Turning on a flashlight seems like it should instantly and directly convert battery energy into light, yet the actual process a flashlight relies on passes through more than one distinct form of energy before any light appears. Why does a flashlight really run through three different forms of energy before any light comes out of it?