Work, Energy & Power: The Physics of Getting Things Done
Understand work, kinetic and potential energy, the conservation of energy, and power — with formulas and the physics background that supports ACT Science.
The Short Version
- Work = force × distance (W = Fd), done only when a force moves something.
- Kinetic energy (½mv²) is energy of motion; potential energy (mgh) is stored energy of position.
- Energy is conserved — it transforms between forms but the total stays the same.
- Power = work ÷ time — how fast work is done. Physics / ACT Science background.
In everyday speech, "work" can mean anything tiring. In physics it means something specific: a force moving an object through a distance. Energy is the capacity to do that work, and it comes in forms that can transform into one another — the stored energy of a raised ball becomes the motion energy of a falling one. Power, finally, is just how quickly work gets done. These ideas, plus the conservation of energy, explain a huge range of physical situations.
This guide covers work, kinetic and potential energy, conservation, and power, with worked and practice questions matched to the level seen in ACT Science and physics at Northside Tutoring.
Why This Matters
Work and energy are central to physics and appear in ACT Science passages about machines, motion, and energy transfer. The conservation of energy is one of the deepest principles in all of science. (The SAT has no science section.)
Work
In physics, work is done when a force moves an object through a distance:
Work is measured in joules (J). Crucially, if nothing moves, no work is done — holding a heavy box still, however tiring, does zero physics work because there's no distance.
Kinetic & Potential Energy
Energy comes in two main mechanical forms:
| Type | Formula | Meaning |
|---|---|---|
| Kinetic energy | KE = ½mv² | energy of motion |
| Potential energy | PE = mgh | stored energy of height |
(m is mass, v is speed, g is gravity, h is height.) A fast or heavy object has lots of kinetic energy; a raised object has gravitational potential energy waiting to be released.
Conservation of Energy
Energy is never created or destroyed — it only changes form. On a roller coaster, potential energy at the top converts into kinetic energy at the bottom; the total stays the same (ignoring friction). This conservation of energy lets you solve problems by tracking energy from one form to another.
Trade PE for KE
When an object falls, its potential energy (mgh) converts into kinetic energy (½mv²). Setting them equal lets you find the speed at the bottom without knowing the time — a common shortcut.
Power
Power measures how fast work is done:
Measured in watts (W), power distinguishes a fast worker from a slow one doing the same job. Two engines can do the same total work, but the more powerful one does it in less time.
How They Connect
The pieces link together: doing work on an object transfers energy to it (the work-energy theorem says work equals the change in kinetic energy). Energy then moves between kinetic and potential forms while the total is conserved, and power tells you the rate at which all of this happens.
Where You'll See This — Test by Test
Work and energy support ACT Science physics passages; the SAT has no science section and the SSAT doesn't test it. They're core high-school and AP Physics.
ACT Science
Energy and machine passages on the ACT Science section rely on work, energy, and power concepts.
Explore ACT Tutoring → K-12 CurriculumPhysics
Work, energy, and power are central to high-school and AP Physics.
Explore Science Tutoring → College AdmissionsSAT
No SAT science section; physics isn't tested there among admissions exams.
Explore SAT Tutoring → K-12 CurriculumSchool Science
The conservation of energy is a unifying principle across the sciences.
Explore Science Tutoring →Watch the Lesson
Sometimes a diagram needs a voice. In the short video below, one of our Northside tutors walks through the core idea and works through test-style problems in real time.
Work, Energy & Power — In Plain English
A live walkthrough from our tutoring team.
— Featuring a Northside Tutoring instructor
Worked Example Problems
These problems are calibrated to the difficulty you'll actually see on test day. Try each one before opening the solution.
A force of 20 N pushes a box 5 m. How much work is done?
Show solution
W = Fd = 20 × 5 = 100 J.
You hold a 10 kg box still for 30 seconds. How much physics work do you do on it?
Show solution
Zero — the box doesn't move, so there's no distance and no work (W = Fd = F × 0).
A 2 kg object moves at 3 m/s. What is its kinetic energy?
Show solution
KE = ½mv² = ½(2)(3²) = ½(2)(9) = 9 J.
As a ball falls, its potential energy decreases. What happens to its kinetic energy?
Show solution
It increases — PE converts into KE, conserving total energy.
A machine does 600 J of work in 3 s. What is its power?
Show solution
Power = work ÷ time = 600 ÷ 3 = 200 W.
Common Mistakes to Avoid
Three points students often miss
- Thinking holding something is work. Physics work requires movement (W = Fd); no distance means no work.
- Forgetting energy is conserved. Energy transforms (PE ↔ KE) but the total stays constant.
- Confusing work and power. Work is total energy transferred; power is how fast it's transferred.
Practice Problems — You Try
Three problems below. Work each before checking the solution.
A 50 N force moves an object 4 m. Find the work done.
Show solution
W = 50 × 4 = 200 J.
What is the potential energy of a 3 kg object held 5 m high? (g ≈ 10 m/s²)
Show solution
PE = mgh = 3 × 10 × 5 = 150 J.
A 2 kg ball is dropped from a height where it has 100 J of potential energy. Ignoring air resistance, what is its kinetic energy just before it lands, and roughly its speed?
Show solution
By conservation, all 100 J of PE becomes KE = 100 J. Then ½mv² = 100 → v² = 100 → v = 10 m/s.
The Northside Method — How We Teach This 1-on-1
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- Assessment. We diagnose which specific skills are slowing your student down — not just whether they "get it" in the abstract.
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