What is Kinetic Energy?
Kinetic energy is the energy an object possesses because of its motion. Any moving mass — a rolling ball, a speeding car, a flying arrow — carries kinetic energy, and the faster it moves, the more it has.
Kinetic energy is KE = ½mv², measured in joules (J), where m is mass in kg and v is velocity in m/s. Because velocity is squared, doubling speed quadruples the kinetic energy.
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Step-by-step worked examples
A 2 kg object moves at 3 m/s. Find its kinetic energy.
KE = ½mv² = ½×2×3² KE = ½×2×9 = 9 J
A 1000 kg car travels at 20 m/s. Find its kinetic energy.
KE = ½mv² = ½×1000×20² KE = ½×1000×400 = 200 000 J = 200 kJ
A 1 kg ball moves at 10 m/s. Find its kinetic energy.
KE = ½mv² = ½×1×10² KE = ½×1×100 = 50 J
Flashcards
Quick quiz
Q1.A 4 kg object moves at 5 m/s. What is its kinetic energy?
Q2.If an object's velocity doubles, its kinetic energy…
Q3.What is the SI unit of kinetic energy?
Q4.A ball at rest (v = 0) has what kinetic energy?
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Common mistakes
Using KE = ½mv instead of ½mv². — Correct: Velocity must be squared: KE = ½mv².
Using weight (N) instead of mass (kg) in the formula. — Correct: KE uses mass in kilograms, not weight in newtons.
Forgetting the ½ factor. — Correct: Without ½, the result is exactly double the correct kinetic energy.
Confusing kinetic energy with momentum. — Correct: Momentum is p = mv (vector); kinetic energy is KE = ½mv² (scalar).
FAQ
What is kinetic energy?
Kinetic energy is the energy of motion — any object with mass and velocity has it, given by KE = ½mv².
What is the kinetic energy formula?
KE = ½mv², where m is mass in kilograms and v is velocity in metres per second, giving energy in joules.
How do you calculate kinetic energy?
Square the velocity, multiply by the mass, then multiply by 0.5. Example: a 2 kg object at 3 m/s has KE = ½×2×3² = 9 J.
What are examples of kinetic energy?
A moving car, a thrown ball, flowing water, wind turning a turbine, and a walking person all have kinetic energy.