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Work, energy and powerPhysicsHigh schoolabout 0:44

A 4 kg ball has 450 J of kinetic energy. How fast is it moving?: animated lesson

A narrated, step-by-step work, energy and power video lesson with captions. Final answer: speed v = 15 m/s.

SpeedLanguage

Video with a synthetic English voice and captions. Pick another language for subtitles in that language, or open the interactive version.

Video narrated by a synthetic voice (Piper TTS, en_US-ljspeech-high voice (LJ Speech dataset, public domain)); the steps come from our maths engine and the answer was checked independently. Download MP4 (0.3 MB) · Captions (WebVTT)

Steps on screen

1. The problem
Worked example
A 4 kg ball has 450 J of kinetic energy. How fast is it moving?
2. Step by step
1. $KE = \tfrac12mv^2$
  Formula: kinetic energy
2. $v = \sqrt{\frac{KE}{\tfrac12\,m}}$
  Rearrange for speed v
3. $v = \sqrt{\frac{(450)}{\tfrac12\,(4)}} = 15\ \text{m/s}$
  Substitute the known values
3. The answer
$\text{speed v = 15 m/s}$
speed v = 15 m/s
4. Check
- Substituted every value back into the formula: both sides agree ✓

Transcript

The problem. Here is the problem: A 4 kg ball has 450 J of kinetic energy. How fast is it moving? Use the kinetic energy formula, rearranged for the unknown, in SI units.

Step by step. Formula: kinetic energy. Rearrange for speed v. Take the positive square root (a magnitude). Substitute the known values.

The answer. So the answer is speed v equals 15 m/s. Kinetic energy grows with the square of speed: doubling v multiplies KE by four. It is never negative, and the speed found from KE is a magnitude.

Check. Substituted every value back into the formula: both sides agree, which checks out.

About work, energy and power

Work done by a force, kinetic and potential energy, and power.

A 4 kg ball has 450 J of kinetic energy. How fast is it…