An object of mass 2000 g possesses 100 J kinetic energy. The object must be moving with a speed of
- A.10.0 m/s
- B.11.1 m/s
- C.11.2 m/s
- D.12.1 m/s
Correct answer
Explanation
The correct answer is A, 10.0 m/s. Using kinetic energy = ½mv² with the mass in kilograms gives a speed of exactly 10 m/s.
First convert the mass to SI units: 2000 g = 2 kg. Kinetic energy is KE = ½mv², so 100 = ½ × 2 × v², which gives v² = 100 and v = 10 m/s. The joule is the SI unit of energy, and 1 J equals 1 kg m²/s², so the mass must be in kilograms and the speed in metres per second for the formula to work. The common slip is to leave the mass in grams, which gives a meaningless answer.
B is wrong: 11.1 m/s would need a kinetic energy of about 123 J for a 2 kg body. C is wrong: 11.2 m/s would need about 125 J, and the figure is better known as the Earth's escape velocity in km/s. D is wrong: 12.1 m/s would need about 146 J.
Exam tip: always convert grams to kilograms before using KE = ½mv²; doubling the speed makes the kinetic energy four times.
Practice Questions
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