Questions: Kinetic Energy: KE = ½mv²

Question 1

A 2 kg ball moves at 3 m/s. What is its kinetic energy?

• A. 6 J
• B. 9 J
• C. 3 J
• D. 18 J

Answer: (B)

KE = ½mv² = ½ × 2 × 3² = ½ × 2 × 9 = 9 J.

Question 2

If you triple the speed of an object, its kinetic energy increases by a factor of nine.

• T. True
• F. False

Answer: True

KE depends on v². Tripling v means the new KE = ½m(3v)² = ½m × 9v² = 9 × the original KE.

Question 3

Why is a car crash at 60 mph so much more destructive than a crash at 30 mph?

Model answer: Because kinetic energy depends on speed squared. Doubling the speed quadruples the kinetic energy, so there is four times as much energy to be absorbed in the crash.

At 60 mph (double 30 mph), the kinetic energy is 2² = 4 times greater. That extra energy causes far more damage to the vehicles and occupants.