Question

Two sprinters A and B of mass 70kg ad 55kg respectively are running with the

same speed in the same direction. Which of the following is NOT true?
1) A has a greater Kinetic energy
Greater force is needed to stop A
The momentum of A is greater than B
iv)
B will finish the race before A
V B has greater kinetic energy
A. ii) and iv) only B. v) only C. iii), iv )and v) only D. iv) and v) only

Answer 1

Only ( IV and V).

A has a greater kinetic energy since the tow runners start with the same speed, in the same direction. K = 1/2mv^2, meaning that the greater value of m (70kg) will constitute for a greater kinetic energy if v is the same speed for both sprinters. I True.

The greater the kinetic energy, the greater the force will be needed to make the sprinter come to a complete stop. II True.

Momentum is defined as p=mv. If both sprinters begin with the same value of v, mv of A will be greater than mv of B, meaning that the momentum of A is greater than B. III True.

If they begin with the same speed, they will finish the race at the same time. Therefore, B will NOT finish the race before A. Statement IV false.

It was stated before that the kinetic energy of A is greater than the kinetic energy of B, so it makes statement V false.

The answer is D (only IV and V).