Question

5. A man of mass 70 kg and a boy of mass 35 kg are standing together (not moving) on a smooth ice surface for which friction is negligible. They push each other apart and the man moves away with speed of 0.3 m/s relative to the ice.

(a) What is their total momentum before pushing each other?

(b) Find the velocity of the boy after pushing

(c) Find the final kinetic energy of the man-boy system (= total kinetic energy of man and boy after pushing). Is kinetic energy conserved from this motion?

Answer 1

Given mass of man, m_m = 70 kg

mass of boy , m_b = 35kg

initial velocity of both man and boy is zero

final velocity of man ,v_m = 0.3m/s

(a) Since both man and the boy was standing still total momentum before pushing each other is ZERO

ANSWER = 0

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(b)Final velocity of boy

Here momentum is Conserved

so P_initial =P_Final

ANSWER:Final velocity of boy after pushing   opposite to mans' direction

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(c)Kinetic energy of whole system

ANSWER:

No Kinetic energy is not conserved ,because before pushing KE is zero

we can say total energy is conserved ,while pushing some kind of work is done which is converted into KE.The situation is similar to a compressed spring pushing two bodies apart.Energy stored in the compressed spring is converted into KE of bodies