Physics: Children and bicycle?

[Cieline J]

A boy of mass 30kg is being given a lift on the back of a 10kg bicycle by a girl of mass 40kg. They are traveling at a steady speed of 2.5m/s.
The boy wishes to get off the back of the bicycle while it is still moving.

a) He knows that if he just puts his feet on the ground and stands up he is likely to fall over. Explain why this is so. ___________________
So instead he pushes himself off the back of the bicycle by pushing forward on the bicycle frame with his hands, so that he lands on the ground with zero horizontal velocity.

b) Calculate the velocity of the bicycle and the girls immediately after the boy has left the bicycle.

c) Calculate the total kinetic energy of the system (bicycle and both children) before and after the boy gets off. Explain the reason for any difference.

 

[N2O Addict]

a) The boy is travelling at 2.5m/s horizontal relative to the ground. When he gets off the back of the bike, the soles of his shoes hit the ground and experiences a large frictional force. This frictional force acts in the direction opposite the direction he was travelling in before. This friction slows the bottom of his body down, with the front still moving at 2.5m/s, he would flip and fall.

See the diagram I drew for this question:

b) Since the boy gave enough energy to reduce his velocity to zero, the energy he has used must be same as the kinetic energy he possessed on the bike.

Given that, the Bike and Girl must have the same amount of energy(because the boy didn't take away any) with a smaller mass, hence they are traveling faster!

Velocity of Bike and Girl = squareroot[2(1/2*80kg*2.5^2=250Joules)/(mass=50kg)]=3.16227m/s

c) Before boy got off:
Bike and girl and boy has mass of 80kg
At 2.5m/s, they had the Kinetic Energy = 1/2*80kg*2.5^2=250Joules

After boy got off,
Girl and Bike had mass of 50kg
At 3.16227m/s, they had Kinetic Energy = 1/2*50kg*3.16227^2=250Joules

No change in Kinetic energy, but a change in velocity due to reduced mass.

WOW LONG QUESTION HOPE IT'S HELPFUL TO YOUR LEARNING. Sorry for the sums if they look complicated, rewrite them on paper and they shouldn't. Keep Excelling!