Question

A 50 kg boy riding on 120 kg motorbike starts moving at a speed of 40 km/h. How long to cover will it take a distance of 800m. what is the the engine avg force applied by of the motorbike?

Answer 1

With the average velocity we can calculate the time:

$v=\frac{d}{t} \implies t=\frac{d}{v}=\frac{0.800\, km}{40\,km/h}=0.02\,h=72\,s$

We use the kinetic energy of the total mass moving with the average velocity to find the average force:

$Fd=\frac{1}{2}m_Tv^2 \implies F=\cfrac{m_T}{2d}\times v^2$

Then we substitute and find

$F=\cfrac{m_T}{2d}\times v^2=\frac{(50+120)kg}{2(800\,m)}\times (\frac{40\,km}{h}\times\frac{10^3\,m}{1\,km}\times\frac{1\,h}{3600\,s})^2$

$\implies F=13.117\,N$

In conclusion, it will take 72 s for the boy to cover the distance, and the average force applied by the engine will be 13.12 N.

Reference:

• Sears, F. W., & Zemansky, M. W. (1973). University physics.