Question

A) Megan was doing time-trials on her bike around 400 metre horizontal track.


ii) Compare the forward force on the bike with the backward force on the bike when Megan was travelling at a constant speed.


B) Megan then crouched down over the handlebars to make herself more streamlined.

She continued to pedal with the same force as before.


i) Compare the forward and backward forces on Megan and her bike now.


ii) Explain your answer.

Answer 1

A) The forward force is equal to the backward force

In this problem:

- the forward force is the force that Megan applies to the pedal to go forward

- the backward force is due to the air resistance and the friction between the wheels and the track

In this case, Megan is travelling at constant speed. This means that her acceleration is zero:

a = 0

According to Newton's second law, the resultant of the forces acting on Megan is equal to the product between mass (m) and acceleration (a):

$\sum F = ma$

However, a = 0, so the resultant of the forces is also zero:

$\sum F =0$

and this implies that the forward force and the backward force are equal in magnitude and opposite in direction.

B) The forward force is larger than the backward force

In this case, Megan crouched down in order to make herself more streamlined. As a result, the air resistance acting on Megan will decrease: so, the backward force will decrease, and therefore the forward force (which has remained the same) will be larger than the backward force.

So, the resultant force

$\sum F$

will be no longer zero, and therefore the acceleration will be different from zero, which means that Megan will increase her speed.