Question

4 Starting from rest on a level road a girl can reach a speed of 5 m/s in 10s on her bicycle. a Find the acceleration b Find the average speed during the 10s. c Find the distance she travels in 10 s. [2] Eventually, even though she still pedals as fast as she can, she stops accelerating and her speed reaches a maximum value. Explain in terms of the forces acting why this happens. ​

Answer 1

a. To find the acceleration, we can use the equation:

acceleration = change in velocity / time taken

Here, the initial velocity is 0 m/s (since she starts from rest), the final velocity is 5 m/s, and the time taken is 10 s. Therefore:

acceleration = (5 m/s - 0 m/s) / 10 s = 0.5 m/s^2

So the acceleration of the girl on her bicycle is 0.5 m/s^2.

b. The average speed during the 10 s can be found by dividing the total distance traveled by the time taken. We don't know the distance traveled yet, so we can use another equation:

distance = (initial velocity x time taken) + (0.5 x acceleration x time taken^2)

Here, the initial velocity is 0 m/s, and the time taken is 10 s. We already calculated the acceleration in part (a) as 0.5 m/s^2. Plugging these values in, we get:

distance = (0 m/s x 10 s) + (0.5 x 0.5 m/s^2 x (10 s)^2) = 25 meters

So the girl traveled 25 meters in 10 seconds.

c. As the girl pedals, she applies force to the pedals, which in turn transfers the force to the rear wheel. This force drives the bicycle forward, causing it to gain speed. However, when the bicycle picks up speed, air resistance (also called drag) comes into play, which increases. Eventually, the force of air resistance becomes equal and opposite to the force that propels the bicycle forward. This means that the net force on the bicycle becomes zero, stopping its acceleration and reaching its maximum speed, known as the terminal velocity. Even if the girl pedals faster, she won't be able to increase her speed because the drag force counteracts the propelling force.