Biking Along
Two bicyclists, A and B, are on a straight road. Biker A, initially at rest, starts accelerating with constant acceleration $a_A=1\ m/s^2$ at $t=0$ until she reaches a speed $v=5\ m/s$. She then cruises at constant speed $v=5\ m/s$. Biker B is initially moving at $12\ m/s$ and is $10\ m$ ahead of biker A. At $t=0$, biker B begins to brake and slows down to a stop with a constant deceleration $a_B=-2\ m/s^2$.
Using graphs only,
1. Determine the distance traveled by biker A when she reaches a speed $v=5\ m/s$.
View answerHide answerWe start by graphing the velocity of biker A and biker B on the graph below:
Biker A reaches a speed of $1\ m/s$ at time $t=5\ s$ and the distance $d_A$ she traveled in that time is equal to the area under the (red) curve $v_A\left(t\right)$ between $t=0$ and $t=5$.
Thus,we conclude that when she reaches a speed $v=5\ m/s$, biker A will have traveled a distance equal to
\boxed{d_A=\frac{5\cdot 5}{2}=12.5\ m}
2. Find when both bikers have the same velocity. What distance has biker B traveled in that time?
View answerHide answerBoth bikers have the same velocity when the two curves intersect i.e. when $t=4\ s$.
3. Find how long it takes biker B to stop.
View answerHide answerBiker B stops when its velocity is equal to zero which occurs at $t=6\ s$.
4. Derive what the acceleration $a_B$ of biker B must be equal to for both bikers to a reach a speed of $5\ m/s$ at the same time.
View answerHide answerFor the bikers to reach a speed of $5\ m/s$ at the same time, the velocities curves must intersect at point $\left(5,5\right)$ as shown in the graph below:
The acceleration of biker B is given by the slope of the new curve which goes through points $\left(0,12\right)$ and $\left(5,5\right)$. The new acceleration of biker B which allows him to reach a speed of $5\ m/s$ at the same time as biker A is equal to
\boxed{a_B=\frac{12-5}{0-5}=-\frac{7}{5}=-1.4\ m/s^2}