Question

Beginner with physics and easily confused by word problems.. bad combo.
Problem: You see a ball roll out into the street while you are driving at 20 m/s. What is the total stopping distance if your reaction tim is 0.31s and your acceleration while stopping is -5.5m/s^2

I solved that and got distance being 36.4m. Yes? No?

My question is part II. How much total time elapses between when the ball rolls out in the street until your car is stopped?

Answer 1

You have forgotten the distance travelled during the reaction time.

Answer 2

Try using this formula:
\[S=\frac{ -u^2 }{ 2a }+ut\]where:
u=initial velocity
a=acceleration (or in this case deceleration)
t=the driver's reaction time.
S=distance travelled

Answer 3

for part II, you have the reaction time, 0.31s, now you need to work out the time when you're stopping
you could use:
\[t=\frac{ v-u }{ a }\]where:
t=stopping time
v=final velocity (which is 0 because you have stopped)
u=initial velocity
a=acceleration

add this answer to your reaction time to get your answer