Simple motion prob.

1. Dec 7, 2006

student12phys

1. The problem statement, all variables and given/known data

Two physics students are trying to determine the instantaneous speed of a bicycle 5m from the start of a 1000m sprint. They use a stopwatch to measure the time taken for the bicyble to cover the first 10m. If the acceleration was constant, and the measured time was 4sec, what was the instantaneous speed of the bicyle at 5m mark?

2. Relevant equations

3. The attempt at a solution

Method 1:
Find acceleration:
a=change in velocity/change in time
=(displacement/time) /time
= displacement/time^2
= (10-0)/(4-0)^2
=.625m/s^2
Find instantaneous velocity at 5sec:
5 x .625 = 3.125m/s

BUt the answer is wrong. WHy?

2. Dec 8, 2006

AlephZero

Accel = change in velocity / change in time - that's right (equation 1).
Velocity = change in distance / change in time - that's also right.

But when you say "velocty = (10-0)/4" you are finding the AVERAGE velocity over the 4 sec, not the FINAL velocity that you need in equation 1.

The correct equation for constant acceleration is starting from rest is displacement = 1/2 a t^2 Check back to your textbook or notes on motion with constant acceleration.