Finding the speed of a bicycle

[drwillis]

Homework Statement

Two physics students are trying to detemine the
instantaneous speed of a bicycle 5.0 m from the
start of a 1000 m sprint. It takes 4s to travel 10m. What is the instananeous speed at the 5m mark? acceleration is constant.
(5m mark is 10 m as they start at -5m)
X=Distance
U=initial speed (U= 0m/s)
Note. This is the first thing I've tried since joining year 11 so i am quite a novice and may have just made a stupid error

Homework Equations

X=ut+1/2at^2
V=U+at

The Attempt at a Solution

x=ut+1/2at^2
10=(1/2).(a).(16)
10=8.a
a=1.25

V=u+at
V=(1.25)(4)
V=5m/s

But the answers say it 3.5m/s.

 

[zm500]

Your acceleration is correct and you used the equation correctly except for the second part. The time t=4s is for 10m displaced not for 5m. So, let's look at the given:

a = 1.25 m/s^2
Vo = 0
Vf = ?
displacement = 5m.

Note: You have three given, so you should be able to solve for Vf (instantaneous speed).

 

[abhishek ghos]

you are right