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Simple Uniform Acceleration Problem

  • Thread starter TheKovac
  • Start date
  • #1
24
0

Homework Statement


A bicycles accelerates from rest, covering 16 metres in 4 seconds. The total mass of the bicycle and its rider it 90kg. What is its average acceleration during this time?

Homework Equations


x = ut + 0.5at^2
v=xt
a=vt

The Attempt at a Solution


v = x/t
v = (16)/(4)
v = 4 m/s

F = mv/t
F = (90)(4)/4
Fnet = 90N

Fnet = ma
90 = 90a
A= 1 m/s^2 <--WRONG!! - How?
A=2ms <--- RIGHT
 

Answers and Replies

  • #2
rock.freak667
Homework Helper
6,230
31

Homework Statement


A bicycles accelerates from rest, covering 16 metres in 4 seconds. The total mass of the bicycle and its rider it 90kg. What is its average acceleration during this time?

Homework Equations


x = ut + 0.5at^2
v=xt
a=vt

The Attempt at a Solution


v = x/t
v = (16)/(4)
v = 4 m/s

F = mv/t
F = (90)(4)/4
Fnet = 90N

Fnet = ma
90 = 90a
A= 1 m/s^2 <--WRONG!! - How?
A=2ms <--- RIGHT
Why didn't you just use x = ut + 0.5at^2 ?
 
  • #3
alphysicist
Homework Helper
2,238
1

Homework Statement


A bicycles accelerates from rest, covering 16 metres in 4 seconds. The total mass of the bicycle and its rider it 90kg. What is its average acceleration during this time?

Homework Equations


x = ut + 0.5at^2
v=xt
a=vt
Your equations are not quite right here and they are leading to some problems. The second and third equations need to be:

[tex]
v_{\rm average} = \frac{x_f-x_i}{\Delta t}
[/tex]

[tex]
a_{\rm average} = \frac{v_f-v_i}{\Delta t}
[/tex]

The Attempt at a Solution


v = x/t
v = (16)/(4)
v = 4 m/s
This quantity you have calculated is the average velocity over the whole time period.

F = mv/t
F = (90)(4)/4
Fnet = 90N
Dividing the average velocity by the time here does not give you the average acceleration; you would need the beginning and ending velocities.


However, if you use your first equation:

[tex]
\Delta x = v_0 t + \frac{1}{2} a t^2
[/tex]

you should get the right answer immediately.
 
  • #4
1
0
You can't use a constant acceleration law - The question never said that the acceleration was constant, it wanted you to find the average acceleration.
Also:
average velocity = displacement / time interval
average acceleration = change in velocity / time interval

16/4 = average velocity
 

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