How do I calculate the magnitude of initial acceleration?

[sarahin3d]

Homework Statement

A system consists of a bicycle and a cyclist traveling at a constant velocity along a horizontal road.

The total resistive force acting on the system in 4.0N and its speed is 8.0 m/s. Calculate the useful power output of the cyclist.

The cyclist stops pedalling and the system comes to rest. The total mass of the system is 70kg.
i) Calculate the magnitude of the initial acceleration of the system.

Homework Equations

P=Fv
s=ut+1/2at

The Attempt at a Solution

Alright so I used P=Fv to find the useful power output and I think I got the answer to that right.
P=Fv
P=(40)(8)
P=320 W

I don't know how to calculate the magnitude of the intial acceleration of the system. I would think that since the velocity is constant that the acceleration would have a magnitude of 0. This question is a 2 mark problem so I don't think that that answer would be right.

 

[gneill]

I don't know how to calculate the magnitude of the intial acceleration of the system. I would think that since the velocity is constant that the acceleration would have a magnitude of 0. This question is a 2 mark problem so I don't think that that answer would be right.

Hi sarahin3d, welcome to Physics Forums.

The velocity was constant while he was peddling. But for this part of the question it says that the cyclist stops peddling.

 

[arst]

Hi

use the formula of force

F=ma

F= 40 N
m=70 kg
a=?

40=70 a
a= 40/70 ==> 0.57ms-2

 

[haruspex]

while he was peddling

What was he selling?

 

[gneill]

What was he selling?

Ha! Oops, that was a brainfart typo!

 

[haruspex]

force acting on the system in 4.0N

P=(40)(8)

Is it 4N or 40N?

 

[arst]

Is it 4N or 40N?

it should be 40

 

[haruspex]

it should be 40

How do you know? It was posted by @sarahin3d, who appears no longer to be a member. Have you seen the source of the question?

 

[arst]

How do you know? It was posted by @sarahin3d, who appears no longer to be a member. Have you seen the source of the question?

Yeah, I have the same source of question. it is 40 Newtons

 

[haruspex]

Yeah, I have the same source of question. it is 40 Newtons

Ok, thanks. But probably should answer as signed: -0.57m/s².