# Average Acceleration and Time Taken to accelerate

• AutumnBeds
In summary: The Attempt at a Solution [/B]1602=20SUP]2[/SUP]2a3001602-202 / 2 * 300 = aa=42 m/s2You could first try solving the problem where the initial and final speeds are ##20m/s## and ##160 m/s## and see what answer you get.1602=20SUP]2[/SUP]2a3001602-202 / 2 * 300 = aa=42 m/s2
AutumnBeds
Hi, I've been asked to solve below as part of an assignment. Below is pretty much what I've submitted but it has been concluded to be incorrect. I am unsure as to why hence the question. Is it due to the fact that I have not taken the mass or forces into consideration?

I have only taken these into consideration for questions (iii) and (iv) as shown below.

Any is greatly appreciated.

1. Homework Statement

A racing car of mass 850 kg accelerates from 20 km/h to 160 km/h over a distance of 300 metres. Frictional forces and wind resistance can be assumed to be 800N.

Determine the following:

(i)The average acceleration

(ii)The time taken to accelerate from 20 km/h to 160 km/h

(iii)The tractive force produced by the car to provide this acceleration - not required for this thread

(iv)The car finally reaches a speed of 250km/h. Friction and wind resistance are 1800 N at this speed. What power output is required to maintain this speed? - not required for this thread

## Homework Equations

(i) v2=u22as

(ii) s= 1/2(u+v)t

## The Attempt at a Solution

[/B]
1602=20SUP]2[/SUP]2a300

1602-202 / 2 * 300 = a

a=42 m/s2

(ii) 300= 1/2(160+20)t

t= 300/90

t= 3.3s

Last edited:
AutumnBeds said:
Hi, I've been asked to solve below as part of an assignment. Below is pretty much what I've submitted but it has been concluded to be incorrect. I am unsure as to why hence the question. Is it due to the fact that I have not taken the mass or forces into consideration?

I have only taken these into consideration for questions (iii) and (iv) as shown below.

Any is greatly appreciated.

1. Homework Statement

A racing car of mass 850 kg accelerates from 20 km/h to 160 km/h over a distance of 300 metres. Frictional forces and wind resistance can be assumed to be 800N.

Determine the following:

(i)The average acceleration

(ii)The time taken to accelerate from 20 km/h to 160 km/h

(iii)The tractive force produced by the car to provide this acceleration - not required for this thread

(iv)The car finally reaches a speed of 250km/h. Friction and wind resistance are 1800 N at this speed. What power output is required to maintain this speed? - not required for this thread

## Homework Equations

(i) v2=uSUP]2[/SUP]2as

(ii) s= 1/2(u+v)t

## The Attempt at a Solution

[/B]
1602=20SUP]2[/SUP]2a300

1602-202 / 2 * 300 = a

a=42 m/s2

(ii) 300= 1/2(160+20)t

t= 300/90

t= 3.3s

You could first try solving the problem where the initial and final speeds are ##20m/s## and ##160 m/s## and see what answer you get.

AutumnBeds said:
The average acceleration
Second time today I have come across this blunder in a question.
There is not enough information to determine average acceleration. If you were told the acceleration is constant then you could find it from the two speeds and the distance, but if it is not then you need to know the time. For further discussion see section 3 of https://www.physicsforums.com/insights/frequently-made-errors-mechanics-forces/. (The discussion there is in respect of average force, but it apples equally to average acceleration.)
AutumnBeds said:
v2=uSUP]2[/SUP]2as
There is a mistake in that equation. I would have thought it just a typo, but you seem to have taken it to be correct.

PeroK said:
You could first try solving the problem where the initial and final speeds are ##20m/s## and ##160 m/s## and see what answer you get.
Thanks. I think this was the answer I was looking for. The comments on the work said Velocities require to be in m/s. Was a simple case of converting km/h to m/s.

## What is average acceleration?

Average acceleration is the rate of change of an object's velocity over a certain period of time. It is calculated by dividing the change in velocity by the time taken to accelerate.

## How is average acceleration measured?

Average acceleration is typically measured in meters per second squared (m/s²) or in other units of distance over time squared. It can also be measured using a tool called an accelerometer, which measures the change in an object's velocity.

## What does a positive average acceleration mean?

A positive average acceleration means that an object's velocity is increasing. This could mean that the object is speeding up, or that it is moving in a positive direction.

## What does a negative average acceleration mean?

A negative average acceleration means that an object's velocity is decreasing. This could mean that the object is slowing down, or that it is moving in a negative direction.

## How does time taken to accelerate affect average acceleration?

The longer the time taken to accelerate, the smaller the average acceleration will be. This is because the change in velocity over a longer period of time is smaller compared to a shorter period of time. Similarly, a shorter time taken to accelerate will result in a larger average acceleration.

• Introductory Physics Homework Help
Replies
20
Views
1K
• Introductory Physics Homework Help
Replies
3
Views
2K
• Introductory Physics Homework Help
Replies
3
Views
870
• Introductory Physics Homework Help
Replies
18
Views
3K
• Introductory Physics Homework Help
Replies
5
Views
1K
• Introductory Physics Homework Help
Replies
6
Views
1K
• Introductory Physics Homework Help
Replies
3
Views
1K
• Engineering and Comp Sci Homework Help
Replies
40
Views
4K
• Introductory Physics Homework Help
Replies
13
Views
20K
• General Math
Replies
2
Views
1K