# Final speed with mass, initial velocity, force, displacement

## Homework Statement

A 1000kg car is moving at 25ms when a net force of 200N acts to slow the car. This force acts while the car moves through a distance of 300m. Find the new speed.

## Homework Equations

Kinetic Energy equation
v2=u2+2as

## The Attempt at a Solution

Attempted the use of Kinetic Energy equation and v2=u2+2as
However the answer I get is 27.4 when the correct answer is 22.5

Last edited:

SteamKing
Staff Emeritus
Homework Helper

## Homework Statement

A 1000kg car is moving at 25ms when a net force of 200N acts to slow the car. This force acts while the car moves through a distance of 300m. Find the new speed.

Unknown

## The Attempt at a Solution

Attempted the use of Kinetic Energy equation and v2=u2+2as
However the answer I get is 27.4 when the correct answer is 22.5
Why don't you show us your calculations?

ehild
Homework Helper

## Homework Statement

A 1000kg car is moving at 25ms when a net force of 200N acts to slow the car. This force acts while the car moves through a distance of 300m. Find the new speed.

Unknown

## The Attempt at a Solution

Attempted the use of Kinetic Energy equation and v2=u2+2as
However the answer I get is 27.4 when the correct answer is 22.5
Are the relevant equations unknown to you? How did you solve the problem then?
You result is correct with the data you gave. Check if they are the same given in the original text. And do not forget the unit next time. What is the unit of the speed? Is it really ms?

Why don't you show us your calculations?
W=Fd
=200*300
=60000
60000=½*1000*v2-½*1000*252
=500v2-312500
372500 = 500v2
v2=745
v=27.3
Are the relevant equations unknown to you? How did you solve the problem then?
You result is correct with the data you gave. Check if they are the same given in the original text. And do not forget the unit next time. What is the unit of the speed? Is it really ms?
Yea see for yourself, its a multiple choice question. And I changed the original post so it makes some sense.

#### Attachments

• 12.3 KB · Views: 428
SteamKing
Staff Emeritus
Homework Helper

## Homework Statement

A 1000kg car is moving at 25ms when a net force of 200N acts to slow the car. This force acts while the car moves through a distance of 300m. Find the new speed.

## Homework Equations

Kinetic Energy equation
v2=u2+2as

## The Attempt at a Solution

Attempted the use of Kinetic Energy equation and v2=u2+2as
However the answer I get is 27.4 when the correct answer is 22.5
The force of 200 N is acting to slow the car, which is traveling initially at 25 m/s. If the force is acting to slow the car, it makes little sense to say that the car is going to speed up to 27.4 m/s. Care to try again? I suggest you use F = ma here, in addition to v2=u2+2as

The force of 200 N is acting to slow the car, which is traveling initially at 25 m/s. If the force is acting to slow the car, it makes little sense to say that the car is going to speed up to 27.4 m/s. Care to try again? I suggest you use F = ma here, in addition to v2=u2+2as
Thanks, I managed to get the answer, all I done was
27.5-25=2.5
25-2.5=22.5

ehild
Homework Helper
W=Fd
=200*300
=60000
60000=½*1000*v2-½*1000*252
=500v2-312500
372500 = 500v2
v2=745
v=27.3

Yea see for yourself, its a multiple choice question. And I changed the original post so it makes some sense.

As the car slows down, the acceleration is negative. The force is opposite to velocity and displacement, so the work W is negative. You should have written W=Fd=-200*300= -60000 = 1/2mv2-1/2 m u2.

You changed the original post by changing the unit of speed from ms-1 to ms. Is it correct?

The answer is correct as ms-1