Calculate Unbalanced Force to Stop a Car in 3 Seconds

[zoezoehzoe]

Homework Statement

A car has a mass of 1350 kg and an initial speed of 21 m s-1. Calculate the magnitude of the unbalanced force applied to the brakes that caused the car to come to a halt in 3 seconds.

Homework Equations

Force = mass x acceleration,
= ( 1350 kg ) x ( -7 m s-2 ),
= -9450 kg m s-2
= -9450 N

My question is this, is it ok to write the answer as a negative value of Newtons, as above, because it doesn't seem right? I'm a little confused, any advice would be appreciated.

The Attempt at a Solution

 

[Doc Al]

My question is this, is it ok to write the answer as a negative value of Newtons, as above, because it doesn't seem right? I'm a little confused, any advice would be appreciated.

Magnitudes are always positive. In this case, the negative sign just indicates the direction--that the acceleration and force were opposite to the velocity.

 

[zoezoehzoe]

Magnitudes are always positive. In this case, the negative sign just indicates the direction--that the acceleration and force were opposite to the velocity.

so

Force = mass x acceleration,
= ( 1350 kg ) x ( -7 m s-2 ),
= -9450 kg m s-2
= 9450 N

would be correct?

 

[Doc Al]

Yes. Your answer would just be the magnitude--leave off any sign.

 

[zoezoehzoe]

Yes. Your answer would just be the magnitude--leave off any sign.

thank you very much for your help.