Calculate Unbalanced Force to Stop a Car in 3 Seconds

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To calculate the unbalanced force needed to stop a car with a mass of 1350 kg traveling at 21 m/s in 3 seconds, the force is determined using the equation Force = mass x acceleration. The calculated acceleration is -7 m/s², leading to a force of -9450 N. It is clarified that the negative sign indicates direction, and the answer should be expressed as a positive magnitude of 9450 N. The final consensus is that only the magnitude should be reported, omitting the negative sign. This approach ensures clarity in presenting the force value.
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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

 
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zoezoehzoe said:
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.
 
Doc Al said:
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?
 
Yes. Your answer would just be the magnitude--leave off any sign.
 
Doc Al said:
Yes. Your answer would just be the magnitude--leave off any sign.

thank you very much for your help.
 
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