Question

What average force is required to stop a 980 kg car in 9.0 s if the car is traveling at 91 km/h ?

Answer 1

Newton's second law of motion:
F=ma
$a = \frac{v}{t}$
You convert from km/h to m/s by dividing by 3.6:
$v = 91 \div 3.6$
$v = 25.3 \: \frac{m}{s}$
Then a is:
$a = \frac{25.3}{9}$
$a = 2.8 \frac{m}{s {}^{2} }$
Then:
F=(980)(2.8)=2744 N