Question

A motorcar of mass $1200kg$ is moving along a straight line with a uniform velocity of $90kmph$. Its velocity is slowed down to $18kmph$ in $4s$ by an unbalanced external force. Calculate the magnitude of the force required.

• A. $6000N$
• B. $10000N$
• C. $21600N$
• D. $77760N$

Answer 1

The correct option is A $6000N$

Mass of the motor car, $m=1200kg$

Initial velocity of the motor car,
$u=90km/h$
$u=90\times 1000/3600m/s$
$u=25m/s$

Final velocity of the motor car,
$v=18km/h$
$u=18\times 1000/3600m/s$
$v=5m/s$

Time taken, $t=4s$

Using equation of motion:
$v=u+at$
$5=25+4a$
$a=-5m/s^2$
Negative sign indicates that its a retarding motion i.e., velocity is decreasing.

Using second law of motion,
$F=ma$
$F=1200\times -5N$
$F=-6000N$
Negative force here is indicating that the force is opposing motion.