Question

Calculate the kinetic energy of a car of mass 750 kg moving with a velocity of 54 km per hour. Find the new kinetic energy of the car if a passenger of mass 50 kg sits in the car.

Answer 1

Step 1, Given data

Mass of the car, m = 750 kg

Velocity, v $=54kmph$ $=54\times \frac{5}{18}m/s=15m/s$

Mass of the passenger = 50 kg

Step 2, Finding the kinetic energy

We know that,

Kinetic energy, $KE=\frac{1}{2}m{v}^2$

Putting all the values
K.E $=\frac{1}{2}\times 750\times 15\times 15$
$=84375J$

After sitting a new passanger

Now, total mass, $m^{\prime }=750kg+50kg=800kg$

Velocity, $v=15m/s$ (same as above)

So, $K{E}^{\prime }=\frac{1}{2}{m}^{\prime }{v}^2$

Putting all the values
$=\frac{1}{2}\times 800\times 15\times 15$
$=90000J$

Hence the new kinetic energy is 90000 J