Question

A puppy weighing 3 kilograms races through the dog park. If she slows from a speed of 2 meters/second to 1 meter/second, what will happen to her kinetic energy?

Answer 1

Answer: Kinetic energy will be decreases by 4.5 Joules.

Explanation:

Kinetic energy is the energy which is possessed by the virtue of objects motion. It is defined as the product of object's mass and square of the objects's velocity.

Mathematically,

$K=\frac{1}{2}mv^2$

Change in kinetic energy will be: $K=\frac{1}[2}m\Delta v^2$

where,

m = mass of the object = 3 kg

$\Delta v=v_2-v_1$ = Change in velocity

$v_1=2m/s\\v=1m/s$

Now, calculating the change in kinetic energy:

$K.E.=\frac{1}{2}\times 3kg\times (1^2-2^2)m^2/s^2=-4.5J$

Negative sign indicates that the kinetic energy is getting reduced.

Hence, kinetic energy will be decreases by 4.5 Joules.

Answer 2

Ans: Thus, the kinetic energy change = 4.5 J

The mass of puppy = 3kg

Kinetic energy initial = $\frac{1}{2} mv^{2}$
                                   = $\frac{1}{2}*3*4 = 6 kg-m/s2$

Kinetic energy final = $\frac{1}{2} mv^{2} = \frac{1}{2}*3*1 = 1.5 kg-m/s2$

Thus, the kinetic energy change = 4.5 J