# What is the kinetic energy of an object with a mass of  5 kg that has been in freefall for  2 s?

Apr 15, 2018

960.4 J

#### Explanation:

The formula of Kinetic energy is $\frac{1}{2} m {v}^{2}$ where m is mass and v is velocity. This simply means that a mass m moving with a velocity v has kinetic energy $\frac{1}{2} m {v}^{2}$.

We know mass, so lets find velocity. It is given that it has been falling for two seconds. So its velocity $= a \times t$ .In this case the acceleration is caused due to gravity and hence acceleration is 9.8 meters per second squared.

Plugging it into the equation, if it has been falling for 2 seconds, then its velocity is $9.8 \times 2 = 19.6$ meters per second

Now since we have velocity, we can find Kinetic energy by simply putting the values of mass and velocity in the first equation

K.E.=$\frac{1}{2} \times 5 \times {19.6}^{2}$= 960.4 J

Apr 15, 2018

$960.4$ joules

#### Explanation:

Well, kinetic energy is defined through the equation,

$\text{KE} = \frac{1}{2} m {v}^{2}$

• $m$ is the mass of the object in kilograms

• $v$ is the velocity of the object in meters per second

The velocity of the free-fall object is defined through the equation,

$v = u + a t$

• $u$ is the initial velocity

• $a$ is the acceleration of the object

• $t$ is the time in seconds

In this case, $a = g = 9.8 \setminus {\text{m/s}}^{2}$, assuming air resistance is negligible.

If the object was dropped from something, then $u = 0$, and so:

$v = 0 + 9.8 \setminus \text{m/s"^2*2 \ "s}$

$= 19.6 \setminus \text{m/s}$

Therefore, the kinetic energy is:

"KE"=1/2*5 \ "kg"*(19.6 \ "m/s")^2

$= 2.5 \setminus {\text{kg"*384.16 \ "m"^2"/s}}^{2}$

$= 960.4 \setminus {\text{kg m"^2"/s}}^{2}$

$= 960.4 \setminus \text{J}$