Kinetic energy:

Energy that a body has when it is in motion.

Kinetic energy KE = \frac{1}{2} mv^2 . where, m - mass of the body and v - body velocity.

The kinetic energy formula is used to measure the body's mass, velocity, or kinetic energy if any of the other two numerics is given.

Step 1: Using the kinetic energy formula, calculate the object's mass.

Given that

Speed of the object, v=21\ m/s

Kinetic energy of the object, KE=140 joules

KE = \frac{1}{2} m v^2     

\therefore Mass m = \frac{2KE}{v^2}

Insert the values in the equation above

m = \frac{2*140 J}{(21 m/s)^2}

m = 0.634 kg

Hence, mass of the object, m = 0.634 kg