Answer:
0.635 kg
Step-by-step explanation:
Recall that Kinetic energy is defined as:
K.E. = (1/2) mv²,
where
K.E = Kinetic energy = given as 140J
m = mass (we are asked to find this)
v = velocity = given as 21 m/s
Substituting the above values into the equation:
K.E. = (1/2) mv²
140 = (1/2) m (21)² (rearranging)
m = (140)(2) / (21)²
m = 0.635 kg