The hardest part of the job is to find the right formula to use, and write it down. You've already done that ! The rest is just turning the crank until an answer falls out.
You wrote. E = m g h.
Beautiful.
Now divide each side by (g h), and you'll have the formula for mass:
m = E / (g h).
You know all the numbers on the right side. Just pluggum in, do the arithmetic, and you'll have the mass.
Answer:
0.5mv^2=50, v=5, 25/2×m=50, m=50×2/25, So, the answer is 4
To find the area, we just need to multiply the length by the width. 200 x 75 = 15,000. The mall has an area of 15,000 sq. feet.
The position of the mass is given by (in cm):

The velocity is the derivative of the position:

Substituting t=0.40 s, we can find the velocity at this time: