Acinder block is sitting on a platform 20m high. it weighs 7.9kg. the block has potential energy. calculate it

The formula for potential energy is mass * gravity * height. With height given as 20m and weight given as 7.9kg, we can fill in 7.9 * gravity * 20. Gravity is 9.8m/s constant so 7.9 * 9.8 * 20 = potential energy. The result is 1549.45, with the unit of energy being joules. Final answer 1549.45 J

By definition, the potential energy is
 U = m * g * h
 Where
 m = mass
 g = accelecarion of gravity
 h = height
 Substituting
 U = (7.9) * (9.8) * (20) = 1548.4J
 The block has a potential energy of 1548.4J
 Note: assuming reference level on the ground

Indoraptor for sure

The figure is missing but we can answer anyway.the correct answer is "releasing the string". in fact, when the person is holding the bow and the arrow, the kinetic energy of the arrow is still zero (because the arrow is not moving). the moment the arrow is released, it acquires a velocity v, so its kinetic energy will become different from zero: where m is the mass of the arrow and v its velocity.  all the other options, instead, do not increase the kinetic energy of the system: -  pulling farther back on the string : this will just increase the elastic potential energy of the system-  holding the arrow higher: this will increase the gravitational potential energy of the system (which is given by u=mgh, where h is the height above the ground)-  decreasing the mass of the arrow: this will decrease the gravitational potential energy of the system (which is given by u=mgh, where m is the mass)