Question

An unruly demonstrator lifts a stone of mass 200 g from the ground and throws it at his opponent. At the time of projection, the stone is 150 cm above the ground and has a speed of 3 m/s. Calculate the work done by the demonstrator during the process. If it takes one second for the demonstrator to lift the stone and throw it, what horsepower does he use?

Answer 1

P =  5.14 × 10⁻³ hp is the work done by the demonstrator during the process.

Explanation:

Step 1

Given values  

The stone's mass m = 200 g = 0.2 kg.

Elevation from the ground, h = 150 cm = 1.5 m

speed = 3 m/s

time = 1 sec.

Now work to lift the stone against gravity = mgh

Step 2:

g is defined as gravity acceleration. The Earth value is 9.8 m / s2. In other words, the earth's surface grav-ity acceleration at sea level is 9.8 m / s2.

= 0.2 × 9.8 × 1.5

= 1.96 × 1.5

= 2.94 J.

Step 3:

Thus the Kinetic Energy of the stone = 1/2 m$v^2$

= 1/2 × 0.2 × $3^2$

= 1/2 × 0.2 × 9

= 1/2 × 1.8

= 0.90 J.

Step 4:

Maximum demonstration work = Work done against gravity + stone K.E.

= 2.94 + 0.90  

= 3.84 J.

Now power by demonstrator = work/time

= W/t

P =   3.84/1

P = 384 W.

384 W Changing  in  hp  

= 384/746     [ 1 hp = 746 W]

= 0.00514

P =  5.14 × 10⁻³ hp.