Question

1. A 50-kg barbell is lifted at a height of 1.4m from the floor in 1.2 seconds. What is the work done? how much power is expended?

2. A 60 kg block at rest was pushed by a 100 N force on a frictionless horizontal.

A. what is the kinetic energy of the block at a distance of 8 meters?

B. what is the velocity of the blockat a distance of 8 meters?

Answer 1

Given:

$m=50\:\rm kg$

$h=1.4\:\rm m$

$t=1.2\;\rm s$

The work done

$$W=mgh=\rm 50*9.8*1.4=686\: J$$

The amount of power

$$P=\frac{W}{t}=\frac{686\:\rm J}{1.2\:\rm s}=572\: \rm W$$

Given:

$m=60\:\rm kg$

$F=100\:\rm N$

$d=8\;\rm m$

The change in kinetic energy is equal to work done

$$\frac{mv^2}{2}=W=Fd=\rm 100*8=800\: J$$

The velocity

$$v=\sqrt{\frac{2W}{m}}=\sqrt{\frac{2*800}{60}}=5.2\: \rm m/s$$