Answer to Question #290067 in Physics for Jurin Junior

Question

1. An object of mass 3kg is dropped from the roof of a two-storey house and reaches the ground with a speed 10 m/s. Neglecting friction, calculate:

i. The kinetic energy of the speed when it reaches to

the ground.

ii. The potentia energy of the object when it is on the

roof.

iii. The height of the roof

iv. The total mechanical energy of the object when it is

1 m above the ground.

2. A man who has a mass of 80 kg can run up a flight of stairs of 20 steps in 5 seconds. Given the each steps in 20cm high, how much power does the man use to run up the flights of stairs?

Answer 1

1 i. $KE_\text{max}=\dfrac12mv^2=150\text{ J}.$

1 ii. $PE_\text{max}=KE_\text{max}=150\text{ J}.$

1 iii. $h=PE_\text{max}/(mg)=5.1\text{ m}.$

1 iv. $ME=PE_\text{max}=KE_\text{max}=150\text{ J}.$

2. $P=W/t=mgH/t=mg(nh)/t,\\ P=80·9.8(20·0.2)/5=627.2\text{ W}.$