Application of conservation of mechanical energy for free fall motion – problems and solutions

1. A 1-kg body falls freely from rest, from a height of 80 m. Acceleration due to gravity is 10 m/s2. What is the kinetic energy when the body hits the ground.

Known :

Mass (m) = 1 kg

Height (h) = 80 m

Acceleration due to gravity (g) = 10 m/s2

Wanted: kinetic energy when the body hits the ground

Solution :

The initial mechanical energy (MEo) = gravitational potential energy (PE)

MEo = PE = m g h = (1)(10)(80) = 800 Joule

The final mechanical energy (MEt) = kinetic energy (KE)

The principle of conservation of mechanical energy :

MEo = MEt

PE = KE

800 = KE

The final kinetic energy is 800 Joule.

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2. A 4-kg body free fall from rest, from a height of 10 m. Acceleration due to gravity is 10 m s–2. What is the kinetic energy and the velocity at 5 meters above the ground.

Known :

The change in height (h) = 10 – 5 = 5 meters

Mass (m) = 4 kg

Acceleration due to gravity (g) = 10 m/s2

Wanted: Kinetic energy at 5 meters above the ground and the velocity at 5 meters above the ground

Solution :

(a) Kinetic energy at 5 meters above the ground

The initial mechanical energy (MEo) = the gravitational potential energy (PE)

MEo = PE = m g h = (4)(10)(5) = 200 Joule

The final mechanical energy (EMt) = kinetic energy (EK)

MEt = KE

The principle of conservation of mechanical energy states that the initial mechanical energy = the final mechanical energy.

MEo = MEt

200 = KE

Kinetic energy at 5 meters above the ground is 200 Joule.

(b) velocity at 5 meters above the ground

The initial mechanical energy (MEo) = the final mechanical energy (MEt)

PE = KE

200 = ½ m v2

2(200) / 4 = v2

100 = v2

v = √100

v = 10 m/s

Body’s velocity at 5 meters above the ground is 10 m/s.

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3. A mango falls freely from rest, from a height of 2 meters. Acceleration due to gravity is 10 m s–2. Determine mango’s velocity when hits the ground.

Known :

Height (h) = 2 meters

Acceleration due to gravity (g) = 10 m/s2

Wanted : mango’s velocity when hits the ground.

Solution :

The initial mechanical energy (MEo) = the gravitational potential energy (PE)

ME = PE = m g h = m (10)(2) = 20 m

The final mechanical energy (MEt) = the kinetic energy (KE)

MEt = KE = ½ m v2

Principle of conservation of mechanical energy states that the initial mechanical energy = the final mechanical energy.

MEo = MEt

20 m = ½ m v2

20 = ½ v2

2(20) = v2

40 = v2

v = √40 = √(4)(10) = 2√10 m/s

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