# Fisika Study Center

## 8 Common Problems of Energy - Junior Physics

physics.fisikastudycenter.com - Learning gravitational energy, kinetic energy and mechanical energy with problems and solutions. This discussion is prepared for grade 7, 8 and 9 of Junor high school students.

Problem 1

A block of 40 kg of mass travels with a speed of 10 m/s. What is the kinetic energy of this block!

Answer

The kinetic energy of a body with mass m and speed v :

Ek = 1/2 m v2
Ek = 1/2 x 40 x 102
Ek = 2000 joule

Problem 2

A bicycle of 40 kg travels with a kinetic energy of 70 J. Determine the speed of the bicycle!

Answer

Ek = 1/2 m v2
720 = 1/2 x 40 x v2
720 = 20 x v2
720 / 20 = v2
36 = v2
v = √36 = 6 m/s

Problem 3

A ball travels with a speed of 8 m/s and its kinetic energy is 128 joule. Find the mass of the ball!

Answer

Ek = 1/2 m v2
128 = 1/2 x m x 82
128 = 1/2 x m x 64
128 = 32 x m
m = 128 /32 = 4 kg

Problem 4

A mango of 0.5 kg of mass free falls from a height of 2 m above the ground. What is the gravitational potential energy of the mango at its initial position?

Answer

The gravitational potential energy of a body:
Ep = m x g x h
Ep = 0.5 x 10 x 2
Ep = 10 joule

Problem 5

A block is at rest at a heigh of 5 m above the ground. The gravitational potential energy of the block is 2500 joule. Find the masss of the block use the acceleration due to gravity g = 10 m/s2!

Answer

Ep = m x g x h
2500 = m x 10 x 5
2500 = 50 m
m = 2500 / 50
m = 5 kg

Problem 6

A bird flies at a height of 10 m with a constant speed of 10 m/s. The bird mass is 2 kg. Find
a) kinetic energy f the bird
b) gravitational potential energy of the bird
c) mechanical energy of the bird

Answer
a) Ek = 1/2 mv2
Ek = 1/2 x 2 x 102
Ek = 100 joule

b) Ep = m g h
Ep = 2 x 10 x 10
Ep = 200 joule

c) EM = Ep + Ek
EM = 200 + 100
EM = 300 joule

Problem 7

A body has kinetic energy of 5000 Joule when its speed is v. Find the kinetic energy of the body when its speed is 0.5 v!

Answer

From kinetic energy
Ek = 1/2 m v2
Ek2 : Ek1 = 1/2 m v22 : 1/2 m v12
Ek2 : Ek1 = v22 : v12
Ek2 = (v2 / v1)2 x Ek1

So then
Ek2 = (0,5 v / v)2 x 5000 joule
Ek2 = 1/4 x 5000 joule
Ek2 = 1250 joule

Problem 8

Given two bodies with masses of m and 3m and speeds of v and 2v. Find the ratio of kinetic energies of both bodies!

Answer

Data
m1 = 1
m2 = 3
v1 = 1
v2= 2

The ratio:

Ek2/Ek1 = m2/m1 x (v2/v1)2

So then:

Ek2/Ek1 = 3/1 x (2/1)2 = 3 x 4 = 12
Finally
Ek2 : Ek1 = 12 : 1

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