## 8 Common Problems of Energy - Junior Physics

- Category: Junior High
- Written by fisikastudycenter

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 v^{2}

Ek = ^{1}/_{2} x 40 x 10^{2}

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 v^{2}

720 = ^{1}/_{2} x 40 x v^{2}

720 = 20 x v^{2}

720 / 20 = v^{2}

36 = v^{2}

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 v^{2}

128 = ^{1}/_{2} x m x 8^{2}

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/s^{2}!

**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} mv^{2}

Ek = ^{1}/_{2} x 2 x 10^{2}

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 v^{2}

Ek_{2} : Ek_{1} = ^{1}/_{2} m v_{2}^{2} : ^{1}/_{2} m v_{1}^{2}

Ek_{2} : Ek_{1} = v_{2}^{2} : v_{1}^{2}

Ek_{2} = (v_{2} / v_{1})^{2} x Ek_{1}

So then

Ek_{2} = (0,5 v / v)^{2} x 5000 joule

Ek_{2} = ^{1}/_{4} x 5000 joule

Ek_{2} = 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

m_{1} = 1

m_{2} = 3

v_{1} = 1

v_{2}= 2

The ratio:

Ek_{2}/Ek_{1} = m_{2}/m_{1} x (v_{2}/v_{1})^{2}

So then:

Ek_{2}/Ek_{1} = 3/1 x (2/1)^{2} = 3 x 4 = 12

Finally

Ek_{2} : Ek_{1} = 12 : 1