The Standard International (SI) unit for density is kilogram per cubic metre (Kg/m³).

The table below shows various substances with their densities:

Substance	Density (Kg/m3)
Helium	0.179
Air (1atm and 20oC)	1.20
Bamboo Wood	400
Ethanol	810
Methylated Spirit	800
Benzene	900
Paraffin Wax	900
Ice	920
Water (Fresh Water)	1000

Please read on density here

Force can be balanced or unbalanced.

When forces from different directions act on an object and consequently bring about the object's motion, then such forces are unbalanced.

Conversely, if theses forces bring about zero motion (static position or no movement) on the object of reference, then it's a balanced force.

In summary,

• An apple hanging on a tree (without any form of movement) has a balanced force.

• An apple falling from a tree has an unbalanced force.

If one covers a distance of 10,000 meters in 50 seconds and another covers the same distance in 5,000 seconds, we can conclude that the first person has a greater speed because he covered the same distance at a lesser time.

• Speed is defined as distance travelled per unit of time. It can also be defined as the rate of change of distance.

Speed = distance/Time

Speed is a scalar quantity since it has only magnitude. Speed does not have a direction.

In physics, motion is defined as a change in the position of an object with reference to time.

Although we sometimes sit or stand in a fixed position, that doesn’t mean motion isn’t taking place. The fact that the earth is constantly rotating implies that we are indirectly moving along with it. We can thus say that motion can be relative to some frame of reference.

In the above paragraph, the earth (constantly rotating along its axis) is our frame of reference and everything present on earth is in motion (with regard to earth as a frame of reference).

Please read on the concepts of motion and speed here

Heat is a form of energy. It cannot be lost according to the first law of Thermodynamics; rather it will be transferred. The first and second laws of thermodynamics are stated below:

• Energy can neither be created nor destroyed but can be transformed or converted from one form to another.

  1^st law of thermodynamics




• In the course of energy transfer, some energy is lost in the form of heat to the surroundings.

  2^nd law of Thermodynamics

Please read on the concept of heat transfer here

If we have water of 100^OC  in an aluminium cup whose temperature is the same as that of its surrounding; let’s say 22^OC; We will observe the followings:

1. The temperature of water inside the aluminum cup will begin to decrease from its initial 100^OC.

2. The temperature of the aluminium cup will increase from its previous 22^OC.

3. After some time, the temperature of the water, aluminium cup and the surrounding will become the same.

From the above instance, it can be deduced that the heat present in the water molecules are first transferred to the aluminium cup and subsequently to the surrounding.

Heat from water >> Aluminium Cup >> Surrounding

Meanwhile, understand that aluminium is a metal; and metals are good conductors of heat.

Eventually, the system (water and aluminum cup) together with the surrounding reaches the same temperature; and the heat transfer ceases. It is at this point that the water, aluminium cup and the surroundings are said to have reached a thermal equilibrium.

Please on the concept of heat and temperature here

Heat is never static. It is always transferred and thus: an object will increase its temperature by gaining heat energy from its surroundings while another object will simultaneously decrease its temperature by losing its heat energy to the surroundings. For this reason, we may also define temperature as:

The measure or ability of an object or substance to transfer heat energy to another object or substance.

The higher the temperature, the greater the ability of an object or substance to transfer heat. Conversely, the lower the temperature, the greater the tendency of an object to receive the transferred heat.

The point is: 'for heat to be transferred, we must have systems or objects or substances with varying temperatures

The earth is governed by the force of gravity and that’s why everything present on it (humans, plants, animals and even air) is always drawn towards it. You will actually need some form of force greater than gravity in order to pull yourself away from the earth.

As an instance, the earth as we know is spherical in shape and held in space. Also, all humans live on a constantly rotating earth and not inside the earth. Since we live on a spherical earth held in space, how come humans are yet to fall off the constantly rotating earth?

Well, the answer is gravity. Even if we attempt to jump off the earth into space, the force of gravity present on earth will still pull us down. In fact, in order to leave the earth; there has to be a force stronger that gravity; which must be applied to whatever is attempting to leave the earth. For this reason:

Whenever one jumps, a certain amount of force is needed to be applied by that person. However, gravity still return the person back to earth because the applied force had been exhausted.

Please read more on the concept of gravity here.

The larger the size of an object, the greater it’s force of gravity. Gravity is therefore affected by the size and closeness of objects.

Since the earth is larger than the moon, it will have a greater gravitational force than the moon. We will be more attracted on the earth than we would on the moon if we were astronauts on the moon. This implies that we will weigh differently on both places. For this reason, our weight will also vary on the various planets (because they are of different sizes). This is shown in the diagram below:

Weight depends on the degree at which the gravity on earth or moon or another planet pulls an object while the Mass of any object or body will always remain the same (constant) since it’s the amount or quantity of matter in the body.

Please read on mass versus weight here

To better understand the Zeroth Law of Thermodynamics, we need to know what thermal equilibrium is. This is explained below:

Two bodies A and B are said to be in thermal equilibrium if each body can transfer heat to each other when placed closed together but ended up not transferring any heat to each other.

This will mean that both bodies are at equal temperature.

Now, according to the zeroth law, assuming we have three bodies namely X, Y and Z:

If X = Y and Z = Y, then X = Z

From the above expression, it will be observed that: if X and Y are in thermal equilibrium and Z and Y are in thermal equilibrium, therefore, X and Z must be in thermal equilibrium too.

So we can see that according to the zeroth law, temperature is a quantity worth measuring.

It can therefore be said that temperature is the quantity that remains constant (the same) for all systems in thermal equilibrium.

Consequently, the zeroth law of Thermodynamics states that if two separate bodies are in thermal equilibrium with another body, then they are also in equilibrium with each other

Please read more on the zeroth law of thermodynamics here

Fundamental units are the units of fundamental quantities.

A fundamental unit is a unit that does not depend on any other unit; neither can it be changed nor is related to another fundamental unit.

A fundamental unit is also referred to as a basic unit.

Fundamental Units are always constant. This means that they don't change, (remains the same) and are standardized all over the world.

The fundamental quantities alongside their respective fundamental units are 7 in number. They include:

1. Mass (m) is a fundamental quantity whose unit is kilogram (kg)

2. Length (l) is a fundamental quantity whose unit is meter, (m)

3. Time (t) is a fundamental quantity whose unit is second (s)

4. Temperature (T) is a fundamental quantity whose unit is Kelvin (k)
   You can read on the Concept of Temperature and Heat here.

5. Electric Current (I) is a fundamental quantity whose unit is ampere (A)

6. Amount of Substance (n) is a fundamental quantity whose unit is mole (mol)

7. Luminous Intensity (|^v) is a fundamental quantity whose unit is candela (cd)

Please read more on fundamental units here

The classical mechanics (laws of motion) is a fundamental part of Physics.

Below are the Newton's laws of motion:

An object at rest will continue in its state of rest; while an object in uniform motion will continue in a straight line, unless an external force acts on it.

Newton's first law of motion.

Please read on motion and speed here.

The rate of change of momemtum is proportional to the applied force and will take place in the direction of that force.

Newton's second law of motion.

To every action, there is an equal and opposite reaction.

Newton's third law of motion.

Please read on Newton's laws of motion here.

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