# Accompanied forces to motion, Forces of inertia, Friction forces & Biological forces

Friction forces are a double-edged weapon because friction forces have benefits and also they have harms, The match is ignited when it is rubbed with a rough surface because friction forces generate heat energy leads to ignition of match, The presence of oil stains on highways is very dangerous because the oil stains decrease the friction forces, so the driver can’t control the vehicle.

### Accompanied forces to motion

There are many accompanied forces to the motion of objects, such as forces that originate due to motion (Forces of inertia and Friction forces), and Forces cause motion (Forces inside living systems).

#### Forces of inertia

When forces act on objects, which are at rest or moving at a constant speed, these objects resist changes in their motion because of their inertia. Inertia is a property of an object that has to resist the change of its state of rest or motion at a regular speed in a straight line unless an external force acted on it.

When you carry small plastic cubes on your plan and stretch your arm forward, then walk forward fast and suddenly stop at once. you will observe that the plastic cubes move forward and fall on the ground. The cubes resist the sudden stopping of the palm of your hand due to inertia, so they continue in the state of motion and fall on the ground. (The cubes move at the same speed of the person who carries them). so, Force of inertia makes objects resist the change of their motion.

When you place a piece of construction paper on the top of a glass cup and put a coin on it and use your forefinger to deliver a quick hit to the paper. you will notice that the coin falls inside the cup, The coin resists the sudden movement of the paper due to inertia, so it remains static and it falls in the cup. So, Force of inertia makes objects resist the change of their rest state.

##### Examples indicating inertia in our life

The passengers and the driver in a moving bus or car (vehicle) are rushed forward when the bus or car stops suddenly due to inertia for the passengers and driver, it makes them resist the sudden stopping of the vehicle to maintain the state of motion, so they rush (force) forward.

The passengers and the driver in a static bus or car (vehicle) are rushed back when the vehicle starts moving forward after it was at rest due to inertia for the passengers and driver, it makes them resist the sudden motion of the vehicle to maintain the state of rest, so they rush back.

A football player rushes forward and falls on the ground if he is tripped during running due to inertia for the football player that makes him resist the sudden stopping of his foot to maintain his state of motion, so he will be forced forward and fall down.

Policemen advise drivers to use safety belts in cars and planes because safety belts work on stopping the forces of inertia to prevent the driver from being injured when a sudden change in motion occurs. The fan is going to turn after the electric current goes off due to inertia, as its arms try to maintain its state of motion.

Friction force

### Friction forces

During the motion of an object, friction occurs between the object and the surrounding medium which generates a force known as friction forces against the motion of the object and resist its motion. Friction forces are resistant forces (against the motion) that originated between the object in motion and the medium touching it.

The surrounding medium may be a gaseous medium such as air, a solid surface as the ground and a liquid surface as water. The relation between the friction forces and the speed of the object is an inverse relationship. (By increasing the friction forces, the speed of the object decreases).

Car tyres are covered with a very coarse substance to increase friction between tyres and the road to help the car in starting motion and stopping. Spare parts of cars are covered with grease, Lubricating and oiling mechanical machines to reduce friction between moving parts of machines and prevent their erosion.

Once you use the brakes of a moving bicycle, its speed decreases gradually until it stops because the friction between the tyre of the bicycle and the brakes generates a friction force against the motion of the bicycle which leads to resist it. Cars that travel on snow have to carry chains that fit around the tyres to increase friction to control the motion.

When you drive a car in city traffic for some time, the brakes become hot because some mechanical energy is transferred into heat energy due to friction. You are able to run over grass much faster than you run over a ground covered with ice because friction with grass is more than friction with ice, so, the motion is more controlled.

#### Benefits of friction forces

• Friction force prevents feet from slipping on roads during walking.
• It helps in stopping and starting car motion.
• It helps in burning matches.

#### Harms of friction forces

• Friction force causes a great loss of mechanical energy because this energy is changed into heat energy.
• It produces heat energy due to friction between some parts of the machines. this heat causes the expansion of these parts and affects their performance.
• It causes the erosion of machines’ parts and damages them as well.
##### Forces inside living systems (biological forces)

There are forces inside living systems (living organisms) whether simple systems such as uni-cellular living organisms or complex systems such as multi-cellular living organisms. These forces enable living organisms to do their different biological operations and keep their survival and vitality. Biological forces are forces inside living systems that enable living organisms to do their different biological operations.

#### Examples of forces inside living systems

Heart muscle contraction and relaxation helps the heart to pump blood all over the body organs and vice versa, [ This is indicated by heart pulses during the movement of blood inside blood vessels].

Blood is pumped all over the body organs due to heart muscle contraction and relaxation. The role of the heart in raising blood from the bottom (lower parts) to top is similar to the role of water pump in raising water from canals and groundwater wells against the Earth’s gravity.

Liquids are transported through pores and the walls of cells from the lower concentration to the higher one. Rising of water and salts from the soil to plant [ from root to stem, then leaves ] against Earth’s gravity force. The concentration and relaxation of muscles help the body organs movement.

Fundamental Forces in nature, Gravitational forces, Electromagnetic forces & Nuclear forces

Force, Newtonâ€™s First Law of Motion, Inertia & Factors that affects the momentum

The advantage and the disadvantage of friction and the ways to decrease the friction

The factors affecting the friction between the solid objects and the water