Elasticity

# Introduction to Force

The wind blows against the sail as a sailboat slides on the water. The wind exerts a force on the sail and causes the sailboat to start moving.

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However, forces can act on a body even though the body is at rest. This does not mean the absence of forces, but that their combined effect on the body is equal to zero. Their sum or resultant in this case is equal to zero and therefore the body is at rest. We say that the forces are in equilibrium.

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Let's get to know about force gradually. We will focus on the basic questions related to it:

• What is force?

• How do we know that a force acts on a body?

• What do we call it?

• What characteristics define it?

• How do we draw it?

• How does the force work by touch and by distance?

• How can the force be distributed throughout a body (point, planar, spatial)?

• How can we measure it?

## What is force?

Force is a physical quantity. Like any physical quantity, it has its own name, notation, magnitude and unit.

Force is most simply described as pushing or pulling an object. When pulling or pushing, we act on an object with force. We cause a body to move, change speed, compress, stretch or bend. As a result of the action of the force, the speed or shape of the body changes, as we learn in more detail below.

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## How do we know that a force is acting on a body?

We know that a force is acting on a body by observing its effects on the body. But we will see that we can also measure the force. By measuring, we determine its size, that is, a numerical value. By observation, also the direction in which it works can be determined.

### Effects of forces

The effects of a force are the changes that are observed in the body due to the action of the force. Let's list them:

• A body, which is at rest, begins to move

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• The speed of motion of a body changes

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• The direction of motion of a body changes

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• The shape of a body changes

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## Name, symbol and unit of force

A force is usually named after the cause of the force. In naming the force, we can also add who it acts on.

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The symbol for force is , after the English word force. We also give the symbol or notation an index that tells who or what is causing the force.

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The chosen unit for force is the newton (unit symbol ). We increase or decrease the unit with prefixes, e.g.:

• with the prefix kilo (k), the unit has been increased a thousand times,

• with the prefix milli (m), the unit has been reduced a thousand times.

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## Properties of force

Force is a physical quantity. Like any physical quantity, force also has its properties. These are:

• Magnitude of force

The magnitude of a force is determined by measurement or calculation. We write it down with a numerical value, as we saw in the example above.

• Point of action of force

The point of action of a force is the point at which the force is exerted on a body.

• Direction of force

The direction of a force on a body is determined by observation. A force can act:

• in the direction of the motion of the body and increases its speed - the force acts in the direction of the body's motion and is therefore positive;

• in the opposite direction to the motion of the body and reduces its speed - the force acts opposite to the direction of the body's motion, so in this case, it is negative.

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If we want to emphasize on the symbol of a force, in addition to the magnitude, we also take into account its direction, we add an arrow to it: . The arrow is called a vector sign.

A force is a physical quantity that causes a body to move, changes the speed or direction of a body's motion, or changes its shape. The symbol for force is , and the unit is newton ( ). A force is named after the cause of the force. We add an index that identifies the cause of the force to the force symbol.

A force is positive if it acts in the same direction as the body is moving. It is negative if it acts in the opposite direction to the body's motion.

If we want to emphasize that we also take into account the direction of action of the force, we add an arrow to the force symbol, .

## Drawing forces

We draw a force using an arrow, as shown in the figure below.

When drawing a force, we must consider all three of its properties:

• The magnitude of the force

The length of the arrow will be proportional to the magnitude of the force. When drawing a force, we first determine the scale. The scale tells how many newtons it represents, e.g. long arrow represents .

• The line of action of the force

In the simplest case, the line of action (point of contact) of a force is the point where the force acts on a body. We draw it as a dot on the arrow.

• The direction of the force

The direction of the force is indicated by the direction of the arrow.

We draw a force using an arrow. On one end of the arrow is the line or point of action of the force. This is the point at which the force acts on a body. The length of the arrow is proportional to the magnitude of the force. We draw it on a scale that tells us how many newtons the selected arrow length represents. The direction of the force is indicated by the direction of the arrow.

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## Contact force or field force

A force can act:

• by making a contact or

• at a distance.

We have already learned about the operation of the contact force. The force generator acts on the object by direct contact. The contact force always has a grip at the point where it acts on the body.

A force can also act on an object at a distance. In this case, the agent of the force does not touch the object on which it exerts the force.

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## Surface and volume distributed forces

A force may act on an object:

• at a single point,

• on the plane or

• on the entire volume of the body.

When someone touches an object over a very small area, the force is said to be acting at a single point. We have already seen such cases above.

In the subsections below, we focus on the other two cases when the force does not act on a very small area.

### Area-distributed or surface forces

The cause of a force often acts on a body over a larger area. We say that the grip of the force is planarly distributed. When drawing area distributed forces, we usually combine all forces into a single force. The grip of this force is at a point called the centre of gravity of the surface.

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### Volume-distributed or body forces

In some cases, the force acts on every little part of the body. We say that the forces are volumetrically distributed. They have a grip on all parts of the body within its volume.

Even in this case, all forces can be combined into a single force. The grip of this force is in the body itself and in its centre of gravity.

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The source of a force can act on the body with a force at a single point, on a surface area or on the entire volume of the body.

In the case of an area or volume-distributed forces, we can draw only one force - the total force, which has the same effect on the body as all the smaller forces together. The point of contact of this force is in the centre of gravity of the surface or body.

## Measuring forces

The magnitude of a force can be determined by measurement. Most of the time, we use a spring balance to help us, which we get to know in more detail in the material, Measuring forces with springs.

A spring balance is a device that consists of a spring and a scale. When a tensile force is applied to the spring, it stretches. The magnitude of the force is read from the scale. The greater the force, the more the spring stretches and the scale shows a higher value.

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material editor: Gabriel Amakhabi