A force is an action which can set a body in motion, modify its movement or deform it.
The unit of measurement for force is the newton (|\text{N}|). This represents the force necessary to give an object with a mass of |1\ \text{kg}| an acceleration of |1\ \text{m/s}^2.|
Graphically, a force is represented with an arrow which indicates:
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its direction, by a line segment;
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its orientation, by the arrow;
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its magnitude, by the length of the arrow or by its value in newtons;
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its point of application, corresponding to the place, the precise point, where the force is applied.
There are different types of forces.
Gravitational force is a force of attraction between two bodies.
It depends upon the mass of each of the two bodies and the distance between them.
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The greater the mass of the two objects, the greater the gravitational force.
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However, the greater the distance between these two bodies, the more this force decreases.
On Earth, the gravitational force acts on any free-falling body an acceleration of |9.8\ \text{m/s}^2.| Weight is one way of measuring gravitational force.
The fall of a person, during a bungee jump, is caused by the gravitational force that pulls that person towards the centre of the Earth.
The gravitational force is also responsible for the tides and the trajectory of the celestial bodies that pass near our planet.
Normal force is the reaction force of a surface preventing an object from sinking into it.
The normal force should always be perpendicular to the surface.
As shown in the image, the force of gravity (|F_g|) should normally bring the object towards the ground (or towards the Earth’s centre). However, the table keeps the object stationary by exerting an upward force which cancels out the force of gravity. This force, exerted by the table, is called the normal force (|F_N|).
In the case of a horizontal surface (such as the book and table above), the normal force is usually equal to the gravitational force. However, if additional force were applied to the book or if the book were placed on an angled surface, the normal force would be different.
Force of friction is a type of contact force that opposes the motion of an object against a surface.
This is a force created by the interaction of two surfaces sliding over each other. Since the force of friction opposes motion between parts, the more friction there is between two surfaces, the less sliding is likely to occur.
The force of friction is determined by 2 factors:
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the type of contact surfaces (a rough surface causes more friction);
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the normal force.
Effective force is the component of a force responsible for moving an object. It corresponds to the force parallel to the object’s motion.
When a person mows the lawn, they apply a force to the mower which is at an angle to the ground. This is the applied force.
Effective force is the component of the applied force that is responsible for moving the lawnmower. Since the lawnmower moves horizontally, the effective force is the horizontal component of the applied force.
The principles of trigonometry are used to determine the actual value of the effective force.
A man pulls on a box using a rope, with a force of |50\ \text{N}| and forming an angle of |30^\circ| relative to the horizontal. What is the effective force exerted on the box?
We form a right triangle, knowing that the vector of |50\ \text{N}| corresponds to the hypotenuse of the triangle (that is the direction in which the force is applied).
Using trigonometry formulas, we can find the effective force, in other words, the force that is parallel to the ground (shown in red in the diagram above).
||\begin{align}cos\ \theta&=\dfrac{\text{adjacent side}}{\text{hypotenuse}}\\ cos\ 30^\circ&=\dfrac{\color{red}{F_{eff}}}{50\ \text{N}}\\ \color{red}{F_{eff}}&=cos\ 30^\circ\times 50\ \text{N}\\\color{red}{F_{eff}}&\approx 43.3\ \text{N}\end{align}||
The effective force is therefore approximately |43.3\ \text{N}.|
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A resultant force is the combination of all forces applied at the same time to an object.
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An equilibrant force is a force that cancels out the resultant force. This means that the object is not in motion or in uniform rectilinear motion.
Tug-of-war is an example of an equilibrium of forces. If the two teams pull with equal force, but in opposite directions, the scarf in the centre will not move. Therefore, the resultant force is zero because the two forces are equal in magnitude and applied in opposite directions.
After a while, if one team pulls harder than another (its force is greater), the equilibrium (or balance) is undone and the scarf moves. Therefore, the resultant force is not zero because one of the forces is not completely cancelled out by the others. So, this system is not in a state of equilibrium.
Electromagnetic force is a force of attraction or repulsion between objects having an electric charge or magnetic poles.
This force acts only on charged particles: protons and electrons. Electromagnetic force also explains why electrons are attracted to protons in an atom. Moreover, the electromagnetic force also explains why atoms and molecules can bind together to form new substances or even very long chains of molecules, like those found in plastics or in DNA.
The electromagnetic force is also felt when magnets are brought together. It allows magnets to attract or repel each other. Some materials, such as iron, can also be attracted to magnets. These materials are then said to be ferromagnetic.
A strong nuclear force is an attraction force that holds protons and neutrons together. It is what keeps nuclei stable.
A weak nuclear force is related to radioactivity phenomena and contributes, for example, to make the Sun shine.