The Forces - LeFonch

What is a Force?

A force is a physical quantity that describes the interaction between bodies.
When bodies interact, forces are experienced by the bodies.
When bodies stop interacting, the forces stop.

Forces, denoted by F, are vector quantities.
However, most forces have their own different symbols, such as weight F₉ and electromagnetic force E.

Weight is the product of mass and the gravitational acceleration of free fall.
On Earth, it is what pulls towards the centre of the Earth.
W = mg
However, keep in mind that the constant g is different for different objects as they have different mass.
On the moon you weigh far less than Earth but on Jupiter you'd way far more; yet you would have the same mass on all three!

Air resistance, or drag is a contact force that acts against anything moving in an air or liquid.
It is caused by all the little particles that make up the fluid colliding with the object moving through it.
If its difficult to visualize, imagine trying to run through a ball pit, yet with trillions of incredibly tiny balls that slow down your movement.

Friction is a contact force that acts against anything moving.

Upthrust, or buoyancy force, is the force that liquids apply onto objects within them.
The denser the fluid, and the more volume displaced by the object, the greater the upthrust.
When buoyancy is equal to weight, the object starts floating!

Restoring force is a force that acts in the opposite direction to displacement, returning the object to its equilibrium position.
Restoring force is seen most commonly in springs.
As you pull on the spring, it gets harder and harder to pull it, as the spring is pulling back as it tries to return to its original shape.

A force can either change the motion of an object by stopping its movement or by setting it in motion.
It is said that a force is applied to, acts on, or is exerted on or by a body.
As forces occur when bodies interact, it must involve at least two different objects.
In physics however this can be simplified, showing the forces acting only on the body we are interested in.
This can be drawn as a free-body diagram.
Forces are vectors and thus are drawn as arrows.
The image below shows a free-body diagram with 3 forces acting upon the body.

Forces can be added together like any other vectors of the same quantity.
The sum of multiple forces is called the resultant force.
The diagram below shows the forces acting on the object with the equation to find the resultant force on the right.

When the resultant force on an object is equal to zero, then it is in equilibrium, and it stays in the same state it is currently in.
If the object is still, it will stay still as long as its in equilibrium.
If an object is moving, it will keep moving in the same direction with the same velocity as long as it is in equilibrium.
That's why the book on your table stays still, or why a meteor in space can keep travelling forever!

If the resultant force is less than or greater than zero, then the object will always be accelerating in one direction or the other.

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