# 9-1 Physics/Forces and their interactions

## Scalar and Vector quantities

Scalar quantities have magnitude only.

Vector quantities have magnitude and an associated direction. Vector quantities are often represented with an arrow. For example:

${\bar {v}}$  represents velocity and $v$  represents speed.

## Contact and non-contact forces

A force is an interaction that causes a change to occur. It is heavily connected to energy. A simpler way of understanding a force is that is a push or a pull that acts on an object due to the interaction with another object.

All forces are either:

• contact forces; the objects are physically touching
• non-contact forces; the objects are physically separated.

#### Contact forces

• Friction - the force that resists the motion of solid surfaces sliding against each other.
• Air resistance (drag) - (friction of the air) the force acting opposite to an object moving in a fluid (usually air).
• Tension - pulling force.
• Normal force - The force that the ground (or any surface) pushes back up with.

#### Non-contact forces

• Gravitational force - Force of attraction between objects of mass.
• Electrostatic force -
• Magnetic force -

All non-contact forces have a "field" or an invisible fabric that surrounds objects in the 3D world we live in.

### Gravity

Weight is the force acting on an object due to gravity. The force of gravity close to the Earth is due to the gravitational field around the Earth. If you suddenly ended up in deep space, there would be no such field and you wouldn't be glued to any floor; instead you would float. As such, the weight of an object depends on the gravitational field strength at the point where the object is:

$W=mg$

Which, if you remember studying direct proportionality in mathematics, would lead us to realise that weight and mass are directly proportional to each other.

The force of gravity between objects, or the weight, can be thought of as acting a central point or the object's centre of mass.

Weight can be measured using a Newton meter.

## Resultant forces

A number of forces acting on an object may be replaced by a single force that has the same effect as all the original forces acting together. This single force is known as the resultant force. When we calculate the resultant force we consider the forces acting ------- (horizontally) and | (vertically) separately. Forces are a vector and so would have a direction.

## Work done and energy transfer

$W={\bar {F}}s=\Delta E$

## Law of elasticity

Also known as Hooke's law, Hooke's law states that the extension (e) of the material is directly proportional to the load or applied force (F), given that the elastic limit of an elastic material is not exceeded.

$F=ke$

Elasticity can be defined as the ability of a substance to regain its original shape and size after being distorted by an external force.