Chapter 3, Topic 1
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# Forces

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What is a force?
A force is either a push or a pull that one object exerts on another object.

• Force is a vector quantity
• Force = mass ✕ acceleration
• SI unit of force is Newtons (N) or kg m s⁻²

### Different Types of Forces

Contact Forces
Contact forces exist between objects that are in contact.

• Applied force:
The push experienced when two objects are pressured together.
• Normal reaction force:
The push exerted by a surface on an object pressing on it. (This push is always perpendicular to the surface).
• Friction:
A force that opposes motion between surfaces in contact.
• Tension:
The pull exerted by a stretched spring, string or rope on an object attached to it.

Non-Contact Forces
Non-contact forces do not require objects to be in contact to exist.

• Air/water resistance:
A force that opposes motion due to an object moving in fluid (AKA drag force).
• Gravitational force/weight:
The pull exerted by the earth’s gravity on any object.
• Electric force:
The attractive or repulsive forces between electric charges.
• Magnetic force:
The attractive or repulsive forces between magnets.

### Classic Questions of Forces

#### 1. Describe ways in which a force might change the motion of a body.

• A force is may make stationary bodies move.
• A force is may moving bodies speed up, slow down or stop.
• A force is may moving bodies change direction. 