non-uniform motion

non-uniform motion occurs when an object’s velocity changes with time. this can be a change in speed, direction, or both.

friction

• friction is a force that opposes motion
• it acts along the surface of contact between two objects
• friction is responsible for non-uniform motion
• friction always acts opposite to the direction of motion

viscous forces (drag)

• frictional force in a fluid is called a viscous force or drag
• fluids include liquids and gases such as water and air
• air resistance is a common example of drag
• drag force increases as the speed of the object increases

terminal velocity

when an object moves through a fluid, it may eventually reach a maximum constant velocity known as terminal velocity.

reaching terminal velocity

• initially, the only significant force acting is weight
• the object accelerates downward with acceleration g
• as speed increases, the upward drag force increases
• the resultant force is weight − drag
• the acceleration decreases as drag increases
• eventually, drag equals weight
• the resultant force becomes 0
• with zero resultant force, acceleration is zero
• the object continues to move at a constant terminal velocity

velocity–time graph

• the gradient of a velocity–time graph represents acceleration
• as drag increases, the gradient decreases
• at terminal velocity, the graph becomes horizontal
• a horizontal line indicates constant velocity and zero acceleration