Friction and Viscous Forces

Overview

This lesson is about non-uniform motion. When resistive forces change with speed, the resultant force changes too, so acceleration is not constant.

What You Need to Know

• Friction and drag oppose motion or attempted motion.
• Air resistance is a drag force and increases as speed increases in the simple AS model.
• In free fall with air resistance, weight is initially larger than drag, so the object accelerates.
• As speed increases, drag increases and the resultant force decreases.
• Terminal velocity is reached when drag equals weight, so resultant force and acceleration are zero.

How to Work Through It

1. Start by drawing force diagrams for objects moving with and without resistance.
2. Compare the motion of a falling object at the start, during acceleration, and at terminal velocity.
3. Link force diagrams to velocity-time and acceleration-time descriptions.
4. Practise written explanations that use resultant force, acceleration, and speed in the right order.

Check Your Understanding

• Why is acceleration greatest at the start of a fall with air resistance?
• What changes as speed increases?
• Why does terminal velocity mean zero acceleration, not zero velocity?
• How would a larger weight affect the terminal velocity if the drag model is unchanged?

Common Mistakes

• Saying that terminal velocity happens when forces disappear.
• Describing speed as constant while also saying there is a resultant force.
• Using SUVAT equations for motion where acceleration is changing.
• Forgetting that drag acts opposite to the direction of motion.

Next Steps

• Practise explaining terminal velocity with both a force diagram and a velocity-time graph.
• Keep the idea of resultant force ready for Newton’s laws.