# 15. Air Resistance & Terminal Velocity

# Success Criteria

Your learning has been successful if you can do the following:

Define acceleration and its units.

Calculate acceleration from speed and time measurements.

# Vocabulary

Learn these so you can communicate this concept well.

Average speed: Calculated by the total distance travelled divided by the total time taken for the journey.

Gradient / slope: How steep a line on a graph is.

Speed: How fast an object is moving.

Velocity: Similar to speed, but also tells us the direction in which an object is moving. It includes both speed and direction of motion.

Do Now: on Pg 55 of your SciPAD (find a space!)

Draw the triangle to show the relationship between distance, speed and time.

Draw the triangle to show the relationship between force, mass and acceleration.

Draw the triangle to show the relationship between speed, time and acceleration.

Do Now: Objects Falling

1. Describe the motion of object A, B and C falling.

2. Use the graph to calculate the average speed of objects A, B and C

Find some space on page 15 of your PESS1.2 SciPAD,

and answer the following questions:

What does each letter in CUTLASS stand for?

## Air Resistance

Air resistance, also known as drag, is the force exerted by air molecules on objects moving through the air. It opposes the motion of the object, affecting its speed and trajectory. Air resistance becomes particularly significant at higher speeds or for objects with large surface areas.

There are several factors affecting air resistance:

Speed: Air resistance increases with the speed of the object. Faster-moving objects experience greater resistance.

Cross-sectional Area: Objects with larger surface areas experience more air resistance.

Shape: The shape of an object affects how air flows around it. Streamlined shapes encounter less resistance compared to irregular shapes.

Density of the Medium: Air resistance also depends on the density of the medium through which the object is moving. For example, objects experience less air resistance in thinner air at higher altitudes.

## What is Terminal Velocity?

TERMINAL VELOCITY is the maximum constant speed that a falling object reaches.

Terminal velocity occurs when the force of AIR RESISTANCE / DRAG acting on a falling object becomes equal in size, but opposite in direction to the WEIGHT FORCE.

## Explaining Terminal Velocity in the Context of a Skydiver

Initially, when the sky diver jumps out of the plane, the downward force (weight force) is larger than the upward force (air resistance). So, there is an unbalanced force acting on them, causing them to accelerate downward.

As the sky diver accelerates, air resistance increases, eventually reaching a point where it becomes equal in size, but opposite the direction to weight force.

At terminal velocity, the downward force (weight force) and upward force (air resistance) are balanced, resulting in a net force of zero and therefore acceleration of zero according to Newton's 2nd law of motion (F=ma).

The sky diver continues to fall at a constant velocity (terminal velocity), since upwward and downwward forces are balanced.