The portal has been deactivated. Please contact your portal admin.

Lesson Explainer: Air Resistance Science • 4th Grade

In this explainer, we will learn how to define air resistance and describe the factors that affect it.

We have already learned about friction and how it resists motion when two solid objects move over each other.

Did you know that you do not only get friction between solid things?

In this explainer, we will discuss how friction can oppose motion between solids and air.

First, let’s recap what we know about friction.

Friction is a force that resists motion between two touching surfaces.

Friction can be represented with arrows in a force diagram, as shown below.

As the man pulls the box, his pulling force moves the box toward him.

As the box is pulled along the ground, friction acts in the opposite direction to the movement of the box. We represent this force with an arrow pointing to the left, labelled Frictional force.

Frictional forces can also occur between a solid object and a fluid.

Definition: Fluid

A fluid is a liquid or a gas.

When an object moves through a fluid, a type of friction acts against it. We call this a drag force.

Definition: Drag Force

A drag force is a frictional force that opposes an object’s motion in a liquid or a gas.

Example 1: Defining Drag Force

Air resistance is a type of drag force.

Fill in the blanks: A drag force is a type of that acts in the an object’s motion through a liquid or gas.

  1. friction, same direction as
  2. magnetism, opposite direction to
  3. friction, opposite direction to
  4. magnetism, same direction as

Answer

We know that a drag force is a friction force that occurs when an object moves through a liquid or a gas.

Magnetism is the force exerted by magnets when they attract or repel each other. A drag force is not a type of magnetism.

So, we can see that options B and D are incorrect.

Drag forces are frictional forces that act in the opposite direction to the movement of an object, not in the same direction. So, option A is incorrect.

The correct answer is option C: a drag force is a type of friction that acts in the opposite direction to an object’s motion through a liquid or a gas.

One type of drag force is air resistance.

You may not notice it, but as you walk or run, air pushes against you and slows you down.

This pushing force is called air resistance.

Air resistance is a type of drag force that happens when an object moves through air.

Definition: Air Resistance

Air resistance is the frictional force that opposes an object’s motion through the air (the drag force in the air).

Example 2: Applying the Definitions of Air Resistance and Drag Force

Fares’s favorite hobbies are skateboarding, cycling, and underwater diving.

  1. During which activities does air resistance act in the opposite direction to Fares’s motion? Select all that apply.
  2. During which activity does drag act in opposition to Fares’s motion?

Answer

Part 1

The first part of this question asks during which activities Fares experiences air resistance.

To answer this question, we need to know what air resistance is.

Air resistance is the frictional force that opposes an object’s motion through the air (the drag force in the air).

When Fares is cycling and skateboarding, he is moving through the air. When Fares is diving underwater, he is moving through water.

Air resistance will act against Fares’s motion only when he is moving through air.

So, air resistance acts in the opposite direction to Fares’s motion when he is skateboarding and when he is cycling, but not when he is diving underwater.

Part 2

The second part of this question asks during which activity Fares experiences drag.

To answer this question, we need to know what a drag force is.

A drag force is a frictional force that opposes an object’s motion in a liquid or a gas.

When Fares is cycling and skateboarding, he is moving through the air. Air is a gas.

When Fares is diving underwater, he is moving through water. Water is a liquid.

In all three activities, Fares is moving through a liquid or a gas.

So, a drag force opposes Fares’s motion during all three of the above activities.

We can draw arrows in force diagrams to represent the force of air resistance.

When drawing force diagrams, the arrow showing air resistance should point in the opposite direction to the movement of an object.

For example, as a skydiver falls downward through the air, air resistance will act against their falling. So, we draw an arrow pointing upward, as shown in the image below.

As a person runs forward, air resistance is acting in the opposite direction to their movement. So, we draw an arrow pointing in the opposite direction to the force of the person running, as shown in the image below.

Example 3: Representing Air Resistance in Force Diagrams

Air resistance is the force of air pushing against a moving object.

Which image correctly shows the force of air resistance?

Answer

We know that air resistance acts in the opposite direction to an object’s movement.

In this case, we can see that the object (the child) is moving to the right.

In option A, the arrow is pointing to the left, in the opposite direction to the child’s movement.

In option B, the arrow is pointing to the right, in the same direction of the child’s movement.

So, the correct answer is option A; this image correctly shows the force of air resistance.

Since air resistance acts in the opposite direction to an object’s motion, it slows objects down.

Sometimes, air resistance can be useful. For example, parachutes use air resistance to make people fall to the ground slowly. This helps people land safely on the ground.

Other times, air resistance is not useful, and we want to reduce it.

For example, when people design airplanes and trains, they want them to travel fast. They do not want too much air resistance to cause them to slow down.

By understanding and changing factors that affect air resistance, we can control the amount of air resistance between objects and the air.

The three factors that affect air resistance are

  1. the surface area of the moving object,
  2. the shape of the moving object,
  3. the speed of the moving object.

First, we will look at how the surface area of an object impacts the amount of air resistance it faces.

Definition: Surface Area

Surface area is the space that the surface of an object takes up. It can be measured in square centimetres (cm2) or square metres (m2).

Let’s compare the surface areas of two parachutes.

Parachute 1 is bigger and has the largest surface area. Its surface takes up more space.

Parachute 2 is smaller and has the smallest surface area. Its surface takes up less space.

Let’s compare how much air resistance these parachutes experience.

Example 4: Identifying How Surface Area Impacts Air Resistance

Sameh is designing parachutes.

  1. He says, “The larger the surface area of the parachute, the faster it will fall to the ground.” Is he right?
  2. Fill in the blank to explain why: A larger surface area creates air resistance.
    1. more
    2. less

Answer

If a parachute has a larger surface area, more air pushes against the object and it experiences greater air resistance.

Part 1

The first part of the question asks if Sameh’s statement is correct.

Air resistance acts in the opposite direction to an objects motion. More air resistance will cause an object to move more slowly.

So, Sameh’s statement is incorrect. The larger the surface area of the parachute, the more slowly it will fall to the ground.

Part 2

The second part of the question asks us to fill in the blank to explain why Sameh’s statement is incorrect.

If a parachute has a larger surface area, more air pushes against the object.

So, option A is correct: a larger surface area creates more air resistance.

Another factor that affects air resistance is the shape of an object. Objects with different shapes experience different amounts of air resistance.

Shapes that cause less air resistance are referred to as streamlined shapes.

Definition: Streamlining

Streamlining is designing the shape of an object to allow fast motion through a fluid, like air or water.

The image below shows some examples of streamlined objects. All of these objects are designed to travel fast.

Notice how the objects in the image have a similar long, thin, and smooth shape. They have a pointed shape, with the point facing the direction of a movement.

This shape minimizes the amount of air resistance they experience and prevents them from being slowed down too much by air resistance.

Example 5: Identifying How Streamlining Impacts Air Resistance

A student takes two pieces of modeling clay. She measures how quickly they fall to the ground.

  1. Fill in the blank: Shape B will fall than shape A.
    1. more quickly
    2. more slowly
  2. Why is this the case?
    1. Because shape B is more streamlined
    2. Because shape A is more streamlined

Answer

Comparing shape A and shape B, we can see that shape B has a longer, thinner, and smoother shape. Shape B is more streamlined.

Part 1

The first part of this question asks us to fill in the blank.

The streamlined shape of shape B will minimize the amount of air resistance the clay experiences while falling to the ground. This means that air resistance will slow it down less.

So, the answer is option A; shape B will fall to the ground more quickly than shape A.

Part 2

The second part of this question asks us to explain why shape B will fall more quickly.

Shape B has a longer, thinner, and smoother shape.

So, the answer is option A: shape B is more streamlined.

We have seen that the shape and surface area of objects impact the amount of air resistance between them and the air.

The third factor that impacts the amount of air resistance is the speed of an object.

Objects that are moving fast experience more air resistance opposing their motion than objects that are moving slowly.

For example, if you run fast, you will feel air pushing against your body. The faster you move, the more air resistance will push against you.

Example 6: Identifying the Relationship between Air Resistance and Speed

The faster an object moves, the more air resistance it has.

Fill in the blank: Air resistance the object.

  1. speeds up
  2. slows down
  3. does not change the speed of

Answer

We know that air resistance opposes an object’s motion.

Objects that are moving faster will experience more air resistance opposing their motion and slowing them down.

So, the answer is option B: air resistance slows down the object.

In this explainer, we have learned how to define air resistance and describe the factors that affect it.

Here are the key points that we have learned.

Key Points

  • A drag force is a frictional force that opposes an object’s motion in a liquid or a gas.
  • Air resistance is the frictional force that opposes an object’s motion through the air (the drag force in the air).
  • An object’s surface area, shape, and speed affect how much air resistance they experience.
  • Objects experience more air resistance when
    • they have a larger surface area,
    • they are not streamlined,
    • they are moving faster.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.