Air resistance, also called drag, greatly affects how things move in a straight line. It pushes against an object’s motion, which changes how fast it travels and how quickly it speeds up or slows down.

What Affects Air Resistance?

1. Speed: The faster an object moves, the more air resistance it faces. We can figure this out using a special equation: $F_d = \frac{1}{2} C_d \rho A v^2$ Here’s what the letters mean:

   • $F_d$ is the drag force,
   • $C_d$ is the drag coefficient (this depends on the shape of the object),
   • $\rho$ is the air density (about $1.225 \, \text{kg/m}^3$ at sea level),
   • $A$ is the front area of the object,
   • $v$ is the speed of the object.

2. Front Area: When the front area of an object is bigger, it faces more drag. For example, a skydiver in a wide position has a front area of about $1.8 \, \text{m}^2$, causing a lot of air resistance.

3. Drag Coefficient: This number shows how much drag an object has based on its shape. For instance, an airplane, which is shaped to cut through the air, has a very low drag coefficient of about 0.02. In comparison, a flat piece of paper has a much higher drag coefficient of around 1.28.

How Air Resistance Affects Motion

• Terminal Velocity: When something falls, it eventually stops speeding up. This point is called terminal velocity, where the pull of gravity is balanced out by the drag force. For example, a skydiver can fall at a maximum speed of about $53 \, \text{m/s}$ when face down. If they position themselves to be more streamlined, that speed can increase to around $90 \, \text{m/s}$.

• Slowing Down: When something is moving through the air, air resistance slows it down. This means that the distance it can travel or how high it can go may be less than expected. For example, if a baseball is thrown at a speed of $40 \, \text{m/s}$, it will immediately slow down because of drag, which affects how far it can go.

In short, air resistance is very important for understanding how things move in a straight line. It depends on speed, front area, and the shape of the object.