Friction is an important force that works against how things move. It happens when two surfaces touch each other. Friction is affected by what those surfaces are made of and how hard they are pressed together. In Year 8 Physics, it’s really important to understand friction. It helps us learn how it affects the speed of moving things.

Types of Friction

1. Static Friction: This is the force that keeps something from starting to move. It works on objects that are not moving and can change up to a certain limit. For example, the amount of static friction (called the coefficient of static friction, or $\mu_s$) can be as low as 0.1 (like ice on ice) to as high as 1.0 (like rubber on concrete).

2. Kinetic Friction: Once something starts moving, kinetic friction kicks in. The amount of kinetic friction (called the coefficient of kinetic friction, or $\mu_k$) is usually less than static friction. Common values are 0.05 for ice on ice and up to 0.8 for rubber on concrete.

The Role of Friction in Motion

Friction affects the speed of moving things in different ways:

• Slowing Down: When an object is moving, friction pushes in the opposite direction, which makes it slow down. For example, if a car is driving on a wet road, friction can really slow it down.

• Limits on Speeding Up: For something to speed up, the push has to be stronger than the friction. For instance, if a car engine pushes with 4000 N and the friction is 300 N, the overall force ($F_{\text{net}} = F_{\text{applied}} - F_{\text{friction}}$) would be $4000 N - 300 N = 3700 N$. This means the car speeds up based on $a = \frac{F_{\text{net}}}{m}$.

Finding the Amount of Friction

We can find the amount of frictional force ($F_f$) using this formula:

$F_f = \mu N$

In this formula, $N$ is the normal force, which is the support force between two surfaces touching each other. For something sitting on a flat surface, $N$ is the weight of that object ($N = mg$, where $m$ is mass and $g$ is the pull of gravity, about $9.81 \, m/s^2$).

Effect on Speed

Friction can have a big effect on speed. For example, it can stop a bicycle moving at 10 m/s completely after a certain distance. We can calculate how much it slows down with:

$a = \frac{F_{f}}{m}$

Friction is really important not just for stopping things but also for figuring out how quickly they can speed up. Understanding this is a key part of Year 8 students learning about force and motion.