Friction

Consider a car travelling along the road.  You can see all the forces on the car in the figure below.  There are 2 frictional forces here, the contact of the tyres on the road and the air resistance of the car.

Friction always opposes the motion of an object.

You need to know how these forces compare if you are to predict what will happen to the speed of a moving object.

If the driving force is greater than the frictional forces, there is a resultant force forwards. This will make the car accelerate and increase its velocity.

If the driving force is less than the frictional forces, there is a resultant force backwards. This will make the car undergo negative acceleration (deceleration) and have lowered velocity.

If the driving force is the same as the frictional forces, there is no resultant force, and so no change in velocity.

Friction is caused by the contact between 2 surfaces.  The rougher they are, the greater the friction is.  Having said that, friction is good because it means that our grip on the ground allows for walking, riding and driving and parachuting.  It also means that you can write with a ballpoint pen and not slide off your chair as you read this.

Unfortunately there are also downsides to friction.  It creates heat and can wear materials out.  This can be solved by smoothing/polishing surfaces and/or using lubricant (in the case of engines for example).