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Forces and Motion 3.1 Friction between solid surface-surface contact and solid objects moving through a fluid such as a gas or liquid

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INDEX for physics notes on acceleration of falling objects, experiments, friction, drag effects, gravity, terminal velocity


3.1 Introduction to the connection between friction and motion

Friction and motion: Solid surface-surface contact and solid objects moving through a fluid

What causes the drag effect in fluids?   What is air resistance?  What has it got to do with friction?

You first appreciate friction when two solid surfaces rub together. When you pull a heavy object across the floor you experience the resistive force between the two surfaces. This resistive force is called friction and is caused by the atoms of the two surfaces bumping in to each other from opposite directions. The moment you stop applying a force, the dragged object ceases moving immediately.

The force of friction always acts in the opposite direction to an object's movement and it can involve ANY type of surface contact effect.

Therefore when a solid object moves through a fluid - gas or liquid, there are collisions between the fluid particles and the surface of the object which creates the resistive force of friction.

In fluids this resistive force is called the drag effect. A boat ploughing through water experience the drag effect of the water brushing over the surface of the hull. An aircraft of skydiver moving through the atmosphere experience a drag effect, which in this case is called air resistance.

If an object is moving at a steady speed the thrust or driving force (engine  or gravity etc.) is being balanced by the opposing force of friction.

e.g. a car moving at a constant velocity means the thrust force from the engine is balanced by the friction of moving parts in the engine, wheel contact with the road surface and the air resistance (the latter being the drag effect).

If you increase the power output of the car by pressing harder on the accelerator pedal you overcome more of the resistive force and move faster, but the friction e.g. the air resistance increases with speed (see next section), so eventually all the opposing forces balance and you attain a higher steady speed.

BUT, if you want a faster constant speed, the engine must work harder and burn more fuel to overcome the increased drag effect as air resistance increases with speed.

The drag effect is due to air resistance, that is the air brushing over the surface of the moving car.

If you take your foot of the accelerator and apply the brake, you increase the force of friction so the car slows down. Theoretically the drag force is reduced with decreased speed, but its a very small effect here compared to the friction between the brake pads and brake discs on the car wheels.

If a moving object has no thrust force acting on it, then it will always slow down and come to a halt e.g. on a level road, if you take your foot of the accelerator, the car will eventually come to a halt as the resistive forces act against the car's forward motion. The total friction effects will bring the car to a halt.

An object can keep moving if it is subjected to a constant force greater than the friction effects e.g. a car will roll down a hill due to gravity.

You should also appreciate that objects will move forever in outer space because there are virtually no molecules to bump into and cause a friction-drag effect.

Ships moving through water experience a drag effect due to the boat hull surface experiencing friction as it moves through the water.

Physics notes index on acceleration of falling objects, experiments, friction, drag effects, gravity, terminal velocity


Keywords, phrases and learning objectives for friction between solid an liquid surfaces

Be able describe the effects of friction between solid surface to solid surface contact and solid objects moving through a fluid such as a gas or liquid and experience friction or drag effects e.g. the motor vehicles on a road, ships ploughing through water.


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Physics notes index on acceleration of falling objects, experiments, friction, drag effects, gravity, terminal velocity

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