 When something slides past something else.

 Doesn’t exist until this happens so is called a reaction force.

 When something slides past something else.  Doesn’t exist until this happens so is called a reaction force.  Always opposes the sliding motion

 When something slides past something else.  Doesn’t exist until this happens so is called a reaction force.  Always opposes the sliding motion  Increases when two objects are pressed together

 When something slides past something else.  Doesn’t exist until this happens so is called a reaction force.  Always opposes the sliding motion  Increases when two objects are pressed together  Rougher the sliding surface, the greater the friction.

- Two solid objects (eg. Dragging a bag of cement across the floor)

- Solid & liquid (eg. A person wading through water)

- Two solid objects (eg. Dragging a bag of cement across the floor) - Solid & liquid (eg. A person wading through water) - Solid & air (eg. Air resistance when cycling into a head wind)

- Two solid objects (eg. Dragging a bag of cement across the floor) - Solid & liquid (eg. A person wading through water) - Solid & air (eg. Air resistance when cycling into a head wind) - Liquid & air (eg. Water falling over a waterfall is turned into spray by friction with the air)

- Lubricating oil and grease between surfaces.

- Wheels or ball-bearings to roll the surfaces past eachother.

- Lubricating oil and grease between surfaces. - Wheels or ball-bearings to roll the surfaces past eachother. - Streamlined shapes to keep air resistance to a minimum.

- Lubricating oil and grease between surfaces. - Wheels or ball-bearings to roll the surfaces past eachother. - Streamlined shapes to keep air resistance to a minimum. - Cushions of air, as in a hovercraft.

 Stops sliding motion from starting.

 Eg – if a block of wood resting on the floor, is pulled gently, friction starts up and matches the pulling force, but in the opposite direction, so the block doesn’t move.

 Stops sliding motion from starting.  Eg – if a block of wood resting on the floor, is pulled gently, friction starts up and matches the pulling force, but in the opposite direction, so the block doesn’t move. The harder you pull, the greater the friction force that is generated.

 Stops sliding motion from starting.  Eg – if a block of wood resting on the floor, is pulled gently, friction starts up and matches the pulling force, but in the opposite direction, so the block doesn’t move. The harder you pull, the greater the friction force that is generated.  Between any two surfaces there is a maximum friction force that can be generated – static friction force.

 Stops sliding motion from starting.  Eg – if a block of wood resting on the floor, is pulled gently, friction starts up and matches the pulling force, but in the opposite direction, so the block doesn’t move. The harder you pull, the greater the friction force that is generated.  Between any two surfaces there is a maximum friction force that can be generated – static friction force.  If the pull is larger than the static friction, then the block will accelerate across the floor.

 When an object is dropped, it accelerates until 10ms -2 (pull of gravity)

 Air resistance (upwards friction) increases as the object moves faster

 When an object is dropped, it accelerates until 10ms -2 (pull of gravity)  Air resistance (upwards friction) increases as the object moves faster – this means that the net downwards force (gravity less friction) gets smaller.

 When an object is dropped, it accelerates until 10ms -2 (pull of gravity)  Air resistance (upwards friction) increases as the object moves faster  Eventually, the downwards pull of gravity is balanced by the upwards air resistance – no more acceleration

 When an object is dropped, it accelerates until 10ms -2 (pull of gravity)  Air resistance (upwards friction) increases as the object moves faster  Eventually, the downwards pull of gravity is balanced by the upwards air resistance – no more acceleration  When the forces are balanced = terminal velocity (200 km h -1 )

 All forces combining and cancelling each other out = 0 net force.

 No net force = no acceleration

 All forces combining and cancelling each other out = 0 net force.  No net force = no acceleration  No acceleration = no movement or constant speed.

 All forces combining and cancelling each other out = 0 net force.  No net force = no acceleration  No acceleration = no movement or constant speed.  Eg – a jetboat moving at a constant speed because the driving force of the engine exactly balances the friction force of the water.