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
When something slides past something
else.
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)
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.
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.
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)
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.
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.
