Game Development Reference
In-Depth Information
mgcosθ
mgsinθ
F
N
= mgcosθ
mg
θ
Figure 3-3.
Normal force on a box resting on an inclined ramp
■
Tidbit
Did you ever wonder why snails and slugs emit their disgusting slime? Among other reasons, the
slime helps the snail move by reducing the coefficient of friction between the snail and the ground.
There are some other interesting characteristics about frictional force. The direction of the
force of friction an object experiences is always in the opposite direction to any net external
force applied to the object. In Figure 3-4, if a box resting on the ground is pulled to the left, the
friction force will counterbalance this force by acting towards the right. If the box is on an
inclined ramp, gravity will pull the box down the ramp, but friction will resist this motion by
exerting a force up the ramp.
Figure 3-4.
The friction force, F
F
, acts in the opposite direction of an applied force, F.
Another interesting thing about friction is that, in a sense, it is a
variable magnitude
force.
Friction will counterbalance an externally applied force up to a maximum possible level defined
by
mF
N
. Let's say the box in Figure 3-4 has a mass of 10
kg
and that the coefficient of friction
between the box and the flat surface it rests on has a value of 0.5. The box will remain motion-
less for any horizontally applied force less than
mF
N
= 0.5*9.81*10 = 49.05
N
. If a 5
N
force is
applied to the box, friction will counterbalance the applied force with a 5
N
force in the oppo-
site direction. If a 40
N
force is applied, the counterbalancing friction force will be 40
N
. If the
applied force exceeds the maximum frictional force value of
mF
N
, friction is no longer able to
counterbalance the applied force and the box begins to move.
