Static friction opposes motion when two surfaces contact each other without movement. Sliding friction is smaller than static friction and resists the motion of surfaces sliding against each other. Elastic forces, such as tension and compression, act on objects when they are stretched or squeezed from their regular shape, pushing back into their original form. The normal force is the supporting force exerted by a stable surface on an object in contact with it, such as a table pushing back on a book resting upon it.

1. Forces Notes # 4
Static Friction – it pushes back as hard as you
push it – no movement – balanced forces
200 Newtons
200 Newtons

2. Sliding Friction
• Sliding Friction – the resistance you feel when
pushing something. Usually smaller than the
static friction.
• Does a skateboard slow down because nobody
is pushing it, or because of friction, or both?
• Does friction always slow things down? Does it
ever speed things up?
• Is friction always bad? Explain how hockey
players need both low friction and high
friction.

3. Elastic Forces
• Elastic Force – the force exerted by a material
when it is stretched or compressed.
• Tension Force – a pulling force that makes an
object stretch, and want to pull itself back into
its regular shape.
• Compression Force – a squeezing force applied
to an object that will make it squish, and want
to push back into its regular shape.

4. Normal Force
• Normal Force - The normal force is the support
force exerted upon an object that is in contact
with another stable object.
– Example: A book on a table is being pushed down by
gravity, but the normal force is the table pushing back.
Sketch the man,
the rope, and
the cliff – then
draw force
vectors and
label the forces
at work.
Which one is
the “normal”
force?

5. Sketch two stick figures pulling on the rope. Draw force
vectors and label the different forces at work. Be thorough!
Don’t leave any force vectors out. Which one is
the “normal”
force?

6. Sketch the “bridge.” Some force vectors are
already drawn, but I want you draw more and label
them - what kind of forces are at work?
Gravity
Which one is
the “normal”
force?