Force, Motion and Friction
What is a force?“May the FORCE be with you.”
“Wildcats Basketball Team is a FORCE to be reckoned with!”
“That lightening storm had a great deal of FORCE.”
You have all heard the word force used in a sentence but
what does it really mean?
In science a force is defined as a push or a pull!
About a force…
Unit of force is the NEWTON (N)– The force required to accelerate a
mass of 1kg at a rate of 1m/s2
– Gravity pulls on a mass of 100g (like an apple) with a force of 1N
– A typical adult has a weight of about 670 N
A force has size and direction
50.5 N
10 N1000 N
10002 N
20
N
5 N
2000 N
20 N
Forces acting on objects…
All forces act on objects. For any push to occur, something has to receive the push. YOU CAN’T PUSH NOTHING!
When you do school work you use your fingers to pull open a book or push buttons on a keyboard.
You can’t always see a force, but you can see it’s results.
Does a force acting upon an object mean that it will move?
• Think of what happens when you sit in your chair during science class. It does not move. Why?
Net Force
The combined forces acting on an object Add if in the same direction
25 N + 20 N = 45 N Subtract if in the opposite direction
Balanced and Unbalanced Forces
The net force tells you whether the forces on the object are balanced or unbalanced.
What do you think the forces on an object would have to equal (in Newtons) in order to have a BALANCED force?
If the forces on an object produce a NET FORCE of ZERO the forces are balanced!
Examples of BALANCED FORCE
If the net forces are balanced the object will not move.
Unbalanced Forces
When the net force on an object is not 0 N the force on the object is said to be UNBALANCED.
What happens to the object if the net force is not zero?
Can you think of an example of an unbalanced force?
Friction
A force that opposes motion between two surfaces that are in contact.
Types of Friction
Static friction: Kinetic friction:
The force keeping 2 adjacent objects from moving.
Sliding and Rolling: The force resisting the movement of one
object sliding or rolling on another.
Kinetic Friction • Friction between moving surfaces (roll vs.
slide)
• Examples: brakes on a bike, writing with a pencil, scratch an itch! Almost all transportation is based on rolling kinetic friction (ie. Cars, trains, planes, bikes, skates)
Static Friction• When a force is applied to an object but does not move!• Forces are balance – if they become
unbalanced, kinetic friction takes over
The direction of the friction force is in the opposite direction of the motion of the object.
What causes friction?
When the hills and valleys of
one surface stick to the hills and
valleys of another surface,
friction is created.
Helpful / Harmful• Helpful:
o Brakes o Writing o Walk and run
• Harmful:o Holes in your socks and jeanso Erosiono Wear and tear on equipment
Geckos – the ultimate in friction technology!
How geckos stick:
StickyBot
Reduce / Increase• Reduce Friction by:
o Lubricants (motor oil, wax, grease)o Switching from sliding to rolling kinetic
frictiono Make a smoother surface
• Increase Friction by: o Make surfaces rougher (sand on icy road or
textured gloves for batting)o Pushing surfaces together (adding force)
• Sanding wood – press harder down as your rub and it will sand down faster due to added friction
Reducing friction
Oil or other lubricant
Ball bearings for axles
Cartilage in joints
Friction Factors:
If sliding object has more mass (hence more weight) = more friction
(Surfaces are pressed together more)
If surface is rougher / stickier = more friction
Surface area… …does NOT affect friction
Friction
