New Material Monday - Forces and Friction
11/10/14 and 11/17/14
What is a force?
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In science, a force is a push or a pull
All forces have two properties:
 Direction
 Size
A newton (N) is the unit that describes the size of a force
A student is pushing down on a chair, but does the chair does not move, so the floor is balancing
the force by pushing on the chair
How do forces combine?
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More than one force often acts on an object
When all the forces acting on an object are added together, you get the net force on the object
An object with a net force with more than zero on it will change its rate of motion
Forces in the Same Direction
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When forces are applied in the same direction, they are added to determine the size of the net
force
Forces in Different Directions
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When two forces act in opposite directions, you subtract the smaller force from the larger force
to determine the net force
The net force will be in the same direction as the larger force
Balanced Forces
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When the forces on an object produce a net force of 0, the forces are balanced
There is no change in motion of the object
Unbalanced Forces
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When the net force is not 0 N, then the forces are unbalanced
Unbalanced forces produce a change in motion
What Causes Friction?
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Friction is the force that opposes motion between two surfaces that touch
The surface of any object is rough
Even a smooth object has tiny bumps
The contact between the bumps of two surfaces causes them to stick, resulting in friction
Types of Friction
 Kinetic friction occurs when force is applied to an object and it moves
Examples:
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Sliding Friction: pushing an object across a surface
Rolling Friction: friction between wheels and a surface
Fluid Friction: friction of objects moving through fluids
Static Friction: friction occurs when a force applied to an object does not move it
Newton’s Laws of Motion
First Law – an object at rest will stay at rest, and object in motion will stay in motion at a constant speed,
unless an unbalanced force acts on them
 Law of Inertia – the tendency of an object to resist a change in motion
 Once airborne, unless acted on by an unbalanced force, an object would never stop!
Second Law – force equals mass times acceleration
 F=mxa
 When mass is in kilograms, and the acceleration is in m/s2, the unit of force is in N, the newton
 One N is equal to the force required to accelerate one kilogram of mass one m/s2
Third Law – for every action there is an equal and opposite reaction
 When you sit on a chair, you exert a force on the chair, and the chair exerts a force up on you;
these are action and reaction forces
Momentum
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Characteristic of a moving object related to its mass and velocity
Momentum is mass x velocity – m x v
Depends on how big an object is and how fast it is moving
The total momentum of any group of objects is the same, or conserved, unless outside forces
act on the objects