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Students will be able to define andunderstand Forces. Students will also beable to draw and interpret Free Body

Diagrams

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Force

Simple Definition of a force : It is a Push or a Pull

A little less simple amF

This second Definition is known as Newtons second Law of Motion

A force is anything that can cause a change in the velocity of anobject. A force is anything that can accelerate a mass.

A force is therefore, a Vector Quantity with the unit being theNewton. Where

sec111 2mkgN

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Some Examples of Common Forces

1) Force of gravity (Fg)There is a force of attraction between all objectsOn earth the force acts downwardsIt is considered to be an 'action at a distance force'

(no physical connection)2) Normal Force (FN)

A force that is perpendicular to two surfaces incontact.

It is the force of a surface 'pushing back on theobject.3) Applied Force (FA)

A force that is applied directly in order to change thevelocity of an object.

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4) Tension (FT)An applied force exerted by materials that can be

stretched (e.g. Ropes, strings, springs, fibers, cables,rubber bands)The more a material is stretched the greater thetension.

5) Friction (Ff)Force between objects that are in contact with eachotherActs in a direction opposite to motion

a) Static Friction (Fs) prevents stationary objects from

moving.

b) Kinetic Friction (Fk) tries to slow down moving objects

c) Air Resistance or Drag (Fair) opposes an object's

motion through the air.

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Free- Body diagrams (FBD)

A diagram of only oneobject showing all of the forces on the object.

Example 1 -Pushing a block across a desk

FN

Ff

Fg

Fa

Force of Gravity: FgNormal Force: FNForce of Friction: FfApplied Force: Fa

+x

+y

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Example #1 Free Body Diagram

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Example #2 Free Body Diagram

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A free body diagrams represent all forces acting on that object.

Forces, which are along the same line should be added, typicallyalong the X and he Y Axis.The sum of all Forces is referred to as the Resultant Force orNet ForceWhen a system has a net force or resultant force of zero, we saythat the system is in Equilibrium. (Dynamic Motion Static NoMotionThe force which places a system in Equilibrium is referred to asthe equilibrant ForceAn object moving at a constant velocity is considered to be inequilibrium

Definitions and More

Consider the motion of car as it accelerates away from a red light.At first the force the engine overcomes friction and air resistance. Asthe velocity of the car increases, friction, especially wind resistanceincreases until The force of friction is equal and opposite to the forcegenerated by the engine. At this point you have an equilibrium of

forces.

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Newtons first Law of Motion

An object will remain at rest or in uniformmotion unless it is acted on by some externalforce.

Because of this we apply:

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Rules for drawing free body

diagrams (FBD's)a) Always show the object removed from its

surroundings

b) The force vectors (arrows) show thedirection of each forcec) The length of each force vector

represents its size compared to the other

force vectorsd) Choose a direction to be the positivedirection and show it with an arrow besidethe diagram

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Example 17 to 19

Pages 36 to 38

Heimbecker et al, Grade 12 Physics:Concepts and Connections

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Homework

Heimbecker et al, Grade 12 Physics: Concepts and Connections

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