Chapter 6

Free Body Diagrams

Chapter 6

Background• the “body” = the object

• “free” = it is shown without other objects around it

• abbreviated “fbd”

Chapter 6

Drawing FBD’s • example: boat on

water being pulled by a rope

• STEPS:• 1. Simplify the

object (dot or box)• 2. Add in all the

forces which apply

Chapter 6

Some Possible Forces

• Gravity

• Normal Force

• Frictional Force

• Tension

• Thrust (Applied Force)

Chapter 6

a. Gravity• points downward towards the center of

the Earth

• use FW

• (commonly called a Weight)

• is equal to: m g

• so Fw = Fmg

Chapter 6

b. Normal Force• points _|_ to surface - use FN

• prevents the object from “falling into” whatever it is sitting on and is always _|_ to the surface upon which it rests

Chapter 6

c. Frictional Forces• runs parallel to motion but in the

opposite direction -

• use: Ff

• Sometimes, to be more specific, you might use FK or FS

• [See next slide]

Chapter 6

“Coefficient of Friction”• a value representing the relative “stickiness” of a surface; the higher the value, the

greater the friction. Values are found in a table and two surfaces must always be compared – such as wood on ice, not just ice.

• has the symbol:

• There are two types of Friction:• 1. Kinetic Friction – the friction which causes moving objects to eventually stop (FK);

this is also known as “Sliding Friction”

• 2. Static Friction – the friction which must be overcome to get a body to move (FS)

Chapter 6

Typical Coefficients of Friction

Surface s k

Rubber on concrete

0.80 0.65

Rubber on wet concrete

0.60 0.40

Wood on wood

0.50 0.20

Steel on Steel (dry)

0.78 0.58

Chapter 6

d. Applied Forces

point in the direction of push or pull use FA

Chapter 6

e. Tension• results if pulling force acts on ends,

such as a rope or wire used to pull a boulder

• use FT

• http://eta.physics.uoguelph.ca/tutorials/fbd/quizzes.html

Chapter 6

Forces Summary (p. 123)Force Symbol Definition Direction

Friction FfThe force that acts to oppose sliding motion b/w surfaces

// to the surface and opposite the direction of sliding

Normal FNThe contact force exerted by a surface on an object

Perpendicular to and away from the surface

Spring FspA restoring force, that is, the push or pull a spring exerts on an object

Opposite the displacement of the object at the end of the spring

Tension FTThe pull exerted by a string, rope or cable when attached to a body and pulled taut

Away from the object and // to the string, rope, or cable at the point of attachment

Thrust Fthrust

or FA

A general term for the forces that move objects such as rockets, planes, cars and people (Force Applied)

In the same direction as the acceleration of the object barring any resistive forces

Weight FgA long-range force due to gravitational attraction b/w two objects, generally Earth and an object

Straight down toward the center of the Earth

Chapter 6

Example 1 – Draw a free-body diagram for…

A box resting flat on a flat surface

Fg

FN

Chapter 6

Example 2 – Draw a free-body diagram for…

An ice puck on a slanted surface

Fg

FN

Chapter 6

Example 3 – Draw a free-body diagram for…

A piano being pulled upstairs by a rope

Fg

FN

FT

Ff

Chapter 6

Example 4 – Draw a free-body diagram for…

A pendulum bob

FT

Fg

Chapter 6

Example 5 – Draw a free-body diagram for…

Take the elevator to the fourth floor

FN

Fg

Chapter 6

Example 6 – Draw a free-body diagram for…

Pendulum swings in a horizontal circle

FT

Fg

Chapter 6

Example 7 – Draw a free-body diagram for… You push 2 boxes

down the hall.

For box A…

FN

Fg

FA

Ff

F due to B

Chapter 6

Example 8 – Draw a free-body diagram for… You push 2 boxes

down the hall.

For box B…

FN

Fg

Fdue to A

Ff

Chapter 6

Example 9 – Draw a free-body diagram for… A truck accelerates

so quickly that a box

in front doesn’t fall.

The F on the box…

FN

Fg

Ff

Chapter 6

Example 10 – Draw a free-body diagram for… Box B accelerates to

the right while A rests

on top. The F on A…

FN

Fg

Ff

Chapter 6

Example 11 – Draw a free-body diagram for… A Physics Text and a

novel are joined by a

rope slung over a knob

FN

Fg

Ff

FT

The force on the Text…

Chapter 6

Example 12 – Draw a free-body diagram for… A Physics Text and a

novel are joined by a

rope slung over a knob

FN

Fg

Ff

FTThe force on the novel…