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 F W (commonly called a Weight) is equal to: m g so F w = F mg

Chapter 6 b. Normal Force points _|_ to surface - use F N 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: F f Sometimes, to be more specific, you might use F K or F S [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 (F K ); this is also known as “Sliding Friction” 2. Static Friction – the friction which must be overcome to get a body to move (F S )

Chapter 6 Typical Coefficients of Friction Surface ss kk Rubber on concrete Rubber on wet concrete Wood on wood Steel on Steel (dry)

Chapter 6 d. Applied Forces  point in the direction of push or pull  use F A

Chapter 6 e. Tension results if pulling force acts on ends, such as a rope or wire used to pull a boulder use F T fbd/quizzes.htmlhttp://eta.physics.uoguelph.ca/tutorials/ fbd/quizzes.html

Chapter 6 Forces Summary ForceSymbolDefinitionDirection Friction FfFf The force that acts to oppose sliding motion b/w surfaces // to the surface and opposite the direction of sliding Normal FNFN The contact force exerted by a surface on an object Perpendicular to and away from the surface Spring F sp A 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 FTFT The 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 F thrust or F A 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 F g or F w A 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 FgFg FNFN

Chapter 6 Example 2 – Draw a free-body diagram for… An ice puck on a slanted surface FgFg FNFN

Chapter 6 Example 3 – Draw a free-body diagram for… A piano being pulled upstairs by a rope FgFg FNFN FTFT FfFf

Chapter 6 Example 4 – Draw a free-body diagram for… A pendulum bob FTFT FgFg

Chapter 6 Example 5 – Draw a free-body diagram for… Take the elevator to the fourth floor FNFN FgFg

Chapter 6 Example 6 – Draw a free-body diagram for… Pendulum swings in a horizontal circle FTFT FgFg

Chapter 6 Example 7 – Draw a free-body diagram for…You push 2 boxes down the hall. For box A… FNFN FgFg FAFA FfFf F due to B

Chapter 6 Example 8 – Draw a free-body diagram for…You push 2 boxes down the hall. For box B… FNFN FgFg F due to A FfFf

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… FNFN FgFg FfFf

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… FNFN FgFg FfFf

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 FNFN FgFg FfFf FTFT 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 FNFN FgFg FfFf FTFT The force on the novel…