Force components and FBD's

1. Lesson 3Lesson 3
Force Components,Force Components,
Free Body Diagrams, andFree Body Diagrams, and
Newton’s First Law of MotionNewton’s First Law of Motion
Rules for drawing FBD’sRules for drawing FBD’s
Nelson TB Reference Pages:Nelson TB Reference Pages:
114, 117-120, 123 -126114, 117-120, 123 -126

2. Breaking a Force Vector intoBreaking a Force Vector into
TwoTwo ⊥⊥ Component VectorsComponent Vectors
U P
W E
3 0 0F
=
30.0
N
[ E30
0 UP
]
 In the first diagram we have the force that we are going toIn the first diagram we have the force that we are going to
break into an east and an up component by usingbreak into an east and an up component by using
trigonometry.trigonometry.
 It is wise toIt is wise to remember that the horizontal component (east)that the horizontal component (east)
is obtained by multiplying the original force (F) by cosis obtained by multiplying the original force (F) by cosθ,θ,
where θ is thewhere θ is the ∠∠ betweenbetween FF and the east component.and the east component.
3 0 0F
=
30.0 N
[ E30
0 UP
]
F E a s t = 3 0 .0 C o s 3 0 0
= 2 6 .0 N
Fup=30.0Sin300
=15.0N
E
U p

3. Free Body Diagrams (FBD’S)
 FBD’s are a very easy way to perform forceFBD’s are a very easy way to perform force
analysis on an object. On the FBD we show allanalysis on an object. On the FBD we show all
forces acting on an object. We then add theseforces acting on an object. We then add these
forces together to determine the total of netforces together to determine the total of net
force (Fforce (Fnetnet) which can be) which can be equal to zero, or not!
 It is important to remember that if FIt is important to remember that if Fnetnet = 0, the= 0, the
object may be at rest or moving with uniformobject may be at rest or moving with uniform
motion (recall Galileo’s thought exp’t).motion (recall Galileo’s thought exp’t).

4.  Example: A boy is pulling a rope attached to a box asExample: A boy is pulling a rope attached to a box as
shown in the initial diagram with a force of 60.0 N [30shown in the initial diagram with a force of 60.0 N [3000
up from horiz]. If the box is moving withup from horiz]. If the box is moving with uniform motionuniform motion,,
determine:determine:
 FFNN, F, Fff, µ and the type of friction acting, µ and the type of friction acting
 Solution: The first step is to draw a FBD. To makeSolution: The first step is to draw a FBD. To make
calculations easier we will immediately break the 60.0 Ncalculations easier we will immediately break the 60.0 N
force into two components, and calculate Fforce into two components, and calculate Fgg..
1 0 .0
k g
D O M
F g = 9 8 .1 N
5 2 .0 N
3 0 .0 N F N
= 6 8 .1 N
F k = 5 2 .0 N

5.  The 60.0 N force is broken into two componentsThe 60.0 N force is broken into two components
30.0 N and 52.0 N. F30.0 N and 52.0 N. Fgg is also placed on theis also placed on the
FBD.FBD.
 The box is moving with uniform motion so theThe box is moving with uniform motion so the
horizontal forces must cancel this means Fhorizontal forces must cancel this means Fkk isis
equal to 52.0 N and is in the opposite directionequal to 52.0 N and is in the opposite direction
to the 52.0 N component of Fto the 52.0 N component of FAA
 Vertical forces must also cancel so FVertical forces must also cancel so FNN mustmust
equal | Fequal | Fgg | - 30.0 N = 68.1 N| - 30.0 N = 68.1 N
 There is no net force; all horizontal and verticalThere is no net force; all horizontal and vertical
forces cancel (or we can say these forces areforces cancel (or we can say these forces are
balanced).balanced).
 µµkk = F= Fkk/F/FNN = (52.0N)/(68.1N)=0.764= (52.0N)/(68.1N)=0.764

6. Rules for Drawing FBD’s
1.1. You may use a dot or a box to represent theYou may use a dot or a box to represent the
object.object.
2.2. ShowShow onlyonly the forces actingthe forces acting onon the object, butthe object, but
show allshow all the forces acting on the object.the forces acting on the object.
3.3. All force arrows are drawnAll force arrows are drawn outout of the box (orof the box (or
the dot) and the arrow lengths must representthe dot) and the arrow lengths must represent
the magnitude of the force.the magnitude of the force.
4.4. You may show the components of a force, orYou may show the components of a force, or
the actual force.the actual force.
5.5. Label all forces with magnitudes and namesLabel all forces with magnitudes and names
(F(Fgg, F, FNN, etc), etc)
6.6. Add any missing forces; such as FAdd any missing forces; such as FNN and Fand FKK

7. Newton’s First Law
(Galileo’s Principle of Inertia)
 An object at rest, or moving with uniformAn object at rest, or moving with uniform
motion,motion, will maintain its current state ofwill maintain its current state of
motionmotion (being at rest or moving with(being at rest or moving with
uniform motion). For this to be true, thereuniform motion). For this to be true, there
must bemust be NONO net force (or unbalancednet force (or unbalanced
force) acting on the object.force) acting on the object.
 (Newton was born the same year Galileo(Newton was born the same year Galileo
died (1642).)died (1642).)

8. Practice Questions
 Work book Page 26Work book Page 26 (New pg 27) - all parts –- all parts – FBD’s,
1st
Law
 WB pg 33WB pg 33 (New pg 35) (left side only)(left side only)
#1,2,3,4,6,9,10 -#1,2,3,4,6,9,10 -FBD’s, 1st
Law
 WB pg 25WB pg 25 (New pg 26) – all parts– all parts –– drawing FBD’s
There are a lot of questions, and we will spend more than
one day working on these!

9. Car Test Videos – Why?
 http://www.youtube.com/user/iihs#p/u/5/4yFvQppaAdc More onhttp://www.youtube.com/user/iihs#p/u/5/4yFvQppaAdc More on
small car crash tests - replace link it does not worksmall car crash tests - replace link it does not work
 http://www.youtube.com/watch?v=C3MPKLy9qHU&feature=c4-overview-http://www.youtube.com/watch?v=C3MPKLy9qHU&feature=c4-overview-
vl&list=PL5RxiGTHpKTGB-J60ezyXDfw2xH623OSZvl&list=PL5RxiGTHpKTGB-J60ezyXDfw2xH623OSZ Car hitting truckCar hitting truck
trailertrailer
 http://www.youtube.com/watch?v=wmCZZ_Ef3Js&feature=c4-http://www.youtube.com/watch?v=wmCZZ_Ef3Js&feature=c4-
overview-vl&list=PL5RxiGTHpKTGB-J60ezyXDfw2xH623OSZ slightoverview-vl&list=PL5RxiGTHpKTGB-J60ezyXDfw2xH623OSZ slight
offset crash for small SUV’soffset crash for small SUV’s
 http://www.youtube.com/watch?http://www.youtube.com/watch?
v=DtGEjXyUOjs&list=PL7A5C4CBD6CB2C789 micro car crashesv=DtGEjXyUOjs&list=PL7A5C4CBD6CB2C789 micro car crashes

10. Car Test Videos – Why?
 http://www.youtube.com/user/iihs#p/u/5/4yFvQppaAdc More onhttp://www.youtube.com/user/iihs#p/u/5/4yFvQppaAdc More on
small car crash tests - replace link it does not worksmall car crash tests - replace link it does not work
 http://www.youtube.com/watch?v=C3MPKLy9qHU&feature=c4-overview-http://www.youtube.com/watch?v=C3MPKLy9qHU&feature=c4-overview-
vl&list=PL5RxiGTHpKTGB-J60ezyXDfw2xH623OSZvl&list=PL5RxiGTHpKTGB-J60ezyXDfw2xH623OSZ Car hitting truckCar hitting truck
trailertrailer
 http://www.youtube.com/watch?v=wmCZZ_Ef3Js&feature=c4-http://www.youtube.com/watch?v=wmCZZ_Ef3Js&feature=c4-
overview-vl&list=PL5RxiGTHpKTGB-J60ezyXDfw2xH623OSZ slightoverview-vl&list=PL5RxiGTHpKTGB-J60ezyXDfw2xH623OSZ slight
offset crash for small SUV’soffset crash for small SUV’s
 http://www.youtube.com/watch?http://www.youtube.com/watch?
v=DtGEjXyUOjs&list=PL7A5C4CBD6CB2C789 micro car crashesv=DtGEjXyUOjs&list=PL7A5C4CBD6CB2C789 micro car crashes