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Common Forces. Physics 12. Joke of the day:. Not physics but still funny?. Review: Free-body diagrams:. Free-body diagrams are used to show the relative magnitude and direction of all forces acting on an object. What if some of the forces were not in the x or y plane???????. - PowerPoint PPT Presentation
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Joke of the day:
Not physics but still funny?
Review: Free-body diagrams:
Free-body diagrams are used to show the relative magnitude and direction of all forces acting on an object. What if some of the forces
were not in the x or y plane???????
Forces at angles:
We will break the forces with angles into their x and y components using trigonometry! AGAIN!
Fy = F sin θ
Fx = F cos θ
Example #1:
A 35kg sign is suspended by two wires that each make a 25° with the horizontal. Calculate the tension in each wire. Draw a FBD.
35kg
25°25°
Free Body Diagram
What do we know? What can we calculate?
NF
smkgF
gmF
g
g
g
340
)81.9(35 2
?2 T
?1 T
Θ=25°
Φ=155°
Fg
Net Force:
Not moving….so no acceleration Thus….
0
0
0
0
nety
netx
net
F
F
F
a
Components:
X components: Is there Fg?
No! Others:
Y components: Is there Fg ?
Yes! (Fgy )
Others:
cos
cos
22
11
TT
TT
x
x
sin
sin
22
11
TT
TT
y
y
So what does that tell us?
Analysis:
X-direction
xxnetx TTF 21
21
21 91.)91.(0
TT
TT
xx TT 210
25cos155cos0 21 TT
Analysis
Y-direction
gyynety FTTF
21
21
21
42.42.340
)340(25sin155sin0
TTN
NTT
gmTTFnety
sinsin 21
Analysis (almost done!):
NT
NT
TN
TTN
TT
TTN
410
410
85.340
42.42.340
42.42.340
2
1
1
11
21
21
Review: Normal force
is perpendicular to the surface of contact contact force exerted on an object For example, the surface of a floor or wall, preventing the
object from penetrating the surface.
Review: Force of friction:
Direction of Ff is always opposite to the direction of motion
Ff = µ FN
µ is the coefficient of friction (“stickiness value”, no units)
Example #2:
Holding a Book Against a Wall
What is the applied force? FA
Fg
FN
Ff
kgm 0.1
65.
Holding a Book Against a Wall
NF
FFF
NF
NF
NF
FF
NF
FFF
NF
smkgF
gmF
A
ANnetx
N
N
N
Nf
f
fgnety
g
g
g
15
15
15
65.
8.9
8.9
8.9
)81.9(0.1 2
FA
Fg
FN
Ff
kgm 0.1
65.
Try it :
Page 467 Questions 8,9
Inclined Planes
Label the direction of FN and Fg.
mgθ
N
Not both in the x or y plane…….
Inclined Planes
Take note of and mark the direction of acceleration (a).
mgθ
N
a
Inclined Planes Choose the coordinate system with x in the same or opposite
direction of acceleration and y perpendicular to x. Draw it on the diagram!!!!!!!
mgθ
N
a
x
y
Inclined Planes
Now some trigonometry!
mgθ
N
a
x
y
90- θθ
Inclined Planes
Find the components of the force of gravity.
mgθ
θ
N
a
x
y
cos
sin
ggy
g
FF
FFgx
Example #3:
A worker places a large plastic waste container with a mass of 84 kg on the ramp of a loading dock as shown in the figure. The ramp makes an angle of 22° with the horizontal. The workers turns to pick up another container before pushing the first one up the ramp.(a) If the coefficient of static friction
is 0.47, will the crate slide down the ramp? (b) If the crate does slide down, what will be its acceleration?
Try it
Page 474 10, 11, 12, 13(b)
