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PHYSICS 1L03
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Reza Nejat
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Physics

PHYSICS 1L03

Reza Nejat

Fall

Description

Forces
Forces
Concepts: What is force?
Force
Category of forces
Free body diagram
Equilibrium
Chapter: 3
Pages: 48-69
1 2
Forces Forces
•ittbj,t
•Werrtlr there are two forces, one on each object
wagon a force is being exerted.
•idnln trll
object, resulting from the interaction between applies a force on the mule, simultaneously; (with
the same magnitude but in opposite direction).
two objects, such as the person and the cart.
3 – Forces always act in pairs, acting on two 4ifferent objects
1 Force Force
Broad Categories of Forces: •Psft:r
• Contact forces: require physical contact –Forceisapushorapullthatoneobjectexertson
another.
between two interacting objects.
–Forceeitherisacontact force or is a contact free
force.
• Contact free forces: require no contact –Forceisavector,havingbothmagnitudeand
direction.
between two interacting objects and acts over
a distance –Forcesarepaired.Twointeractingobjectsexerta
force on each other, at the same time.
• These forces may be long range forces –Forcesareadditive.Theeffectoftwo
simultaneous force on a same object is the same
•Contactfreeloclldareas field forces as the net force of the two forces.
5 6
Force
Force:
What does a force do? •Wealrr
1
on the ball while the two are in contact ,
changing the velocity (the speed and/or the
direction) of the ball.
1
• :-- Force is an interaction between two
objects.
2
• :-- Force (external) changes the velocity,
causes acceleration , of the object.
7 8
2 Force: Force:
What does a force do? Action and effect of a force:
•ranote
9 10
Force
Brief Review on adding vectors
• Unit of Forces:
•uortmbrwtd
•li(.) vectors in component form, you
should watch the tutorial video on
•Nd)iI vectors posted on AVENUE
1N ▯ 1 Kgm
s2 •DOOTSIHISEORYOU
WILL BE LOST FOR THE REST OF
THE TERM!
11 12
3 Brief Review on adding vectors
Brief Review on adding vectors
▯ ▯ ▯
•W eanlays A ▯ A ▯xA y
break any 2D
Ax▯ A cos▯
vector up into
components Ay▯ A sin▯ ▯
▯
along two 2 2 2
A ▯ A ▯xA x
perpendicular
2 2
axes, such as x A ▯ A x A x
and y: A ▯ ▯ ▯
tan▯ ▯ y A ▯ A ,xA y▯
A
x ▯ ▯
A ▯ ▯ A, ▯ ▯ A ▯ A ,xA y
13 14
y
Review: Adding two vectors: Adding two vectors:
Fseot faerhnid 1 and F2, shown below
F 2x▯ F 2os ▯ Ry
▯ ▯ ▯
R ▯ F ▯1F 2 F ▯ F sin▯
2y 2
A simple way to find the resultant of two F1 R F1x ▯ F1cos ▯ F
1y R
vectors is finding the components of it. F1
1. Find the x and y components of each F2 F1y ▯ F 1in ▯
vector.
2. x-component of resultant is the Rx▯ F 1x▯ F 2x
summation (algebraically) of x- Ry
▯
components of vectors. F1y R y F 1y ▯ F2y F2y ▯ F2
3. y-component of resultant is the F1 R
▯
summation (algebraically) of y- R ▯ R x R y F R x
components of vectors. F2y F2 F 1x 2x x
tan ▯ ▯ R R
Rx▯ ▯ Fix R ▯ F F F 15x y x 16
y ▯ iy 1x 2x
4 Practice: Total Force on an object Practice: Total Force on an object
fo eulav tah w rof ,woleb erugi f eht n •I ▯ will •Fitnie ▯ ▯ ▯
C = A + B have: (at home) and direction of
D ▯ A ▯ B ▯ C
at home
a. A minimum value y
b. A maximum value
o
c. Find C for ▯= 90 . B=12
Co6
23
▯
A=5
23
Force Force
Categories of Forces: Fundamental types of forces: Only four types:
•FnamFcse
• Gravity (long ran)e
– contact-free forces – the force between masses
– holds planets and stars together, makes things fall, etc
– cannot be explained as any component of
• Electromagnetic ( long rang)
other forces – the force between charges, magnets,
– responsible for all familiar “everyday” forces (except
• Convenience Forces (non-fundamental) gravity)
– Can be explained in terms of fundamental • Strong (short rang)
– holds atomic nuclei together
forces
• Weak ( short rang)
27 – Beta decay (radioactivity) Force Force
•Goi()i
Gravitational force (Gravity):
eno stcar t ta taht ecrof eh •t ▯
F
mass to another g
▯
t rexe stcejbo o wt y •A Fg
gravitational force on each
other.
ttrli
orbiting the Sun.
29 30
Force Force
•ewfli:v Newton's law of universal gravitation:
• An attractive gravitational force between
two objects is proportional to the product of
their masses and inversely proportional to
m 1 2 ▯11 2 2 the square of the distance between them.
F ▯ G 2 G ▯ 6.67▯10 N m kg
r
•W:r F ▯ G m 1 2
r2
– m 1nd m a2e the objects masses,
– r is the distance between them,
– G is the universal gravitational constant.
32 33 Force Weight
Example: •W wtomm nalrll
•tttj the force of gravity acting on a body
40 cm from each other center. What is the force
of gravity of each acting on the other?
mM E M E
w ▯ F ▯g 2 ▯ m(G 2 ) ▯ mg
F ▯ Gm1m2 G ▯ 6.67▯10N m kg2 R E R E
r2
•NetEtuf: w ▯ mg
• where g is the acceleration of an object due
to gravity. On the Earth, it is approximately
34 constant everywhere on the planet. 36
Force Force
Example: Example:
,elrala g , at ’2thrip
M
Mars’s surface, if the mars has a mass of and Mars’s surface.
6.42 x 103 kg and a radius of 3.40 x 10 m. w ▯ mg
F ▯ Gm1m 2 G ▯ 6.67▯10▯1N m kg 2
r2
37 39
7 Force Force
Example:
•iolt
m ftll the 20 kg sphere at Mars’s
g ▯ 9.8 2 surface, if the mars has a mass of 6.42 x 103kg
s and a radius of 3.40 x 10 m.
• On the Earth surface, it is approximately
constant everywhere. F ▯ Gm1m2 G ▯ 6.67▯10N m kg2
r2
•ttr
gravitational field weakens, so g decreases.
g ▯ 1
r2
41 42
Force Force
Effect of gravitational force:
Electromagnetic force
.rftgsre
tmete
cctirs
(electric & magnetic force) on each other.
Puffy-face and bird-leg look
mytptlo
body is raised under the influence of gravity.
48 49
8 Force Force
•Clb’w
Coulomb’s Law
• An electric force between two charges is
proportional to the product of the magnitude
of their chrges and inversely proportional to
q q
F ▯ k 1 2 k▯ 8.99▯10 N m C 2 2 the square of the distance between them.
r 2
•W:r q1q2
– q and q are the magnitudes of the charges (in F ▯ k 2
1 2 r
coulomb “C”),
– r is the distance between them,
– k is the Coulomb constant.

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