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9TH CLASSES
PHYSICS
DAILY PLAN
SUBJECT: Force and the laws of motion
GOALS:
DURATION:
IN PRACTICE:
If object A exerts a force F on object B, then object B
exerts a force –F on object A. These two forces are
equal in magnitude but opposite in direction.
Ex 8:
PRESENTATION:
DYNAMICS
Dynamics is about the relation between force and motion.
Force: A force is a pull or push on an object. A
force can cause a change in the shape or velocity of
the object, start it to move or stop the motion.
Net Force (or Resultant Force): is the vector sum
of all forces acting on that object.
Fnet=F1+F2+F3+… (vector sum)
F
F’
F = - F’
Unit of force: is Newton (N) or Dyne (dyn)
Newton’s laws of motion:
N
First Law of Motion:
a) An object at rest will remain at rest if there is zero
resultant force acting on it; and;
b) If the net force on a moving object is zero, the
object continues to move with constant velocity (=on
a straight path)
v
This object stops.
When an object is at rest on a
graund, the forces acting on
the object are the normal
force, N, and the force of
gravity, W. Here ;
W = N = mg
W
N
N
F
F
v
pump
air
This object
does not
stop.
The reason is: friction force stops the first object.
But there is no net force to stop the second object.
Therefore:
Fnet  0  v  const
If an object is moving, no force is needed to keep it
moving. The object will move forever without any
force on it. This is because of inertia.
W
W
N = W+ F
N=W-F
Second Law of Motion:
Newton’s second law answers the question of what happens to an
object that has a nonzero resultant force acting on it.
Newton’s second law states that; if a net force acts on a body, the
body will accelerate. The acceleration of an object is directly
proportional to the resultant force acting on it and inversely
proportional to its mass.
[
a
m
F
m
2a
2F
aF
Third law of motion:
Units:
SI: 1 Newton = 1 kg m/s2
m
F
a/2
2m
and
F net  ma
3F
a/3
Inertia:
Inertia is the tendency of an object at rest to remain at
rest; and of an object in motion to remain in motion
with its original velocity.
Mass:
Mass is measure of inertia. (unit : kg)
Big mass big inertia
a
3a
m
F
1
a
m
3m
F
]
F
a
m
(Fnet and a are in the same direction)
Therefore 1 N is the force which gives a 1-kg mass an
acceleration of 1 m/s2.
Also we have the cgs unit:
1 dyne = 1 gr cm/s2.
So: 1N=100000 dyn
Forces of Friction
When a body is in motion on a rough surface, or through a viscous
medium such as air or water, there is resistance to motion because of
the interaction of the body with its surroundings. We call such
resistance a force of friction.
Ex 10: A 2 kg object accelerates from rest to 5 m/s in 0.4 s.
What is the net force on the object?
Ex 11: A force of 20 N acts on a 4 kg object. What distance
does the object travel in 3 s? (v0=0)
Mass and Weight
Mass is the amount of matter. Mass is scalar. (unit: kg)
Weight is the gravitational force pulling the object down.
Weight is vector. (Unit: Newton)
Mass
Static and kinetic friction: When there is friction, it is easier to
keep an object moving than to start it to move.
Ffrictio
n
starts to move
maximum
static friction
Weight
kinetic friction
Fapplie
d
The Earth pulls each 1-kg object with a 9.8 N force. This is
called gravitational constant (g)
2 kg
1 kg
3 kg
Kinetic friction depends on two factors:
1) The force pushing the objects together (N)
2) Material of the object (k)
k: coefficient of friction
g=9.8 N/kg
v
g  10 N/kg
10 N
Ff = k N
20 N
30 N
Ff
W=mg
k
N
Ex 37:
Inclined Plane
Forces on the object:
v0=0
F=25 N
5
y
N
k=0.8
[Sol: kN gives 40 N but friction can not be 40 N (or it will start
to move back). Ff= 25 N.]
mg Sin

x
Therefore: Formula Ff=kN gives us the friction if the object is
moving.
If object does not have any intention to move then Ff=Fext.
a
mg Cos
mg

Ex 38: What is the acceleration of the object in figure?
v0=0
Ex 32:
g
m
a) Find the accelerationn
of the object.
b) In which direction is
it moving?
37°
Find the tension in
therope and the acceleration.
Ex 35: What is the
final velocity of the
object after sliding for
5m on a smooth
inclined plane, if it is
released from rest?
Lmax=?
F=15 N
2
k=0.5
HOMEWORK:
MULTIMEDIA:
DEMONSTRATION:
EXPERIMENT:
v0 = 8 m/s
TEACHER:
30°
DIRECTOR: