Desvita Roza

1. JU R U SA N FISIK A
FA K U LTA S MATEMATIK A D A N ILMU PEN GETA H U A N A LA M
U N IVER SITA S N EGER I PA D A N G
2 0 1 7
LAWS OF MOTION AND
CONNECTED MASSES

2. Force and Motion
 An unbalanced force sets an object an object in motion.
 An unbalanced force acting on an object in motion.
 An unbalanced force changes the states of rest or state of
motion can change its state of motion.
 An unbalanced force changes the state of rest or state of
motion of an object. This statement is called the first law
motion.
 An object remains in its state of rest or of uniform motion
uncless acted upon by an external net force.
 A force is a vector quantity, it has both magnitude and
direction.
 All the rules of vector addition are applicable to force
vectors.

3. Inertia
 In the absence of an unbalanced force an object at rest will
continue to be at rest and an object in uniform motion will
continue to be uniform motion.
 This property is called inertia.
 Since inertia is the tendency to resist a change in the state of
rest or state of motion, it can be defined as the tendency to
resist acceleration.
 The first law of motion is also called the law of motion
 Law of inertia helps us to define force.
 A force is something that change or tends to change the
stateof rest or of uniform motion along a straight line of an
object.

4. The Second Law of Motion
aF net 
maF net 
 When an object at rest begins to move, it has an
acceleration since the velocity chnages.
 When an object change its state of motion, it has an
acceleration since because the velocity change.
 Whenever an unbalance force acts on an object, it produces
an acceleration on the object.
 The second law of motion states that:
 The unbalanced force acting on a particle is proportional to
the acceleration it prodces on the particle.

5. Unit of Force
 The second law of motion helps us to measure force using
the relation:
Fnet =m . a
 m is the mass of the particle and it is measured in kilogram
(Kg)
 Acceleration is measured in ms-2
 Therefore, force is measured in Kg.ms-2
 Kg.ms-2 is called a newton (N)
 unit of force is the newton (N)
 The amount of that produces an acceleration of 1 ms-2 on
particle of mass 1 kg is called one newton

6. Mass and weigth
 It is the mass of an object that determines its
inertia. A more massive object is more
reluctant to move than a ligth object.
 Mass is a measure of the inertia of the object
 Weigth is something that makes an object fall
 It is a measure of the force an object
experiences due to the gravitational pull of
the earth.
 The net force on a freely falling object is its
weigth
 The accelaration of free fall -g is produced by
this net force. Fnet =ma Weigth=-mg
 The negative sign indicated that the direction of the
weigth force is downward.
 Mass is a scalar quantity while weigth is a vector quantuy

7. Third Law of Motion
 The third law of motion states that forces always
come in pairs.
 What are the forces acting on the meditatingg
girl?
 Her weigth mg acts vertically down
 If this is the only force acting on her, she must be
felling freely with an acceleration -gms-2
 There is another force acting on her keeping her at rest
 This force is applied by the floor and directed upward
 This force is called the force of reaction and is called the
normal force (N)
 Look at the force pairs on the box resting on the floor
 Since the net force on the box is zero
N - mg = 0

8. Reaction Forces
 Reaction forces come in many forms.
 Look at the traffic ligth supported by two
cables.
 What are the force acting on it keeping it in
position?
 The weigth of the ligth mg acts vertically down
 To prevent the ligth from falling freely the cables apply force
upward
 These forces are called tension forces
 The tension on the left cable is T1
 The tension on the rigth cable is T2
 The vector sum of these there force is zero
 Here the action and reaction forces add to zero because all
the forces the acting on the same object keeping it at rest.

9. Connected Masses
 In order to do problems using connected masses
1. we isolate each object
2. Identify the force acting on each
3. Apply the laws of motion to form equations in each
4. Solve the equations for the unknown
 There blocks arre pulled along a frictionless horizontal by
applying a force of 18 N as shown belown. (a) what is the
acceleration of the system? (b) what are the tension for the
two pleces of strings connecting m1 ,m2 and m3 ?

10. 2
3
6
18 
 ms
Kg
N
m
F
a net
 The tension on the string connecting masses m1 and m2 is
T1
 Notice here that T1 pulls m1 to the rigth and m2 to the left.
 The tension on the string connecting masses m2 and m3 is
T2
 T2 pulls m2 to the rigth and m3 to the left
 a total force Fnet = 18 n pulls a total mass m = 1+2+3=6 Kg
Fnet =ma
 Since the masses are connected, each mass has the same
acceleration, namely 3 ms-2

11.  To find the tension T1 , we isolate the mass m1
 The net force acting on m1 is T1
 the tension T1 produces an acceleration 3 ms-2 on m1 = 1 Kg
 Applting Fnet = ma on m1 gives:
T1 = 1.0 kg x 3.0 ms-2 =3.0 N
 To find T2 , we isolate m2
 The forces on m2 are T2 to the rigth and T1 an T2
 Applying Fnet = ma on m2 gives
T2 -T1 = 2.0 Kg x 3.0 ms-2= 6.0 N
T2 = T1 + 6.0N = 3.0 N + 6.0 N = 9 N

12.  Two mass m1 =100 g m2 = 1001 g are
connected by a string that passes over a
frictionless pulley. find the acceleration of the
masses and the tension on the string.
 Since the same string supports the two
maased and there is no friction on the pulley,
the tension on the same as the tension on the
left = T
 We now isolate the mass m1 = 100 g = 0.1 Kg
 The weigth of m1 = m1g=0.1 kg x 9.8 ms-2 = 0.98 M acts
vertical down
 The tensiont on the string acts vwrtically up
 Since m2 > m1 . We will assume that m1 moves up and m2
moves down
 The net force on m1 is T-m1g=t-0.98N
 If a is the acceleration of the system, then
Fnet=m1 a T-0.98 = 0.1 a T- 0.1a = 0.98

14. Object on an Inclined Plane
 Consider a mass m resting on a plane
surface.
 Its weigth mg acts vertically down
 This is the force pressing the object to the
surface.
 what happens if the plane is titled to form
an inclined plane of inclination θ ?
 The weigth mg is still vertically down
 Only part of this is pressing on the surface
 The other part pulls it down the plane
 A line drawn at rigth angles to the plane makes angle θ
with mg
 The vector component of mg at rigth angles to the pane
is mg cos θ
 The component of mg down the plane is mg sin θ

15.  The component mg cos θ presses the object on the surface.
 The surface in turn in turn applies a force of reaction equal
and opposite to mg cos θ
 This is the normal force Fn
 There is the normal force at rigth angles to the plane.
Fn = mg cos θ
 mg sin θ pulls the object down the plane.
 This is the net force on the object.
 It produces an acceleration a on the
 object down the plane.
mg sin θ = ma a= g sin θ
Fnet = ma
 When the angle θ inreases, the force pressing the object to
the surface decreases and the force that pulls it down the
increases.

16. Istilah istilah fisika
 Force and Motion
 first law motion
 a vector quantity
 Inertia
 Acceleration
 the velocity
 a newton (N)
 Reluctant
 a scalar quantity
 Third law of motion
 Normal force (N)
 Reaction forces
 connected masses
 frictionless pulley
 The net force
 a plane surface.
 at rigth angles
 the normal force