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DR. MOHAMMED MOSTAFA EMAM
INAYA MEDICAL COLLEGE
(IMC)PHYS 101 - LECTURE 5
FORCE, WORK, ENERGY & POWER
1
FORCE
• What change the state of object is called “force”.
• We mean by saying state, shape or position of the
object.
2
Newton's law of Motions
(All objects save their state of motion)
(Force causes acceleration)
(Every actions cause reactions)
3
Newton's law of Motions
(All objects save their state of motion)
1st law of motion
(Force causes acceleration)
2ed law of motion
(Every actions cause reactions)
3rd law of motion
4
• In summary, you observe and apply force every day.
• Force is a vector quantity having both magnitude
and direction.
• We understand that cause of motion is “force”.
• The unit of force is Newton or kg.m/s².
5
FORCE
• You can kick the ball and apply a force then you give
speed to the ball and changes its position or you can
press the spring apply a force and give potential energy
to it, we can increase the number of example.
6
FORCE
• Newton's First Laws of Motion
(all objects save their state of motion)
• In his first law of motion Newton stated that
all objects save their state of motion.
• In other words, if an object is at rest it
continuous to be at rest and if it is moving it
continuous to move unless a nonzero force
is applied on it.
• What we mean by a word “nonzero force”? Look at
the following picture to understand what we mean.7
We called nonzero force “net force” also
8
FORCE
• Newton’s Second Law of Motion
9
FORCE
• In the previous topics we said that force causes acceleration.
• Moreover, we also learned the net force concept in the last
section.
• Now, we deal with the relation between force and acceleration.
• Newton’s Second Law of Motion
(Force causes acceleration)
10
FORCE
• As you remember, acceleration is the rate of change in the
velocity of the object. This change occurs because of the net
force. Thus, we can say that there is a linear relation
between the net force acting on the object and acceleration.
We show this relation like;
11
FORCE
• If we increase the amount of net force than the acceleration
also increases in the same amount.
• If we decrease the net force than acceleration also
decreases. Let’s see it from the following pictures.
12
FORCE
• If we increase the amount of net force than the acceleration
also increases in the same amount. If we decrease the net
force than acceleration also decreases. Let’s see it from the
following pictures.
13
FORCE
• From this relation:
•
14
FORCE
•Example
• Find the acceleration (a) of the block given in the picture below.
where, F is the force and its unit is Newton, m is mass and has the unit
kg and a is the acceleration has unit m/s².
15
FORCE
•Example
• Find the acceleration (a) of the block given in the picture below.
where, F is the force and its unit is Newton, m is mass in Kg, and has
the unit kg and a is the acceleration has unit m/s².
16
FORCE
•Example
• Find the acceleration (a) of the block given in the picture below.
where, F is the force and its unit is Newton, m is mass in Kg, and has
the unit kg and a is the acceleration has unit m/s².
17
FORCE
•Example
Given in the picture below, a horse is pulling the horsebox having
8 kg mass in it with a force of 40N; if the applied force has an
angle of 37º to the horizontal; calculate the acceleration of the
horsebox.
18
FORCE
19
FORCE
20
FORCE
21
FORCE
22
FORCE
23Direction of the acceleration is in the direction of
Fx which is the net force in direction of motion.
FORCE
•Example
Find the weight (W), of the object having mass 15 kg.
24
FORCE
•Example
Find the weight of the object having mass 15 kg.
25
FORCE
• Equilibrium Conditions:
Equilibrium in physics means, forces are in balance. The net
force should be zero.
In other words, forces acting downward and acting upward,
and forces acting right and acting left should be equal in
magnitude. Look at the illustration shape to understand that.
26
FORCE
In the given picture there are five forces acting on our block.
Let us analyze them, whether this block is in equilibrium or not.
Forces acting downward are; 10N+5N=15N Force acting on
upward is; 15N.
Thus, we can say that,
27
FORCE
In the given picture there are five forces acting on our block.
Let us analyze them, whether this block is in equilibrium or not.
Forces acting downward are; 10N+5N=15N Force acting on
upward is; 15N.
Thus, we can say that,
our block is in equilibrium in +y, -y direction.
28
FORCE
• Now, look at the forces acting right and left. As you see,
they are also equal in magnitude and opposite in direction.
Thus, we can say that,
29
FORCE
• Now, look at the forces acting right and left. As you see,
they are also equal in magnitude and opposite in direction.
Thus, we can say that,
our block is in equilibrium in +y, -y direction.
our block is in equilibrium in +x, -x direction.
30
FORCE
•Example
If the boy is in equilibrium, find the G from the given data in picture.
31
FORCE
•Example
If the boy is in equilibrium, find the G from the given data in picture.
As we see;
T at each arm is = 30 N
32
FORCE
•Example
If the boy is in equilibrium, find the G from the given data in picture.
33
FORCE
34
FORCE
35
FORCE
36
FORCE
37
(sin 30º =0,5)
FORCE
38
(sin 30º =0,5)
FORCE
39
(sin 30º =0,5)
FORCE
• Newton’s Third Law of Motion:
(Every actions cause reactions / normal force)
• In this law Newton states that, when we apply a force on an object then it also apples force in same magnitude but opposite in direction.
• In general, all actions have reactions in the same
magnitude but opposite direction. Suppose that, when
you swim you push the water to the backward direction
and water also pushes you to the forward direction.40
FORCE
• We can increase the examples;
1- When birds fly they push the air with their wings and the
air also push the bird in the opposite direction, in this way
birds can stay in the air and fly.
• In every where, we can see this couple of forces. Everyactions cause reactions.
41
FORCE
• 2- Force exerted by the rifle to the bullet gives acceleration
to the bullet, moreover, the bullet also exert a force to the
bullet which is the reaction of action force and as a result
rifle is recoiled.
42
FORCE
• Look at the picture given above.
• 3- The book applies a force because of its weight, and the
table shows a reaction to this action. These force pairs are
same in magnitude but as you see their directions are
opposite. In the next section we examine this example in
detail.
43
FORCE
Normal Force:
• We call this force as “normal force” which is same in
magnitude and opposite in direction with the applied force
(weight of the book).
• For different situations; in general, force is the reaction to
the perpendicular force exerting on it.
44
FORCE•Example
Find the normal force that the inclined plane exerts on the box.
(sin37º=0,6 cos37º=0,8),
(m=4kg, g=10m/s²).
45
FORCE
46
FORCE
47
FORCE
48
FORCE
49
FORCE
50
CHECK YOUR UNDERSTANDING
• 1. Who was the scientist who gave us the Laws of Motion?
51
CHECK YOUR UNDERSTANDING
• 1. Who was the scientist who gave us the Laws of Motion?
• Answer: Sir Isaac Newton
52
CHECK YOUR UNDERSTANDING
• 2. How many Laws of Motion are there?
53
CHECK YOUR UNDERSTANDING
• 2. How many Laws of Motion are there?
• Answer: three
54
CHECK YOUR UNDERSTANDING
• 3. What is another name for the first law of motion?
55
CHECK YOUR UNDERSTANDING
• 3. What is another name for the first law of motion?
• Answer: Law of Inertia
56
CHECK YOUR UNDERSTANDING
• 4. Which law explains why we need to wear seat belts?
57
CHECK YOUR UNDERSTANDING
• 4. Which law explains why we need to wear seat belts?
• Answer: First Law of Motion
58
CHECK YOUR UNDERSTANDING
• 5. Which law says that force is equal to mass times acceleration (F=MA)?
59
CHECK YOUR UNDERSTANDING
• 5. Which law says that force is equal to mass times acceleration (F=MA)?
• Answer: Second Law of Motion
60
CHECK YOUR UNDERSTANDING
• 6. Which law says that heavier objects require more force than lighter objects to move or accelerate them?
61
CHECK YOUR UNDERSTANDING
• 6. Which law says that heavier objects require more force than lighter objects to move or accelerate them?
• Answer: Second Law of Motion
62
CHECK YOUR UNDERSTANDING
• 7. Which law explains how rockets are launched into space?
63
CHECK YOUR UNDERSTANDING
• 7. Which law explains how rockets are launched into space?
• Answer: Third Law of Motion
64
CHECK YOUR UNDERSTANDING
• 8. Which law says that for every action there is an equal and opposite reaction?
65
CHECK YOUR UNDERSTANDING
• 8. Which law says that for every action there is an equal and opposite reaction?
• Answer: Third Law of Motion
66
Work, Energy & Power
67
Work
• The work done by force is defined
as the product of that force times
the parallel distance over which it
acts.
cosFsWThe unit of work is the newton-meter,
called a joule (J).
Energy
• The amount of energy transferred
to the object is equal to the work
done.
• Types of Energy
• Kinetic Energy = “Motion Energy”
• Potential Energy = “Stored Energy”
Kinetic Energy
• Kinetic Energy is the energy
possessed by an object
because it is in motion.
2
21 mvKE
Gravitational
Potential Energy• Gravitational Potential Energy is
the energy possessed by an
object because of a gravitational
interaction.
mghPEG
Conservation of Energy
• Energy can neither be
created nor destroyed, but
only transformed from one
kind to another.
finalinital )PEKE(W)PEKE(
Power
• Power is the time rate of doing work.
SpeedForce
work thisdo taken totime
force aby donework erAveragePow
Power
• The unit of power is;
a joule per second, called a Watt (W).
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