Applied Physics Chapter #5

1. Applied Physics
Lecture # 1
Chapter # 5
Force and Motion-1

2. Content
• Lecture # 1
• Newton’s laws of motion
• Some common forces (Weight, Normal Reaction, Tension,
Force of Friction
• Application of Newton’s 2nd law (Problem solving strategy)
• Sample problems

3. Force: The measure of interaction between
two objects (pull or push). It is a vector
quantity – it has a magnitude and
direction
Mass: The measure of how difficult it is to
change object’s velocity (sluggishness or
inertia of the object)

4. Types of Forces
• There are two main types of forces
• Contact
• Field

5. Contact Forces
Contact forces result from physical contact between two objects
• Think About a Book on a Table
• If you push it, you are exerting a contact force
• If you put it down, no longer interacting… so no more force from
you
• But table is touching it- table is now exerting a force

6. Field Forces
• An object can move without something directly
touching it or
• Field forces act between disconnected objects
• Also called “action at a distance”
• What if you dropped the book?
• It falls due to gravity
• Gravitational Force is a field force.
• They affect movement without being in physical
contact
• Can you think of other field forces?
• Magnetic fields
• Electric Forces
• Nuclear Forces

7. Two Types of Forces
• Example of Contact Forces
• Friction
• Tension
• Examples of Field Forces
• Gravitational
• Electric
• Magnetic

8. Force and mass
• Mass – measurement of how difficult it is to change the objects
velocity or it is the quantity of matter in a physical body.
• Inertia – resistance to change in velocity or it is the property of a body by
virtue of which it opposes any agency that attempts to put it in motion or, if it is moving,
to change the magnitude or direction of its velocity.
• So mass is a measurement of an object’s inertia

9. Question
•What is the relationship between mass and
inertia?
•Mass is a measure of how much inertia
something has.

10. Newton’s Laws
how it was derived……???

11. Background
Sir Isaac Newton (1643-1727)
an English scientist and a
mathematician famous for
his discovery of the law of
gravity also discovered the
three laws of motion.
Today these laws are known as
Newton’s Laws of Motion and
describe the motion of all objects on
the scale we experience in our
everyday lives.

12. Newton’s laws
• 1st Law: An object at rest will remain at rest and an object in
motion continues in motion with constant velocity if no external
force acts on it.
• 2nd Law: The acceleration of an object is directly proportional to
the net force acting on it and inversely proportional to its mass
𝐹𝑛𝑒𝑡 = 𝑚𝑎
• 3rd Law: Every action has an equal but opposite reaction
𝐹12 = −𝐹21

13. If objects in motion tend to stay in motion, why don’t
moving objects keep moving forever?
Things don’t keep moving forever because
there’s almost always an unbalanced force
acting upon it.
A book sliding across a table slows
down and stops because of the force
of friction.
If you throw a ball upwards it will
eventually slow down and fall
because of the force of gravity.

14. Question
• A force of gravity between the sun and its planets holds the
planets in orbit around the sun. If that force of gravity
suddenly disappeared, in what kind of path would the
planets move?
• Each planet would move in a straight line at constant speed.

15. Question
• The Earth moves about 30 km/s relative to the sun.
But when you jump upward in front of a wall, the wall
doesn’t slam into you at 30 km/s. Why?
• both you and the wall are moving at the same speed, before,
during, and after your jump.

16. Acceleration
• An unbalanced force causes something to accelerate.
• A force can cause motion only if it is met with an unbalanced force.
• Forces can be balanced or unbalanced.
• Depends on the net force acting on the object
• Net force (Fnet): The sum total and direction of all forces acting on
the object.
• Net forces: Always cause acceleration.

17. Balanced Versus Unbalanced
Balanced forces cause no
acceleration.

18. Unbalanced forces
cause acceleration.
Balanced Versus Unbalanced

19. In Other Words…
F
a
Small Force = Small Acceleration

20. In Other Words…
Large Force = Large Acceleration
F
a
So….if you push twice as hard, it accelerates twice as much.

21. But there is a twist….
• Acceleration is INVERSELY related to the mass of
the object.

22. In other words…..using the same amount of force….
F
Large Mass a
Small acceleration
F
Small Mass
Large acceleration
a

23. More about F = ma
If you double the mass, you double the force. If you
double the acceleration, you double the force.
What if you double the mass and the acceleration?
(2m)(2a) = 4F
Doubling the mass and the acceleration quadruples
the force.

24. What does F = ma say?
F = ma basically means that the force of an
object comes from its mass and its
acceleration.
Force is measured in
Newtons (N) = mass (kg) x acceleration (m/s2)
Or
kg m/s2

25. Solving Newton Second Law Problems
• 1.Draw a free body diagram
• 2.Break vectors into components if needed
• 3.Find the NET force by adding and subtracting forces that are on
the same axis as the acceleration.
• 4.Set net force equal to “ma” this is called writing an EQUATION OF
MOTION.
• NOTE: To avoid negative numbers, always subtract the smaller
forces from the larger one. Be sure to remember which direction is
larger.

26. Example
• A 50 N applied force drags an 8.16 kg log to the right across a
horizontal surface. What is the acceleration of the log if the force of
friction is 40.0 N?

27. Tougher Example
• An elevator with a mass of 2000 kg rises with an acceleration of 1.0
m/s/s. What is the tension in the supporting cable?

28. Check your understanding
Net Forces and Newton’s Second Law

29. Question
• Suppose that the acceleration of an object is zero.
Does this mean that there are no forces acting on it?
• No, it means the forces acting on it are balanced and
the net force is zero.
• Think about gravity and normal force acting on
stationary objects.

30. Question
• When a basketball player dribbles a ball, it falls to the floor
and bounces up. Is a force required to make it bounce?
Why? If a force is needed, what is the agent.
• Yes, when it bounced it changed direction. A change in
direction = acceleration. Acceleration requires a force. The
agent was the floor.

31. Newton’s third law describes the relationship
between two forces in an interaction.
• One force is called the action force.
• The other force is called the reaction force.
• Neither force exists without the other.
• They are equal in strength and opposite in
direction.
• They occur at the same time
(simultaneously).
Newton’s Third Law

32. When the girl jumps to shore, the boat moves backward.
Newton’s Third Law

33. When action is A exerts force on B, the reaction is simply B exerts force on A.
Identifying Action and Reaction Pairs

34. Some common forces
Here are some forces which we must deal with in our daily life.
• Weight of an object
• Normal reaction
• Tension in a string
• Friction between two surfaces

35. Weight: It is the gravitational force
with which earth attracts every object
towards its center, therefore, weight is
always directed straight downward in
every problem.
𝐹
𝑔 = 𝑊 = 𝑚𝑔
W=mg

36. Normal force: It is the reaction force
from the floor or any other surface
against which the object is being
pushed, therefore, normal reaction
is always directed at 90 degrees to
the surface.
W=mg
N

37. Tension: In a single string
magnitude of tension force is
same at each point and is
directed away from the object to
which the string is connected.
W=mg
N
T

38. Application of Newton’s 2nd law to find unknown force or
acceleration in a problem
Problem solving strategy:
• Draw the free body diagram (FBD) for the problem, i.e., indicate all the
forces acting on the body.
• Then write down known and unknown quantities (N, mg, T, fs, a, ).
• For a body under the action of several forces we can write Newton’s
2nd law
∑𝐹 = 𝑚𝑎
• For 2-dimensional problem, we can write the above equation for x and
y directions separately as
∑𝐹
𝑥 = 𝑚𝑎𝑥 , ∑𝐹
𝑦 = 𝑚𝑎𝑦
• This gives us two equations which can be solved together to find the
values of unknowns. For example,
𝑇 cos 𝜃 − 𝑓𝑠 = 𝑚𝑎𝑥 (1)
𝑇 sin 𝜃 + 𝑁 − 𝑚𝑔 = 𝑚𝑎𝑦 (2)
N
fs
W=mg
T

40. N
mg
T

∑𝐹𝑥 = 𝑚𝑎𝑥
𝑇 − 𝑚𝑔 sin 𝜃 = 𝑚𝑎𝑥
put 𝑎𝑥 = 0
𝑇 − 𝑚𝑔 sin 𝜃 = 0
𝑇 = 𝑚𝑔 sin 𝜃
Substituting the values, we get
𝑇 = 8.5 × 9.8 × sin 30°
𝑇 = 8.5 × 9.8 × 0.5
𝑇 = 41.65 𝑁
∑𝐹𝑦 = 𝑚𝑎𝑦
𝑁 − 𝑚𝑔 cos 𝜃 = 𝑚𝑎𝑦
put 𝑎𝑦 = 0
𝑁 − 𝑚𝑔 cos 𝜃 = 0
𝑁 = 𝑚𝑔 cos 𝜃
Substituting the values, we get
𝑁 = 8.5 × 9.8 × cos 30°
𝑁 = 8.5 × 9.8 × 0.866
𝑁 = 72.13 𝑁
Solution
Part (a) Part (b)

41. Part (c)
N
mg

Now the string is removed, so the block
will accelerate down the plan.
∑𝐹𝑥 = 𝑚𝑎𝑥
𝑚𝑔 sin 𝜃 = 𝑚𝑎𝑥
𝑎𝑥 = 𝑔 sin 𝜃
Substituting the values, we get
𝑎𝑥 = 9.8 × sin 30°
𝑎𝑥 = 4.9 𝑚/𝑠2

43. Applying Newton’s second law on the traffic light signal, we get
𝑇3 = 𝑊 (1)
Now applying Newton’s second law to the joint, in x-direction
𝐹𝑛𝑒𝑡,𝑥 = 𝑚𝑎𝑥
𝑇2 cos 53° − 𝑇1 cos 37° = 𝑚𝑎𝑥
𝑇2 cos 53° − 𝑇1 cos 37° = 0 (2)
And in y-direction
𝐹𝑛𝑒𝑡,𝑦 = 𝑚𝑎𝑦
𝑇2 sin 53° + 𝑇1 sin 37° − 𝑇3 = 𝑚𝑎𝑦
𝑇2 sin 53° + 𝑇1 sin 37° − 𝑇3 = 0 (2)
Solving equations 1, 2 and 3, we get 𝑇1 = 73.4 𝑁 and 𝑇2 =
97.4 𝑁, therefore, the strings will not break, and the traffic light
will remain hanging.

50. Practice problems
Chapter#5 Problems: 15, 19, 32, 46, 47, 51, 52, 53, 55, 59,66,77
From the book:
Fundamentals of Physics, 8th edition
Authors: Halliday, Resnick, Walker