1. General Physics 101 PHYS Dr. Zyad Ahmed Tawfik
Website : zyadinaya.wordpress.com

2. بسم الله الرحمن الرحيم
Lecture No. 3
Newton's laws

3. Objectives
1. Definition of force, resulting force and its calculation.
-2. Equilibrium states and application of Newton's first law
-3. Newton's second law and its application.
-4. Reactive force and Newton's third law.
-5. Fundamental force in nature .
- Weight.
-Friction, static, kinetic (solving related problems).

4. Newton's Laws

5. Newton’s Laws

6. Force
Push can change the state of the object from one place to another.

7. Force (contd..)
Pull can change the state of the object from one place to other.

8. Facts about FORCE Force unit is the NEWTON (N).
Its definition a push or a pull.
What change the state of object is called “force”.
means that we can control the magnitude of the applied force and also its direction, so force is a vector quantity, just like velocity and acceleration.

9. Adding Forces
Forces are vectors (They have both magnitude and direction) and so add as follows:
In one dimension, note direction using a + or – sign then add like scalar quantities (regular numbers with no direction associated with them)
Examples: = =
+3 N
+3 N
+6 N
0 N
+3 N
-3 N

10. We experiences some effect called forces.
We can’t see forces but we can see the effect of a force.

11. Newton’s First Law Newton’s First Law
An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force.

12. Newton’s First Law
What does this mean?

13. Newton’s First Law An object remains in its state of rest
if no external force is acting on it.
An object remains in its state of motion
if no external force is acting on it.
An object changes its state of rest or uniform motion if some external force acting on it.

14. Newton’s First Law
An object in motion remains in motion in a straight line and at a constant speed.
An object moves with a velocity that is constant in magnitude and direction, unless acted on by a nonzero net force.
Consider a body on which no net force acts. If the body is at rest, it will remain at rest. If the body is moving with constant velocity, it will continue to do so.

15. Newton’s First Law In other words
The above three statement can be written as a single statement which is the Newton’s first law
An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless an external force is acting on it.
In other words
Every body continuous in its state of rest or in uniform motion Unless an external force is acting on it.

16. What is meant by unbalanced force?
Newton’s First Law
What is meant by unbalanced force?
If the forces on an object are equal and opposite, they are said to be balanced, and the object experiences no change in motion. If they are not equal and opposite, then the forces are unbalanced and the motion of the object changes.

17. Some Examples from Real Life
Newton’s First Law
Some Examples from Real Life
A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion.
Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.

18. Newton’s First Law is also called the Law of Inertia
Inertia: the tendency of an object to resist changes in its state of motion
The word “inertia” comes from the Latin word inertus, which can be translated to mean “lazy.”
The First Law states that all objects have inertia. The more mass an object has, the more inertia it has (and the harder it is to change its motion).

19. Inertia is a term used to measure the ability of an object to resist a change in its state of motion.
An object with a lot of inertia takes a lot of force to start or stop; an object with a small amount of inertia requires a small amount of force to start or stop.

20. This can be said in a single statement
Inability of an object to change its position by itself is called Inertia.

21. Newton’s First Law (The Law of Mass)
Unit for MASS = kilogram.
Weight (W) or Force due to Gravity is how our MASS (m) is effected by gravity (g).
NOTE: MASS and WEIGHT are NOT the same thing. MASS never changes When an object moves to a different planet.

22. Newton’s First Law, cont.
There are Three conditions here,
Condition #1 – The object CAN move by a
CONSTANT SPEED.
Condition #2 – The object is at REST.
Condition #3 – The forces are BALANCED! (SUM of all the forces is ZERO).
There is NO ACCELERATION in this case AND the object must be at EQILIBRIUM.

23. Under this condition (no net force acting on the body):
If the body is at rest, it will remain at rest.
If the body is moving with constant velocity, it will continue to do so.

24. Equilibrium The condition of zero acceleration is called equilibrium.
In equilibrium, all forces cancel out leaving zero net force.
Objects that are standing still are in equilibrium because their acceleration is zero.

25. Equilibrium
Objects that are moving at constant speed and direction are also in equilibrium.
A static problem usually means there is no motion.

26. Types of acting force Weight(mg) – straight down.
Norma Force (Fn) – always drawn perpendicular to up.
Tension (T or FT) – always drawn from object.
Friction(Ff)- Always drawn opposing the motion. FN T Ff T
W = mg
m2g

27. Normal Forces
Normal force: this force acts in the direction perpendicular to the contact surface and opposite the weight. N w

28. Normal Force, cont.

29. Example, no angle
A 10-kg box is being pulled across the table to the right at a constant speed with a pulling force of 50N. 1. Calculate the Friction Force 2. Calculate the Normal Force Solution: In this case the friction force is opposite the pull So, Ff = - Fpull Ff = 50 N FN Fp Ff mg

30. Friction Forces
Fx = F cos θ &
Fy = F sin θ

31. Example, with angle
A 10-kg box is being pulled across the table to the right at a constant speed with a pulling force of 50N with angle of 30 degrees above the horizontal.
Calculate the Friction Force
Calculate the Normal Force
Solution: In this case Fa analysis in x and y direction. FN Fa
Fay Ff 30
Fax mg

32. Force equals mass times acceleration.
Newton’s Second Law
Newton’s Second Law
Force equals mass times acceleration.
F = ma
Acceleration: a measurement of how quickly an object is changing speed.

33. Newton’s Second Law What does F = ma mean?
Force is directly proportional to mass and acceleration.

34. Newton’s Second Law It is very easy It is too hard
Smaller force is needed to move smaller mass
It is too hard
Larger force is needed to move larger mass

35. Newton’s Second Law 10 n 30 n Smaller force give smaller acceleration
Larger force give Larger acceleration

36. Newton’s Second Law
Smaller force (F) is needed to move smaller mass (m)
Larger force (F) is needed to move larger mass (m)
So, Force is directly proportional to mass
F α m
Smaller force (F) give smaller acceleration (a).
Larger force (F) give smaller acceleration (a).
So, Force is directly proportional to acceleration (a).
F α a
Acceleration is produced when a force acts on a mass.
The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).
Newton’s second law states that the net force on an object is proportional to the mass and the acceleration that the object undergoes.
F = ma

37. MFMcGraw
PHY Ch 04b - Revised: 6/9/2010

38. Net Force
The net force is the vector sum of all the forces acting on a body.

39. Net Forces

40. Example 1: What resultant force F is required to give a 6 kg block an acceleration of 2 m/s2?
a = 2 m/s2
F = ma = (6 kg)(2 m/s2)
F = 12 N
Remember consistent units for force, mass, and acceleration in all problems.

41. Example
A 10-kg box is being pulled across the table to the right by a rope with an applied force of 50N. Calculate the acceleration of the box if a 12 N frictional force acts upon it. Solution FN Fa Ff mg

42. If the surface is smooth, the friction force, Ff= 0
Note
If the surface is smooth, the friction force, Ff= 0

43. For every action there is an equal and opposite reaction.
Newton’s Third Law
Newton’s Third Law
For every action there is an equal and opposite reaction.

44. Newton’s Third Law
What does this mean?

45. Newton’s Third Law
The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object

46. Coefficients of friction
IiCoefficients of friction is the ratio between friction force and normal force.
Symbol is the Greek letter mu (μ)
μ= Ff / FN
The coefficient of friction has no units.

47. Or Ffriction = Fnormal
4/18/2008
Friction Force = Coefficient of friction  Normal Force
Ffriction = Fnormal
Lecture 8

48. Example Fk Solution (a FK = Fn FK = 0.2 X150= 30N Solution (b
A girl is pulling a suitcase of 15Kg mass on a rough horizontal floors show figure.The pulling force =75N and the coefficient of between the suitcase and the floor is 0.2
What is the force of kinetic exerted on the suitcase?
What is the acceleration on the suitcase? N W Fk
75N
Solution (a
FK = Fn FK = 0.2 X150= 30N
Solution (b

49. Review Newton’s First Law:
Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force.
Newton’s Second Law:
Force equals mass times acceleration (F = ma).
Newton’s Third Law:
For every action there is an equal and opposite reaction.

50. Vocabulary
Inertia:
the tendency of an object to resist changes in its state of motion or Inability of an
object to change its position by itself
Acceleration:
•a change in velocity
•a measurement of how quickly an object is changing speed, direction or both
Velocity:
The rate of change of a position along a straight line with respect to time
Force: Its definition a push or a pull.

51. Thank You for your Attention