General Physics 101 PHYS

1. General Physics101 PHYS Dr. Zyad Ahmed Tawfik Email : zmohammed@inaya.edu.sa 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 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. 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.

15. Newton’s First Law 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. 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. Newton’s First Law Some Examples from Real Life 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. A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its 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. Inertiais 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) NOTE:MASS and WEIGHT are NOT the same thing. MASS never changes When an object moves to a different planet. Unit for MASS = kilogram. Weight(W) or Force due to Gravity is how our MASS (m) is effected by gravity (g).

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

28. Normal Force, cont.

29. Example, no angle FN Fp Ff mg 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

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

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

32. 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 Larger force is needed to move larger mass

35. Newton’s Second Law 10 n Smaller force give smaller acceleration 30 n 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. PHY 1401- 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. a= 2 m/s2 F = ? 6kg Example 1:What resultant force F is required to give a 6 kg block an acceleration of 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 FN Fa Ff mg 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

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

43. 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 Friction Force = Coefficient of friction  Normal Force Ffriction = Fnormal

48. N W Example • 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? 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.