3. Chapter 3 Newton’s Laws

4. Every object possesses inertia. Inertia is the tendency of a body at rest to remain at rest, and of a body in motion to continue moving with unchanged velocity. or Inertia is the sluggishness of an object to changes in its state of motion. Mass - a measure of the inertia of an object THE MASS

5. Demo - Inertia rocks Demo – Hoop and chalk

6. slug. slug.slug A slug weighs 32.2 lb. The standard kilogram is an object whose mass is defined to be one kilogram. standard kilogram standard kilogram Abbreviation is kg. There is an English unit of mass. The THE STANDARD KILOGRAM

7. In general force is the agency of change. In mechanics forces cause accelerations. It is a vector. An external force is one whose source lies outside of the system being considered. FORCEFORCE

8. The net external force acting on an object causes the object to accelerate in the direction of that force. THE NET EXTERNAL FORCE

9. The combination of forces that act on an object is the net force. (Only the net force is shown in the figures on this slide.) Fa Fa Fa The acceleration of an object is directly proportional to the net force. mmm This symbol means proportional to

10. F a m Fa F a The acceleration is inversely proportional to the mass of the object. Consider the same net force applied to different mass objects. mmmmm

11. The acceleration is proportional to the force and inversely proportional to the mass of the object.

12. The Newton (N) is the SI unit of force. A resultant 1 N force will give a 1 kg mass an acceleration of 1 m/s 2. The pound (lb) is the English (US customary) unit of force. US customaryUS customary A resultant 1 lb force will give a 1 sl mass an acceleration of 1 ft/s 2. THE NEWTON

13. An object at rest will remain at rest; an object in motion will continue in motion with constant velocity, except insofar as it is acted upon by an external force. “If you leave an object alone, it has constant velocity.” NEWTON’S FIRST LAW

14. Newton stated it in terms of momentum. A less rigorous form of the second law will be used here. If the net external force acting on an object is not zero, then the acceleration of the object is directly proportional to the net external force and inversely proportional to the mass of the object. NEWTON’S SECOND LAW

15. The force in each of these equations represents the vector sum of all of the forces acting on the object of mass m. or

16. This vector equation can be written out as three separate scalar equations.

17. Forces represent interactions between one piece of matter and another. Therefore, forces come in pairs. For each force exerted on one body, there is an equal, but oppositely directed, force on some other body interacting with it. “You cannot touch without being touched.” Paul Hewitt This is often called the Law of Action and Reaction. NEWTON’S THIRD LAW

18. THE LAW OF UNIVERSAL GRAVITATION r m1m1 m2m2

20. Gravity is the most dominant force in nature. Yet it is the weakest.

21. 1 unit 2 units 3 units 4 units Let’s look at the inverse-square nature for the force of gravity.

22. The weight of an object is the gravitational force acting downward on the object. Because the Earth is not a perfect uniform sphere, and because it is spinning, the weight measured by a scale (often called the effective weight) will be very slightly different from that defined here. THE WEIGHT

23. Look at the force of gravity on a freely falling object. We call that the weight of the object. A 1 kg object would weigh 9.81 N or 2.20 lb. RELATIONSHIP BETWEEN MASS AND WEIGHT

24. It is an applied force that tends to stretch things. It’s magnitude is called the tension. THE TENSILE FORCE

25. It is a tangential force acting on an object that opposes the sliding of that object on an adjacent surface with which it is in contact. The friction force is parallel to the surface and opposite to the direction of motion or of impending motion. Only when the applied force exceeds the maximum static friction force will an object begin to slide. THE FRICTION FORCE

26. The normal force is a part of the contact force between surfaces in contact. The normal force is perpendicular to the surfaces in contact. The frictional force is parallel to the surfaces in contact. THE NORMAL FORCE

27. For surfaces in contact that are sliding with respect to each other, the coefficient of kinetic friction is the ratio of the friction force to the normal force. THE COEFFICIENT OF KINETIC FRICTION

28. For surfaces in contact on the verge of sliding with respect to each other the coefficient of static friction is the ratio of the maximum static friction force to the normal force. THE COEFFICIENT OF STATIC FRICTION

29. FRICTION Friction opposes the motion between surfaces in contact with one another. When there is a tendency for movement between two surfaces and yet there is no motion, the friction is static friction. Static friction has an upper limit. When there is motion between the two surfaces, the friction is kinetic (sliding) friction.

30. FAFA FAFA On the verge of slipping Maximum Static Friction Applied Force, F A Friction, F Kinetic (sliding) Friction Sliding F F F FAFA FAFA F FAFA

31. DIMENSIONAL ANALYSIS: Fundamental Dimensions Length - L Mass - m Time - t Derived dimensions are combinations of the fundamental dimensions. For example: length/time 2 Remember that you can only add or subtract things that are alike. For example velocity plus velocity

33. MATHEMATICAL OPERATIONS WITH UNITS: When numbers are placed into equations, their units must appear with them. Units undergo the same mathematical operation as the numbers do. Conversion from one form of the unit to another may be necessary. For example meters to centimeters. See examples in text, page 30.

34. Free Body Diagrams (FBD) This is a diagram showing some object and the forces applied to it. It contains only forces and coordinate information, nothing else. There are only two kinds of forces to be considered in mechanics: Force of gravity Contact forces

35.  Example FBD A car of mass m rests on a 30 0 incline. mg N F  x y FBD

36. Newton’s Second Law NSL A car of mass m rests on a 30 0 incline. mg N F   NSL x y What if friction is smaller? FBD

37. Newton’s Second Law NSL A car of mass m rests on a 30 0 incline. mg N F  NSL oops x y  FBD