Chapter 5 Newton’s laws of motion (Chap. 3 in the textbook page 48)

1. Chapter 5Newton’s laws of motion(Chap. 3 in the textbook page 48) Dr. Haykel Abdelhamid Elabidi March2014/JuU 1435

2. Units of Chapter 5 (3) • Force weight and gravitational mass • Density • Newton’s first law of motion • Equilibrium • Newton’s third law of motion • Newton’s second law of motion • Someexamples of Newton’slaws • Gravitational forces • Friction

3. Force, weight and gravitational mass • A force is a push or a pull. • The force is a vector (defined by its magnitude and its direction). • The net or total force on an object is the vector sum of all the forces acting on the object. • Forces that are exerted only when two objects are in contact are referred to as contact forces. • The S.I. unit of force is « Newton » (N)

4. Force, weight and gravitational mass

5. Density

6. Density Example 3.1. page 50: A cylindrical aluminum rod has a radius of 1.2 cm and a length of 2.0 m. What is its mass? Solution Example 3.1. page 50: For a cynlinder :

7. Newton’s first law of motion Newton’s first law states that: Every object continues in a state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces acting upon it. An equivalent statement of the first law is that if there is no force on an object, or if there is no net force when two or more forces act on the object, then: an object at rest remains at rest, and an object in motion continues to move with constant velocity.

8. Equilibrium The state of an object remains unchanged whenever the net force on the object is zero. An object in equilibrium if the state of its motion is unchanged The normal force is a force exerted by a surface on an object. It is always perpendicular to the surface.

9. Equilibrium Example 3.2 page 52: A woman has a mass of 60.5 kg. She is standing on a floor and remains at rest. Find the magnitude of the normal force exerted on her by the floor. Solution Example 3.2 page 52:

10. Equilibrium Types of equilibrium: 1- Unstable equilibrium: a small displacement leads to an unbalanced force that further increase the displacement from the equilibrium location (ball in position A). 2- Stable equilibrium: a small displacement leads to an unbalanced force that tends to restore the object to the equilibrium location (ball in position B). 3- Neutral equilibrium: it is in equilibrium at any location near C.

11. Newton’s third law of motion

12. Newton’s second law of motion

13. Newton’s second law of motion Example 3.6 page 55: A child (1) pushes a sled across a frozen pond with a horizontal force of 20 N. Friction is negligible. What is its mass if the acceleration is 0.5ms-2 . A 60 kg child (2) sits on the sled. Give its acceleration if the same force is exerted. Solution Example 3.6 page 55: a) The sled does not move vertically, so its weight and the normal force exerted by the ice cancel. The same force produces a small acceleration when applied to a more massive object.

14. Some examples of Newton’s laws Example 3.9 page 58:

15. Some examples of Newton’s laws Solution Example 3.9 page 58:

16. Some examples of Newton’s laws

17. Gravitational forces

18. Friction • Friction is a force that always act to resist the motion of one object on another. • Frictional forces in fluids are called viscous forces. • When we walk or run, we are not conscious of any friction in our knees or other leg joints. These joint, really, are lubricated by synovial fluid, which is squeezed through the cartilage lining the joints when they move. This lubricant tends to be absorbed when the joint is stationary, increasing the friction and making it easier to maintain a fixed position.

19. Friction

20. Friction

21. Friction Example 3.17 page 66: Solution Example 3.17 page 66:

22. Homeworks: Exercises 3.17; 3.49; 3.54