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# Forces

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### Forces

1. 1. Learning outcomes <ul><li>Describe how balanced and unbalanced forces affect body </li></ul><ul><li>Describe how a force may change motion of a body </li></ul><ul><li>Identify and draw free body diagrams of forces acting on object </li></ul><ul><li>Solve problems using a graphical method for a static point mass being acted on by 3 forces for 2-D cases </li></ul><ul><li>Use resultant force = mass × acceleration </li></ul><ul><li>Explain effects of friction on motion of a body </li></ul>
2. 2. 3.1 Balanced and Unbalanced Forces <ul><li>Forces </li></ul><ul><li>Newton’s first law of motion states that an object at rest will remain at rest and an object in motion will continue in motion at constant speed in a straight line in the absence of a resultant force acting on it </li></ul>
3. 3. 3.1 Balanced and Unbalanced Forces <ul><li>Balanced Forces </li></ul><ul><li>Newton’s first law of motion implies that when no resultant force acts on a body, the body will continue with whatever motion it has </li></ul><ul><li>If it is at rest, it will remain at rest </li></ul><ul><li>If it is moving, it will continue moving at constant speed in a straight line </li></ul>
4. 4. <ul><li>Rock at rest, net force acting </li></ul><ul><li>on it must be zero. </li></ul><ul><li>Weight of rock acting downwards. </li></ul><ul><li>So there must be a second force pushing it upwards. </li></ul><ul><li>This second force is the reaction of the table and it balances the weight of the rock. </li></ul>3.1 Balanced and Unbalanced Forces Balanced Forces
5. 5. <ul><li>The weight of the aircraft is balanced by the lift from the wings, so the aircraft flies at constant height. </li></ul><ul><li>Air resistance encountered on moving forward is balanced by the thrust of the engines, so the velocity of the aircraft is constant. </li></ul>3.1 Balanced and Unbalanced Forces Balanced Forces
6. 6. <ul><li>A falling object will reach a point when air resistance = weight. </li></ul><ul><li>Object falls with terminal velocity. </li></ul><ul><li>Resultant force acting on it is zero. </li></ul><ul><li>No resultant force does not mean </li></ul><ul><li>that no forces act on body. </li></ul><ul><li>It means all forces are balanced . </li></ul>3.1 Balanced and Unbalanced Forces Balanced Forces
7. 7. <ul><li>Balanced Forces </li></ul><ul><li>When the forces acting on an object are balanced, the object is either at rest or moving with uniform velocity. </li></ul>3.1 Balanced and Unbalanced Forces Rest Constant velocity or
8. 8. <ul><li>Unbalanced Forces </li></ul><ul><li>If the forces on an object are unbalanced, there will be a resultant force acting on it and changing its velocity. </li></ul>3.1 Balanced and Unbalanced Forces
9. 9. <ul><li>A resultant force will cause </li></ul><ul><li>(i) a stationary object to move </li></ul><ul><li>(ii) a moving object to </li></ul><ul><ul><li>stop </li></ul></ul><ul><ul><li>move faster </li></ul></ul><ul><ul><li>slow down </li></ul></ul><ul><ul><li>change direction </li></ul></ul>3.1 Balanced and Unbalanced Forces
10. 10. 3.1 Balanced and Unbalanced Forces <ul><li>Newton’s second law of motion states that the resultant forces acting upon an object is equal to the product of the mass and the acceleration of the object. </li></ul><ul><li>The direction of the force is the same as that of the object’s acceleration. </li></ul>
11. 11. <ul><li>Force = mass × acceleration </li></ul><ul><li>F = ma </li></ul><ul><li>SI unit for force is the newton (N) </li></ul><ul><li>One newton (N) is defined as the force which produces an acceleration of 1 m s −2 when it is applied to a mass of 1 kg. </li></ul><ul><li>1 N = 1 kg m s −2 </li></ul>3.1 Balanced and Unbalanced Forces 1 N 1 kg a = 1 m s –2
12. 12. 3.1 Balanced and Unbalanced Forces
13. 13. 3.1 Balanced and Unbalanced Forces
14. 14. 3.1 Balanced and Unbalanced Forces
15. 15. <ul><li>Resultant Forces </li></ul><ul><li>When several forces act on an object, these forces can be replaced with a single resultant force. </li></ul>3.1 Balanced and Unbalanced Forces Resultant force
16. 16. <ul><li>Resultant Forces (Cases of Parallel Forces) </li></ul><ul><li>For cases of parallel forces, the magnitude of the resultant force can be calculated by simple addition or subtraction </li></ul>3.1 Balanced and Unbalanced Forces
17. 17. <ul><li>When forces acting on an object are balanced(or no resultant forces are acting on an object): </li></ul><ul><li>(a) If it is at rest, it will remain at rest. </li></ul><ul><li>(b) If it is moving, it will continue moving </li></ul><ul><li>at constant speed in a straight line. </li></ul><ul><li>When the forces on an object are unbalanced, a resultant force acts on the object and it accelerates (or decelerates). </li></ul>
18. 19. <ul><li>Action and Reaction </li></ul><ul><li>Newton’s third law of motion states that for every action there is an equal and opposite reaction </li></ul>3.2 Free-body Diagram
19. 20. <ul><li>Action and Reaction </li></ul><ul><li>If object A exerts a force on object B, then B exerts an equal but opposite force on A </li></ul><ul><li>Action and reaction act on different objects. </li></ul>3.2 Free-body Diagram
20. 21. <ul><li>Free-body Force Diagrams </li></ul><ul><li>According to Newton’s 3 rd law, forces always occur in pairs with one force acting on a body, and an equal and opposite force acting on another body. </li></ul><ul><li>If both forces and bodies are drawn in the same diagram, it can cause confusion about which force acts on which body. </li></ul><ul><li>A free-body force diagram shows forces acting on a single body only. </li></ul>3.2 Free-body Diagram
21. 22. Example Free-Body Diagram 3.2 Free-body Diagram
22. 23. <ul><li>Mass under Action of 3 forces in 2 dimensions </li></ul><ul><li>(a) Unbalanced forces </li></ul><ul><ul><li>When we apply 3 forces to a point object, the resultant force acting on the object can be obtained by adding the force vectors </li></ul></ul>3.2 Free-body Diagram A B C A B C R
23. 24. <ul><li>Vector A is drawn according to scale. </li></ul><ul><li>Vector B is drawn according to scale with its starting point touching the ending point vector A. </li></ul><ul><li>Vector C is drawn according to scale with its starting point touching the ending point of B . </li></ul><ul><li>Draw an arrow from the starting point of A to the ending point of C to obtained the resultant R . </li></ul><ul><li>The same R can also be obtained by starting with vector B or C . </li></ul>Procedure for Adding Vectors A B C R 3.2 Free-body Diagram
24. 25. <ul><li>(b) Balanced forces </li></ul><ul><li>When 3 forces acting on an </li></ul><ul><li>object are balanced, the object will not accelerate. </li></ul><ul><li>When we draw the vector diagram the 3 vectors form a closed triangle. </li></ul><ul><li>Resultant is zero. </li></ul>3.2 Free-body Diagram
25. 26. 3.2 Free-body Diagram
26. 27. <ul><li>For every action there is an equal and opposite reaction </li></ul><ul><li>Action and reaction forces act on different objects </li></ul><ul><li>A free-body force diagram shows the forces that act on a single body </li></ul><ul><li>The resultant of 3 or more forces that act on an object can be determined using vector addition </li></ul><ul><li>When the 3 forces that act on an object are balanced the object will not accelerate. The 3 forces will form a closed triangle. </li></ul>
27. 28. 3.3 Friction <ul><li>Friction as a Useful Force </li></ul><ul><li>Friction is needed for walking and rolling </li></ul><ul><li>No friction will cause slip </li></ul>
28. 29. <ul><li>Friction as a Nuisance </li></ul><ul><li>Friction causes surfaces to heat up, results in wear and tear. </li></ul><ul><li>In machinery, friction reduces efficiency as large amount of energy wasted as heat. </li></ul><ul><li>Much research carried out in industries on ways to reduce friction. </li></ul>3.3 Friction
29. 30. <ul><li>Ways to reduce friction: </li></ul><ul><ul><li>Moving parts made as smooth as possible </li></ul></ul><ul><ul><li>Materials with very low frictional resistance is used </li></ul></ul><ul><ul><li>Ball and roller bearings are placed between moving parts </li></ul></ul><ul><ul><li>Surfaces separated by lubricant </li></ul></ul><ul><ul><li>Surfaces separated by air cushion </li></ul></ul>3.3 Friction
30. 31. <ul><li>Frictional force between two surfaces on a horizontal plane: </li></ul><ul><ul><li>Depends on material in contact </li></ul></ul><ul><ul><li>Depends on roughness of surfaces </li></ul></ul><ul><ul><li>Is proportional to the force pressing the surfaces together </li></ul></ul><ul><ul><li>Is independent of the area of contact </li></ul></ul>3.3 Friction
31. 32. 3.3 Friction
32. 34. <ul><li>Friction is a force that opposes sliding motion between two surfaces in contact. </li></ul><ul><li>For a body on a horizontal plane, the frictional force depends </li></ul><ul><ul><li>(a) the nature of the contact surfaces </li></ul></ul><ul><ul><li>(b) the weight of the body </li></ul></ul><ul><li>Friction does not depend on the area of contact of the surfaces. </li></ul>
33. 36. <ul><li>What happens when there is no resultant force on a body? </li></ul><ul><li>How are force and acceleration related? </li></ul><ul><li>How can an object move if action equals reaction? </li></ul><ul><li>Is friction a useful force or a nuisance? </li></ul>Questions