2. Chapter 6 Forces Force - A push or a pull on an object. The symbol F is used to represent the force. It is a vector quantity which means it has both direction and magnitude. Contact Force - A force that acts on an object only by touching it. Long Range Force - A force exerted without contact.

3. Four Fundamental Forces of Nature 1.Gravity 2.Electromagnetic 3.Strong Nuclear 4.Weak interaction Chapter 6 Forces

4. The agent is the specific, identifiable, immediate cause of the force.

5. Chapter 6 Forces

6. Free Body Diagram Chapter 6 Forces

7. ForceSymbolDefinition FrictionFfFf The contact force that opposes motion NormalFNFN The contact force exerted by the surface of an object SpringF sp A restoring force TensionFTFT The pull of a string or rope ThrustF thrust A force that moves objects WeightFgFg A force due to gravitational attraction

8. Chapter 6 Forces 1. An object at rest or in motion will stay that way unless acted upon by an unbalanced force. 2. The acceleration on an object is directly proportional to the fore and inversely proportional to the mass. a=F/m 3. For every action there is an equal and opposite reaction. Newton’s 3 laws

9. Chapter 6 Forces Inertia - The tendency of an object to resist motion. Equilibrium - When the net force on an object is zero. Equilibrant - The force equal to and opposite to the resultant.

10. Chapter 6 Forces Mass vs Weight Mass is the amount of matter in an object and weight is the force of gravity acting on the object. Mass never changes.

11. Chapter 6 Forces Newton A unit of force. F = ma = kg m/s 2 To change mass into weight, multiply the mass by 9.8 m/s 2

20. Chapter 6 Forces What force is required to accelerate an automobile weighing 20,000 N from 40 m/s to 70 m/s in 10 seconds?

21. Chapter 6 Forces Net Force: The sum of all the forces

22. Chapter 6 Forces

23. a.the mass of the football. b.the net force. c.the acceleration of the football. A force of 100 N is exerted by a punter on a football that weighs 45 N. Find: a. b. Net Force = F up - F down = 100 N - 45 N = 55 N up c. = 4.6 kg Joe not blocking

24. Friction: A force that opposes the direction of motion. Static Friction: The force on one surface by the other when there is no relative motion between the two surfaces. Kinetic Friction: The force on one surface when the surfaces are in relative motion. Chapter 6 Forces

25. Kinetic Friction Force F f, kinetic = µ k F N Static Friction Force 0< F f, static < µ k F N Chapter 6 Forces

26. Surfaceµsµs µkµk Rubber on concrete.80.65 Rubber on wet concrete.60.40 Wood on wood.50.20 Steel on steel (dry).78.58 Steel on steel (with oil).15.06 Teflon on steel.04 Coefficients of Friction Chapter 6 Forces

29. A 50-kg box is pushed across a wooden floor at 2 m/s. How much force is exerted on the box? Kinetic Friction Force F f, kinetic = µ k F N m = 50 kg F N = 490 N µ k =.2 F f = (.2)(490 N) = 98 N Chapter 6 Forces

30. A force of 60 N accelerates a 15 kg block at 3 m/s 2 along a horizontal surface. a.How large is the frictional force? b.What is the coefficient of friction? a. F net = F applied - F f (15 kg)(3 m/s 2 ) = 60 N - F f F f = 15 N FfFf F applied b. µ s = F f /F N µ s = 15 N/(15 kg*9.8m/s 2 ) =.102 Chapter 6 Forces

31. Air Resistance The force on a moving object when moving through air. Terminal Velocity The fastest something can fall because of air resistance. Simple Harmonic Motion (SHM) Motion that returns an object to its equilibrium position as a result of a restoringforce that is directly proportional to its displacement. Chapter 6 Forces

34. Simple Pendulum T period (s) l length (m) g acceleration due to gravity 9.8 m/s 2 Find the period of a pendulum with a length of 2 meters. T = 2.84 m Chapter 6 Forces