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3a. dynamics newtons laws
- 1. Dynamics
- 2. What is Dynamics Where kinematics DESCRIBES motion ... Dynamics studies the CAUSES of motion. All motion can be explained by … Force Force is a VECTOR –
- 3. Four fundamental forces There are 4 fundamental forces: • Gravitation • Electromagnetism • Weak nuclear force • Strong nuclear force All forces that we experience in life come either from gravitation or electromagnetism.
- 4. Types of forces 1. Applied 2. Thrust 3. Weight 4. Normal 5. Elastic 6. Tension 7. Friction 8. Air resistance 9. Electric 10. Magnetic Some kinds of forces you will encounter include:
- 5. http://www.physicsclassroom. com/class/newtlaws/u2l2c.cfm Summary and exercises on drawing free body diagrams.
- 6. Force and Newton’s First Law
- 7. Force and Newton’s First Law An object will remain at rest or move with constant velocity unless acted upon by a net external force.
- 8. What does this mean? A ball at rest will stay at rest unless someone kicks it.
- 9. … and … An astronaut who is moving through space will continue to move with constant velocity unless acted upon by a force (What kind of forces could alter the astronaut’s velocity?)
- 10. … so … What causes this stone to stop after it leaves the athlete’s hand?
- 11. Net forces Repulsion There are forces on this ball. What are they and why don’t they cause motion?
- 12. This is similar to a ‘tug of war’, which will experience no movement if both forces are equal
- 13. … similarly … Air resistance •As a sky diver accelerates during freefall … •Air resistance increases •When air resistance equals the sky diver’s weight …
- 14. Task 1 • Draw graphs for • displacement and time, • velocity and time • acceleration and time • for an object in freefall • under air resistance
- 15. Task 2 Magnetic forces are often at least as powerful as gravitational forces. Consider a 5 kg piece of iron suspended in mid-air by a powerful magnet above the piece of iron. How much force does the magnet exert on the iron?
- 16. Solution •The iron does not move, implying a constant velocity ( v = 0 ). Thus, by Newton's First Law, the sum of the forces on the iron must be zero. In this case, there are two forces acting upon the iron: the gravitational force of the earth, and the magnetic force of the magnet. Thus F G + G M = 0 . We can calculate the gravitational force using the fact that the gravitational acceleration on earth is 9.8
- 18. Newton’s Second Law F = ma Net Force = mass x acceleration 1N = 1kg x 1ms-2
- 19. 1. What does this mean? A shuttle which applies a constant force in space will accelerate indefinitely at a constant rate A constant force will cause a constant acceleration, NOT a constant velocity.
- 20. so why is it that a constant force … …usually causes a constant VELOCITY, not a constant ACCELERATION? Propulsion V=VC a = 0 Rolling Friction
- 21. … similarly … If there is no net force (ie: F=0), then there is no acceleration (ie: a=0). This supports Newton’s first law… … in the absence of a net external force, (ie: F=0) • Bodies at rest will remain at rest (since a=0) • And bodies moving at velocity ‘v’ will continue to move at velocity ‘v’. (since a=0)
- 22. 2 … also … Under the same force, larger masses will undergo smaller accelerations … … and vice versa
- 23. 3. finally If: F = ma, (and W = mg) Then: g = W/m Since: W is proportional to m Then: g is constant for all values of m (and W).
- 25. What does this mean? • It means that in a vacuum … • All objects fall with the same acceleration • (eg: a feather and a bowling ball). • On earth, different accelerations are caused by air resistance. Video http://article.wn.com/view/2014/11
- 26. Air resistance. • Air resistance is a force which always opposes motion • It’s magnitude depends on … • The size of the body • The velocity of the body • The viscosity of the fluid • And is represented by the formula F= 6[pi]Nrv
- 28. Mass Mass is a SCALAR. Its magnitude is measured in kilograms, (kg) It has no direction Mass is the amount of matter, or ‘stuff’ that makes up a body
- 29. Weight •Mass is not weight •Weight does not measure the matter (stuff) in your body •It measures the FORCE that is exerted on your MASS due to GRAVITATIONAL acceleration (g) •However, weight is used to calculate your mass
- 30. Weight (a force) is a VECTOR –Weight’s magnitude is measured in … –Weight’s direction is always … From: F = ma W = mg Weight = mass x gravitational acceleration
- 31. Weight will change in different locations due to different gravitational accelerations: Location Earth Moon Space g = 9.8 1.6 0 Weight 80kg x 9.8 ms-2 = 784 N 80kg x 1.6 ms-2 = 128 N 80kg x 0 ms-2 = 0 N
- 32. In contrast … Mass, whose matter (stuff) remains the same in all locations …
- 33. Mass and Inertia •Mass has a property called INERTIA •Inertia is the tendency to resist any change in motion (including direction). •The tendency to keep moving in a straight line at constant linear velocity. •The larger the mass, the larger its inertia.
- 34. Inertia is why … …great effort is required to start something moving … and less effort is required to keep it moving at the same speed
- 35. …you keep moving when your car stops … and this happens
- 36. … (most) magicians are able to pull a table cloth from under things without pulling them all off the table.
- 37. … ships of great mass (and inertia) need to start braking at least half an hour before they want to stop.
- 38. Cars turning a corner will try to continue in its original direction (having the effect of leaning right when turning left, and vice versa)
- 39. Test on second law Remember that Force is proportional to acceleration and not to velocity Note also that Force and acceleration can be in opposite direction to velocity. Try the following exercises to check whether you really understand this Interactive: http://www.physicsclassroom.com/Physics-Interactives/Newtons-Laws/Free-Body-Diagrams/Free-Body-Diagram-Interactive Hard copy
- 40. Problem (part 1) A 10 kg mass, initially at rest, experiences three forces: one North with magnitude 10 N, one East, with magnitude 20 N and one Northeast with magnitude 30 N. Find the resulting acceleration and direction of movement. (Start by drawing a diagram)
- 41. Problem (part 2) Assuming the forces continue to act while the object is in motion, after 10 seconds … a) What is the object's velocity? b) How far has it traveled?
- 42. • • • • • Solution (part 1)
- 43. • θ • •
- 46. Newton's third law Every action has an equal and opposite reaction If object A exerts a force on object B, then B exerts a force which is equal in size and opposite in direction on A.
- 47. What are the action and reaction forces in the following pictures?
- 50. WARNING! Balanced • Relates to 1st law • Is rarely the same type of force • Relates to the same object • Only applies during constant velocity Equal and opposite • Relates to 3rd law • Is always the same type of force • Relates to 2 different objects • Applies in all kinds of motion Balanced forces are not the same as equal and opposite forces.
- 51. Practise1. What are the equal and opposite forces related to this egg?
- 52. Practice A plane is flying at constant velocity. Draw in all forces that act on the plane, together with their equal and opposite forces.
- 53. Task 1 Refer to the 12 questions on Newton’s second law. Add in all the equal and opposite forces
- 55. Summary of Newton’s Laws 1. An object will remain at rest or move with constant velocity unless acted upon by a net force. 2. A constant force will cause a constant acceleration, the magnitude of which is inversely proportional to the body’s mass 3. Every action has an equal and opposite reaction. If F = 0 Then a = 0 v is constant F = ma