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Forces (Part 2)

Newton’s Laws of Motion. Forces (Part 2). Recall from ICT Package. Only the resultant force affects motion, not the component forces. If forces are balanced (no resultant force) Object at rest remains at rest Object moving in a straight line at constant speed remains doing so

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Forces (Part 2)

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  1. Newton’s Laws of Motion Forces (Part 2)

  2. Recall from ICT Package • Only the resultant force affects motion, not the component forces. • If forces are balanced (no resultant force) • Object at rest remains at rest • Object moving in a straight line at constant speed remains doing so • If forces are unbalanced (got resultant force) • Object moves faster • Object moves slower • Object changes direction

  3. Demo 1 • Textbook on Penny Board (stationary) • Learning Point: An object at rest will want to remain at rest – this effect is sometimes referred to as “inertia” • Another illustration: when you are standing still in a stationary bus, and the bus suddenly accelerates forward

  4. Demo 2 • Textbook on Penny Board (moving) • Learning Point: an moving in a straight line will want to continue moving in a straight line. • Another illustration: when a vehicle suddenly brakes • Application: Safety features of a Car (seatbelt and headrest)

  5. Newton’s First Law • When there is no resultant force (i.e. balanced forces) • An object at rest remains at rest • An object in uniform motion remains in uniform motion • uniform motion = travelling in a straight line AND constant speed

  6. Half Time • BMW Motorbike Table Cloth Trickhttp://www.youtube.com/watch?v=vfnt8Sdj7cs • Mythbusters Attempt (Part 1)http://www.youtube.com/watch?v=_faLNmcAReA • Mythbusters Attempt (Part 2)http://www.youtube.com/watch?v=cuSAQX5ZeYg • How it’s really done?http://www.youtube.com/watch?v=j4cbYkm2NUk

  7. Demo 2 • Pull a light person sitting on a pennyboard (constant force) • What do you observe while the person is being pushed? Is the speed constant? • Push a heavier person with the same force. • Push the same person, but with a larger force. • What do you observe?

  8. Newton’s Second Law • When there is a resultant force Fresultant, • The object experiences an acceleration in the same direction of Fresultant • This acceleration is the ratio of Fresultant to the mass of the object • Equation form: Fresultant = ma

  9. Exercise 1 • Textbook Pg 58, Work Example 3.2 • A boy pushes a stationary box of mass 20 kg with a force of 50 N. Calculate the acceleration of the box (assume no friction) • F= ma • a = F/m • = (50)/(20) • = 2.50 ms-2in the direction of the applied force

  10. Exercise 2 • An elevator weighs 1000 kg and is pulled upwards by a cable. • (a) Draw the free body diagram of the elevator • (b) If the elevator is accelerated upwards at 0.1 ms-2, calculate the tension in the cable.

  11. Demo 3 • Magnet and Paperclip • Zero Gravity Demo: http://www.youtube.com/watch?v=cP0Bb3WXJ_k

  12. Newton’s Third Law • When object A exerts a force F on object B (i.e. FA onB), object B exerts an equal and opposite force back to object A (FB onA) • These two forces are called an “action-reaction pair” • “For every action, there is an equal and opposite reaction” • Action-reaction pairs ALWAYS act on two separate objects, they never act on the same one object

  13. Exercise 3 • Consider a book resting on the table. • What is the action-reaction pair? • Draw • (i) the FBD of the book • (ii) the FBD of the table • (iii) the FBD of the book AND table (as one object)

  14. Newton’s Third Law • Newton’s Third Law is only applicable when there is an interaction between two objects • Newton’s Third Law is really not about motion, so it is a misnomer to call it Newton’s 3 Laws of motion • Important note: Newton’s Third Law deals with component forces, while the 1st and second law deals with resultant forces

  15. Putting it altogether • For a more complex question which combines N2L, N3L and FBD, look at Textbook Page 65, Worked Example 3.4

  16. Recap • Newton’s First Law: • Object at rest remains at rest • Object in uniform motion remains in uniform motion • Newton’s Second Law: • F = ma • Newton’s Third Law • When A exerts force F on B, B exerts equal and opposite force back on A.

  17. Homework! • Textbook Page 68 • Section A: Q1-2. Section B: Q1-2 • Do on foolscap paper • Estimated Time: 30 min • Due 23 Apr (next Tuesday)

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