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Forces (Part 1)

Newton’s Laws of Motion. Forces (Part 1). Scene from what movie?. What is a Force?. A force is a push or a pull A force is a vector Force has both magnitude and direction Unit for force is Newtons (N). Component & Resultant Forces.

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Forces (Part 1)

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

  2. Scene from what movie?

  3. What is a Force? • A force is a push or a pull • A force is a vector • Force has both magnitude and direction • Unit for force is Newtons (N)

  4. Component & Resultant Forces • When a more than one force acts on a body at the same time, each individual force is called a component force • The total sum of all the component forces acting on an object is the resultant force • For any one object, there can be several component forces, but only be one resultant force. • Sometimes, resultant force is also called “net force”

  5. Balanced & Unbalanced Forces • When there is no resultant force (or resultant force = 0), the object is said to be balanced • When there is a resultant force, the object is said to be unbalanced. • We will discuss more about balanced and unbalanced forces in this chapter, and later in chapter 5.

  6. 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

  7. 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)

  8. 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

  9. 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

  10. 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?

  11. 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

  12. Exercise 1 • Textbook Pg 54, 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

  13. 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

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