Newton’s Laws of Motion

 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

 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

 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)

 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

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 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?

 When there is a resultant force F resultant,  The object experiences an acceleration in the same direction of F resultant  This acceleration is the ratio of F resultant to the mass of the object  Equation form: F resultant = ma

 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 -2 in the direction of the applied force

 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.

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 When object A exerts a force F on object B (i.e. F A on B ), object B exerts an equal and opposite force back to object A (F B on A )  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

 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)

 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 1 st and second law deals with resultant forces

 For a more complex question which combines N2L, N3L and FBD, look at Textbook Page 65, Worked Example 3.4

 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.

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