Force 1 It is as natural for a moving object to keep moving with a constant speed along a straight line as for a stationary object to remain at rest. Next Slide Aristotle’s old belief Constant speed motion requires constant force Inertia Inertia 1 Galileo’s law of inertia Photo

Force 1 Next Slide Galileo’s thought experiment Newton’s first law of motion: Every object remains in a state of rest or uniform speed along a straight line (constant velocity) unless acted on by an unbalanced force. Inertia and Newton’s first law of motion Diagrams Photo Inertia 2

Force 1 Next Slide Force : changes the state of rest or uniform motion of an object (vector! Why?) Inertia : the resistance of an object to a change in its state of rest or uniform motion in a straight line. Mass : is a measure of inertia (scalar! Why?) Inertia and Newton’s first law of motion Mass can be considered as a measure of inertia Inertial balance Diagrams Inertia 3

Force 1 Examples of smooth surface Next Slide Origin of wrong concept of Aristotle’s old belief Nature of friction Direction : opposite to the motion (velocity) Friction Demonstration of Newton’s first law on smooth surfaces Diagrams Photo Force 1

Experiment to study resultant force Next Slide Falling of object in liquid (terminal velocity) Its relation with Newton’s first law of motion Friction-compensated inclined plane Results Unbalanced force (resultant force, net force) Photo Diagram Calculation Force 2 Photo Diagram

Force 1 3.  F  ma or F = constant  ma Next Slide Deductions from the results 1. Acceleration  force (mass is constant) 2. Acceleration  1/mass (force is constant) Newton’s second law of motion Acceleration is not zero if the resultant force is not zero. Force 3

Force 1 Next Slide The acceleration of an object is directly proportional to, and in the same direction as, the unbalanced force acting on it, and inversely proportional to the mass of the object Newton’s second law of motion Definition: Unit of force : Newton (N) Force 4

Force 1 Next Slide constant in F = constant  ma becomes 1 Mathematical form of Newton’s second law Newton’s second law of motion 1 newton of force will give a mass 1 kg an acceleration 1 Direction of resultant force = direction of acceleration Force 5

Force 1 Next Slide The force of gravity acting on the object is called the weight of the object and is measured in newton Weight and Mass Definition: Acceleration in free falling = 10 m s -2  1 kg of mass has a weight 10 N (downwards) Weight & Mass 1

Force 1 Next Slide Weight and Mass Instruments to measure weight and mass Weightlessness Discussion about the restrictions of the above machine Photo Calculation Weight & Mass 2

Force 1 Next Slide Addition of Forces (vectors) More than one force acting on an object Add them together to get ONE resultant force F = ma can only be applied for resultant force Tip-to-tail method (Revision) Example 1 Example 2 Calculation Addition of Force 1

Force 1 Next Slide Addition of Forces (vectors) Method of resolving components Adding forces or vectors without drawing diagrams Example Examples for components of forces Calculation Addition of Force 2

END of Force 1

Force 1Inertia 1 Galileo Galilei ( ) Back to Click Back to

Force 1Inertia 2 A ball reaches point C which is of the same height Same situation for D and E If the track is infinite long, the ball will never stop. Small bearing is released from rest on a smooth track at A. A CDE F Next Slide

Force 1Inertia 2 Galileo’ pin-and-pendulum experiment Consider the swing of a simple pendulum Next Slide

Force 1Inertia 2 The bob rises to the same height as before Even we have a pin, the bob rises to the same height Back to Click Back to

Force 1Inertia 2 Isaac Newton ( ) Back to Click Back to

Force 1Inertia 3 We set the platform into vibration and record the period. Fix load on the platform and repeat the vibration, we find that a longer period can be found. The larger the load, the longer the period. Back to Click Back to

Force 1 Friction is caused by the interlocking of surface irregularities. Back to Click Back to

Force 1 A mass placed on a thin layer of polystyrene beads on a glass plate A balloon is blown up and attached to a short pipe Next Slide

Force 1 Air-layer Ball Motion on a air track Click Back to Back to

Force 1Force 2 Back to Click Back to An object is falling inside liquid.

Force 1Force 2 The object is falling downwards with constant velocity. Do you know why? Liquid resistance is equal to the weight No unbalanced force liquid resistance weight Back to Click Back to

Force 1Force 2 An inclined plane is prepared so that when we give the trolley a hard push, it moves down with constant velocity. It is called to be friction-compensated. Careful adjustment for the plane is needed to achieve this situation. Back to Click Back to

Force 1Force 2 Identical elastic strings are used to pull the trolley. At first, we use one string and then two, and three. We always maintain the same length for all the strings so that each string produces the same force. The accelerations in each case are recorded. Friction-compensated inclined plane trolley elastic string Next Slide

Force 1Force 2 One elastic string is used to pull several trolleys. At first, we use one trolley and then two, and three. We always maintain the same length for the string so that the string produces the same force in each case. The accelerations in each case are recorded. friction-compensated inclined plane elastic string Click Back to Back to

Force 1Force 2 Different tape charts for different no. of strings with one trolley are shown. We find that the acceleration is directly proportional to the no. of strings used (Force) when the mass of trolley is kept constant. Next Slide 1 string2 strings3 strings a = 2 m s -2 a = 4 m s -2 a = 6 m s -2

Force 1Force 2 Different tape charts for different no. of trolleys with one string are shown. We find that the acceleration is inversely proportional to the no. of trolleys used (mass) when 1 string is used (constant force). Back to Click Back to 1 trolley2 trolleys3 trolleys a = 2 a = 1a = 0.67

Force 1Weight and Mass 2 Beam balance (measure mass) Spring balance (measure weight) Back to Click Back to

Force 1Weight and Mass 2 Can we use the beam balance or spring balance on Moon to get correct readings of mass and weight of an object with 1 kg mass? 1 kg slot-mass is still needed to balance the object. The reading from the spring balance = 1  1.8 = 1.8 N! Mass is the same anywhere while weight depends on position and is not a constant even for the same object. The acceleration due to gravity on Moon is only about 1.8 m s -2. Back to Click Back to

Force 1Addition of Force 1 Two forces 3 N and 4 N are acting on an object (2 kg) as shown below. What are the resultant force and acceleration? Use a scale of 1 cm to 1 N to draw the forces in the form of arrows. The direction of the force is indicated by the arrow. 2 kg 3 N 4 N N 4 N (4 cm in length) 3 N (3 cm in length) Back to Click Back to

Force 1Addition of Force 1 Attach the tip of an arrow to the end of another arrow. (Tip-to-tail method) 4 N (4 cm in length) 3 N (3 cm in length) Draw an arrow from the starting point to the end point. It is the net force. Length of the arrow : 5 cm Direction : N 53.1°E 5 N (5 cm) 53.1° Direction of net force : N53.1°E Magnitude of net force : 5 N (Why?) Direction of acceleration : N53.1°E (Why?) Magnitude of acceleration : Back to Click Back to

Force 1Addition of Force 2 By using the concept of tip-to-tail method, one force can also be separated into two different forces, for example, Use a scale of 1 cm to 1 N (10 sin 60°cm) 60° 10 N (10 cm) 10 cos 60° N (10 cos 60° cm) 10 sin 60°N Next Slide

Force 1Addition of Force 2 We want to add the following two forces using the method of resolving components. Scale : 1 cm to 1 N 60° 30° 5 N 6 N Next Slide

Force 1Addition of Force 2 We place them together on xy-coordinate plane with both the nails at the origin. 60° 30° 5 N 6 N y x Each force can be represented by a force (component) along x-axis and a component along y-axis. 5 sin 60°N 30° y x 5 cos 60°N6 cos 30°N 6 sin 30° N Next Slide

Force 1Addition of Force 2 5 sin60° N y x 5 cos 60° N6 cos 30° N 6 sin 30° N Add the components along x-axis together. Then add the components along y-axis. They are of the same direction and we can add them like scalars. Next Slide + -

Force 1Addition of Force 2 Combine the components of force along each axis to form the net force vector. 5 sin 60° N y x 5 cos 60° N6 cos 30° N 6 sin 30° N  Next Slide - +

Force 1Addition of Force 2 Magnitude of the net force: Direction of force (  ): Click Back to Back to

Force 1Addition of Force 2 HK Convention & Exhibition Centre By resolving components, the roof will not fall even no support is directly below the roof Next Slide

Force 1Addition of Force 2 Hong Kong Space Museum By resolving components, the roof will not fall even no support is directly below the roof Next Slide

Force 1Addition of Force 2 The top of a tunnel Back to Click Back to