2. Keith Warne TeachBomb www.TeachBomb.com Keith Warne TeachBomb www.TeachBomb.com Newton’s First Law ”A body will................................ of...... or..................................... in a straight line, unless acted on by a......................................"  This law describes -................ IMPORTANT POINTS: (A moving object…) Continues in its state Rest or uniform motion in a straight line Unless acted upon by (external) forces

3. Newton’s Laws Second Law A..............................on an object causes it to...................in the direction of that force. The magnitude of the acceleration will be directly proportional to the...................and inversely proportional to the..............of the object. F res = F + f and F res = m x a F m a = ? F m=1kg T = ? FgFg F res = Fg Fg + T = m x a And Fg Fg = mg These are the two most basic scenarios - any of the variables can be calculated if all the others are known. a=2m.s -2

4. Inertia A body has a natural tendency to resist any changes to its state of motion. This resistance is known as Inertia. Examples: –What will happen if the card in the picture is flicked? Explain why. –The moon moves around the sun in a circular orbit. Explain why. Every object in a state of uniform motion tends to remain in that state of motion unless an external (unbalanced)force is applied to it. Gravity Motion Earth 1. 2. Motion Inertia Friction

5. Inertia 1. A body has a............................................ any changes to its state of motion. This resistance is known as........................ If the card in the picture is flicked......................................................... Inertia keeps the peg stationary when the card is moved quickly. The peg’s Inertia is overcomes.................................. which try to keep it’s position on the card. 2. The moon was moving past the earth in a straight line but became................. by the............................ Gravity does not act against the direction of motion (90 o ) so the motion continues because............................................... to..................... (The question is who threw it in the first place!) Every object in a state of uniform motion tends to remain in that state of motion unless an external (unbalanced)force is applied to it. Gravity Motion Earth 1. 2. Motion Inertia Friction

6. The Normal or Reaction Force This is the Reaction Force (Newton …..) exerted by a surface on any object that rests (exerts a force on) that surface. Always EQUAL in magnitude and to the initial force and at 90 0 to the SURFACE. F┴F┴ N = ………. Normal Reaction Force (N) Weight (F g ) Normal Reaction Force (……) N = …….. Calculate the normal force in each example here if the ball has a 2 kg mass.

7. FRICTION Friction forces exists between two surfaces in contact. There are two types …………… FRICTION and ………………(moving) FRICTION. Static friction is ………………. than dynamic friction. The maximum force exerted on a stationary object …………..it begins to move (accelerate) is equal to the maximum static frictional force. The force required to ………. an object moving with ……………… velocity on a surface is equal to the dynamic friction. Max force Static friction V = 0 Force dynamic friction constant velocity

8.  = = Coefficient of FRICTION STATIC FRICTION and Can vary from zero to a MAX IMUM of F s = μ s........ μ s = coefficient of.......... Friction DYNAMIC (moving) FRICTION. Always constant value given by F d = μ d N μ d = coefficient of.......... friction....... force Static friction V =.... Constant Force Dynamic friction constant velocity N = -F g FfNFfN FAFgFAFg

9. μ s = = gradient F N (N)..... μ s = = gradient F f (N)..... DRY surfaceWET surface............ Graphs of Frictional force against normal force. 0 1 2 3 F f (N)............................. The force of friction F f is directly proportional to the Normal force (weight) The.................of these graphs would be EQUAL to the coefficients of friction. F N (N)

10. Friction Examples 5kg FAFA A block is held against a vertical surface by a horizontal force. What is the coefficient of friction if the minimum applied force is 20 N?

11. Friction Examples Object on an inclined plane. 2 kg  What is the maximum angle that the slope can have before the block begins to slide if the coefficient of friction is 0.8? 2 kg

12. A Exam Question The diagram shows a truck with a crate immediately behind the cab in position A. The truck suddenly accelerates forward and the crate slides towards the back of the truck and comes to rest in position (B) a)Draw a force diagram, indication and labeling the horizontal force(s) acting on the crate while the truck accelerates.(2) b)Briefly explain why the crate slides towards the back of the truck. (4) c)Name and state the law or principle that you have applied in order to reach an answer in (b).(4) WCED Nov 2001 TOTAL [10]

13. AB Exam Question The diagram shows a truck with a crate immediately behind the cab in position A.The truck suddenly accelerates forward and the crate slides towards the back of the truck and comes to rest in position (B) a)Draw a force diagram, indication and labeling the horizontal force(s) acting on the crate while the truck accelerates. (2) Friction between the crate and truck

14. Newton’s 3rd Law "For every action there is an ………… and ………………… reaction.” The gasses experience a force ………………… out of the rocket, this has an equal but opposite …………………. which drives the rocket forward! The force exerted by the hand on the head is the same …………………. as the force exerted by the head on the hand! The head hits the hand just as hard as the hand hits the head!

15. An apple on a table. Non contact forces Gravity Exerted by the ……… on the apple Reaction by the ……….. on the earth. Contact forces: Table Apple exerts a force ……… on the table. Table exerts a …………. or …………. force on the apple. All these forces are …………. so there is no …………… force. Forces on apple: gravity (down) & reaction (up)

16. Walking & Pushing Draw sketches to show and explain all the reaction pairs of forces that occur when 1.a person is walking 2.A person is pushing a car.

17. Person Walking The person pushes ……………. on the ground. The ground pushes …………….. on the person. The person has an …………. force so moves in the direction of that force. (The earth………… movement Forces on person

18. Pushing a car The person pushes backwards on the ground. The ground pushes forwards on the person. The person pushes forward on the car. The car pushes back on the person. The forces on the car are less than the forces on the ground. The person has an unbalanced force so moves in the direction of that force. Movement Forces on person

19. Newton’s 3rd BOB (who isn't very bright) is tired of pushing his van to the petrol station. He gets an idea -- "Hey! I go much faster when I am roller-blading than when I am walking. Why don't I wear my roller-blades!" So BOB gets his roller blades from the back of his van, and starts pushing. Will this work? Explain using Newton's laws what will happen. (Calculate the acceleration and displacement each will experience. BOB weighs 50 kg, and the van weighs 2,000 kg.)

20. Newton’s 3rd ANS: Since every action has an equal and opposite reaction, when BOB pushes the van, the van pushes BOB with the same size force but opposite direction. Assume he pushes for one second. BOB weighs 50 kg, and the van weighs 2,000 kg. Taking right as positive calculate the acceleration of both Bob and the van as well as the distance moved by both:

21. Pairs of forces

22. 2. Pairs of forces Action-reaction pairs 1.Man against ground - ground against man – greater for …. than for ………. 2.Man on rope – rope on man – all ………….. but less than ……... ground forces and greater than ….. ground forces. 3.A and B both experience a …………………………. 1. AB

23. Inclined Plane The system shown is in equilibrium. What is the magnitude and direction of the friction force acting on the block? 50kg 25 o

24. Pulley system The system shown is in equilibrium. What is the magnitude and direction of the friction force acting on the block? 10kg 250 N 30 o F nett = F // + F A + F f 0 = (-250 sin 30) + (98) + F f Down = ( - )

25. Keith Warne TeachBomb www.TeachBomb.com Keith Warne TeachBomb www.TeachBomb.com Eg - Hanging Weight A weight of 30 N hangs on a rope fron a ceiling. A horizontal force pulls it to the side. The angle between the rope and ceiling is 60 0. Determine: the tension in the rope (T) and the horizontal force. 30N 60 o Force Diagram 30N T Fg T F 60 o

26. Newton’s - Connected objects In both these cases the tension gets ………………… as more blocks are connected I.e. T 1 ….. T 2 …. T 3 The systems can be treated as a …….. …………: F … = ………………………….. Where F … = ……….orF … = ……….. So …………………………………….. T1T1 T2T2 T3T3 m1m1 m2m2 m3m3 f m3m3 m2m2 m1m1 T3T3 T2T2 T1T1 Fg To find the tension between blocks you must consider them ……………. I.e. ……………………………………………….

27. Newton’s Laws - Pulley systems The accelerations of these systems can be easily found by considering them as a …………….: F res = …………wherem total = ………… And F res = ………………….. (always opposed) To find tension; ……………………………….. Individual masses or tensions can be found by considering masses individually: ……………………………………... Tension in any one string is the same throughout! m2m2 m1m1 TT F g1 F g2 m1m1 m2m2 m3m3 T1T1 T2T2 F g1 F g2

28. Examples Newton 1 & 2 1. A 25kg container is acted upon by a horizontal force of 65N. The frictional force between the container and the floor is 15N. Calculate the acceleration of the container. (a = 2m.s -2 )

29. Examples Newton 1 & 2 2.A ball with a mass of 18kg is suspended by a string. a)Calculate the tension in the string if the ball is i)stationery, ii)accelerates upwards at 3m.s -2 ; iii)accelerates downwards at 3m.s -2. T = 176.4N upwards T = 230.4 upwards T = 122.4N upwards

30. Examples Newton 1 & 2 3.Two blocks rest on a frictionless horizontal surface connected by a string. A force of 80N is exerted on the 7kg block. Calculate: a)the acceleration of the blocks b)the force exerted by the string on the 13 kg block. 13 kg 7kg 80 N

31. Examples Newton 1 & 2 4.A constant force of 120N is used to lift two masses of 4 kg and 6kg which are attached to each other with string. Ignore any mass of the string and calculate the acceleration of the system and the tension in the string. 4kg 6kg 120 N

32. Examples Newton 1 & 2 &3 5.The frictional force on the system is 10 N (6 N & 4 N). Calculate the force that the 4kg block exerts on the 6kg block. 4kg 6kg a = 2m.s -2 (Ff=10N) (6N of 6kg block 4N on 4kg block) F