1. Force and Work You will learn about ● Force and its units ● Types of force ● Weight and difference between it and mass ● Friction ● Hooke's law ● Work ● Power

3. Force ● A force is something which causes an object to accelerate.

4. Force A force may cause an object to slow down, speed up, stop or change direction. a force always acts in a certain direction ex. if you push something, the force is in the direction of the push Sometimes is seems there is no force because they cancel each other out as in a tug of war and no team is moving. (show lever forces each side no movement)

5. Measuring Force Force is measured using a spring balance (Newton-meter) Force is measured in Newtons (N) 1N is weight of 100g.

6. T ypes of forces Pushing force Pulling force Force of tension

7. Types of Forces Weight Friction Magnetic Electric

8. Types of Forces Stretching force Twisting force Compression force Reaction force

9. Weight Weight is a force due the pull of gravity. As it is a force it is measured in Newtons (N) Weight is a force. Mass is the amount of matter in a substance measured in Kg Weight depends on where you are but your mass always stays the same

10. Weight and mass difference Your weight on Earth may for example be 600 N about 60 kg of mass On the moon your mass is still 60 kg but your weight is only 100 N Out in the vacuum of space your weight would be 0 N and your mass still 60 kg.

11. Calculating Weight (Hons) On Earth objects fall with increasing speed in fact 10 m/s every second. This is the acceleration due to gravity which is 10 m/s 2 This is different depending on gravity, on the moon it is only 1.6 m/s 2 and in space is 0 m/s 2. On Earth the weight of an object: weight = mass (kg) x 10

12. Find the weight of a block of mass 7.5 kg. ANSWER wt.= mass x 10 = 7.5 x 10 75 N What is the weight of the same block on the moon. ANSWER wt. = mass x 1.6 =7.5 x1.6 = 12 N Calculating Weight (Hons)

13. Weight examples (hons) What is the weight of the same block in space? ANSWER wt. = mass x 0 = 7.5 x 0 0 N What is mass of a block that weighs 20 N ? ANSWER wt. = mass x 10 20 = mass x 10 Mass = 2 kg

14. Weight examples (hons) What is the weight of a 500g bag of sugar? ANSWER wt. = mass (kg) x 10 =0.5 kg x 10 = 5 N Note the grams must be converted to kg first.

15. What is friction? Friction force that opposes the motion between two objects in contact. the force of friction works in the opposite direction of the force of motion

16. What causes Friction? the source of friction is the contact between two surfaces, which are not perfectly smooth. The tiny humps and hollows catch off each other opposing the motion.

17. Advantages of Friction Used to stop or slow things down, e.g brakes on a car or a bicycle. Without friction we could not walk or kiss, car tyres would have no grip on the road. Without friction you could not pick things up with your fingers.

18. Disadvantages of Friction Friction slows things down, ships travel much slower than planes because of the friction due to the water. Energy is often wasted e.g in car engine energy is wasted as heat and sound due to friction Friction wears away surfaces, brake pads wear, tyres lose their thread.

19. Friction friction makes motion possible friction also makes it hard to move objects reducing friction makes it easier to move objects

20. How can friction be reduced? by polishing and making surfaces smoother by using lubricants Lubricants – substances that reduce friction

21. Hookes Law.

22. Robert Hooke 1635-1703 ● Researched and wrote papers on Geometry, Snowflakes, Heat, Astronomy, Fossils, Air Pumps, Light, Watches, Telescopes and Silkworms…..among other things ● Continually squabbled with Newton with whom he was miffed because Newton seemed to get all the attention and credit..Hookes Law was one of the few things he was popularly credited with ● A rather crabby individual but credited with the invention of universal joints, the balance wheel and the iris diaphragm

23. What did Hooke discover ? ● the more force that was put on materials the more they extended (Stretched) and return to their original shape when the force is removed. ● Materials which can be stretched and return to their original shape are said to be elastic. ● With some materials they also extended in a regular way eg if the force was doubled so did the extension ● this was true as long as their elastic limit was not exceeded (not over stretched)

24. What does this mean? The more you stretch a spring the longer it gets. When you stop stretching the spring it returns to its original length. Is there any exception to returning to the original length or shape? What practical use is hooke's law in your own house?

25. Yes the exception is if you stretch the spring too much it then will not return to it's original shape, it has gone beyond it's elastic limit. A practical use of hooke's law in your house is the bathroom scales or kitchen scales. a) If a force of 1N stretches a sping by 3 cm how much will it be stretched by 3 N? b) What will the extension on the same spring for 5 N? c) What force would cause the same spring to stretch by 5 cm? Answers a) 9 cm b) 15 cm c) 1.66 N

26. What the graph shows ● Use the results in the table to draw a graph. ● Put the extension on the vertical axis and force (weight) on the horizontal axis. ● Use the graph to find out the force for an extension of 7 cm. ● Use the graph to find out the extension for 7.5 N.

27. Hooke's Law Graph O 1 2 3 4 5 6 7 8 9 2 4 6 8 10 12 14 16 Extension/cm Force/N Answers (a) for 7cm the force is 3.5 N (b) for 4.5 N the extension is 9 cm

28. What is the elastic limit? ● The material no longer shows elastic behaviour (ie does not return to original size when stretching force is removed) ● The material is permanently deformed ie is larger or longer than originally

29. Hooke's Law The extension of a spring is directly proportional to the force stretching it. This means that if you double the force the extension is doubled If you treble the force the extension is trebled. If you half the force the extension is halved etc. What does directly proportional mean?

30. What does k mean in F=kx? ● k is called the spring constant and is a measure of the stiffness of the spring or material ● It has units of Nm -1 (newtons per metre) ● The higher the k the stiffer the spring ● Materials with a high k need a large force to for a given extension

31. So what? ● Understanding Hookes law is critical to the construction of any structure eg bridges buildings ● Deformation of materials is also critical in vehicle design though in practice the terms stress ( the force per square metre) and strain (the extension per unit length) are more commonly used instead of simply force and extension

32. A material is said to obey Hookes Law if its extension is directly proportional to the applied force More info on Hooke at http://www.roberthooke.org.uk/ http://www.roberthooke.org.uk/ Summary:

34. Work, Energy and Power ● Work done(J) = Force (N) x distance moved (m) ● When you swim you are doing work as you are exerting a force and moving. ● Example ● What is the work done when a force of 6 N moves a glass a distance of 0.5 m? ● Answer ● Work = force x distance ● = 6 x 0.5 ● = 3 J

35. Work example Example (honours) ● Find the work done by a weightlifter lifting a 90 kg mass 1.6 m of the floor? ● Answer ● First wt = mass x 10 (the weight is the force) ● = 90 x 10 ● =900 N ● Now work = force x distance ● = 900 x 1.6 ● = 1440 J

36. Energy and Power ● Energy = is the ability to do work. ● Since the work you do = the energy you use enery is measured in joules (J) as well as work. ● Power is the work done per unit time. ● work done ● time taken ● Power is measured in Watts (W) Power =

37. Power Example ● A boy used a force of 25 N to push a lawnmower a distance of 20 m. It 8 seconds to do this. What is the average power. ● ANSWER ● First work = force x distance ● = 25 x 20 ● = 500 N ● Then power = = = 62.5 W Work time 500 8