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**CHAPTER 7: DYNAMIC Push and pull are types of force.**

We cannot see force but we can see the effects of force. Activities such as pushing a box, climbing, riding a bicycle, pulling a rope involve the use of force. Force can change the shape, position, speed, size, stop and direction of the movement of an object.

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Type of force

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**Not work done Push a wall Sit on the chair Sleep on a bed Read a book**

Stand up

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FORCE TYPE OF Magnetic Force Electro- static force friction Electrical Gravitational MEASURING Spring Balance FRICTIONAL Direction Magnitude WORK Work = Force X Distance Power X Time

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**Frictional Force (friction)**

friction occurs whenever two surfaces rub against each other. Friction is a force that always oppose motion.

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**Gravitational Force (gravity)**

gravitational force or gravity is the force of attraction that pulls objects towards the Earth. The force of gravity acts equally in all objects. Thus, in a vacuum, all objects whether heavy or light will fall to the Earth with the same velocity(halaju). Viscosity - kelikatan

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**Electrostatic Force produced by charged substances.**

Are caused by the presence of static electrical charges. The electrical charges are produced when two different objects are rubbed against each other. Like charges repel while unlike charges attract. Example, it can attract small pieces of paper, stream of water flowing from a tap.

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Electrical force Is produced when electrons move through a conductor placed in a magnetic field. electron flows from negative terminal to the positive terminal

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**Magnetic force Are the attractive and repulsive forces exerted**

by magnets. Like poles of magnets repel each other while unlike poles of magnets attract each other.

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**THE MEASUREMENT OF FORCE**

The unit of force is measured in Newton (N). Force is measured using the spring balance. 1 N shown on a spring balance is equal to nearly 100g. (10N = 1kg) Example, if

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**FRICTIONAL FORCE AND ITS APPLICATION. **

A force which slows down moving objects is called frictional force or friction. Frictional force can also be used to start or stop a motion. Frictional force acts when two surfaces come into contact, sliding over each other. Friction can at times be useful but in other instances it an also be a nuisance (GANGGUAN). Friction is a force that opposes motion. It acts in the opposite direction to movement. Friction always produces heat. Friction has direction and magnitude.

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**How different types of surface affect frictional force.**

Friction has magnitude and direction. Magnitude depend on: 1. Nature surface – smooth or rough 2. Weight – light or heavy 3. Surface area not influenced magnitude of force. (big or small) Different types of surfaces affect the magnitude of frictional force. Rough surfaces have more friction than smooth.

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Perlis 2012

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**ADVANTAGES AND DISADVANTAGES**

Advantages of friction (friction is useful) Friction plays an important role in our daily activities. Friction between a piece of rough metal and a flint produces sparks. These sparks ignite the gas in a lighter. Friction enables the mechanical belt in car engines and machines to turn engine parts. This moves the vehicle and machines. Disadvantages of friction (friction is a nuisance). Friction produces heat. For example, machine parts that move produce heat. This heat can damage certain parts of the machines if the machine is not carefully controlled. Friction opposes and slows down movement. For examples ridding a bicycle up a slope or paddling a boat. Friction wears away materials. For examples, friction wears out the soles of shoes and the tyres of vehicles.

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PMR 10

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PMR 2012

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PMR 2012

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PMR 2012

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PMR 2012

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FORMULA Work is done when a force moves an object through the distance in the direction of the force. Work (Nm) = Force (N) X Distance (m) Work (J) = Force (N) x Distance (m) Joule = Newton metre 1 newton-metre (Nm) = 1 joule (J)

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test (20 kg + 10 kg) , 2m 10 n , 1 km (1000m) 30 kg + 20 kg naik tangga 5 anak tangga, tinggi 5 anak tangga ialah 8 m, kira WD 15m 8 m 20 kg 10 m

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test 15m 8 m 20 kg 10 m

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**TRY WD (Nm) = force (N) x distance (m) = (40kg + 5 kg ) x 10N x 4m**

= 1800 J

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Power is the rate of doing work, that is the work done in one second (or the energy used per second).

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Check ?

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Selangor 2011

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**TEST 1 45 kg 7 m 8 m 10 m Calculate work done?**

Calculate Power if time taken is 5 s

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**TEST 2 45 kg 20 cm 8 m 20 cm 20 cm 20 cm a. Calculate work done? 10 m**

b. Calculate Power if time taken is 5 s 10 m

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TEST 3 20 cm 45 kg 20 cm 8 m 20 cm 20 cm 20 cm a. Calculate work done? b. Calculate Power if time taken is 5 minute 10 m

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**Test 4 45 kg 12 m 7 m 10 m 5 m a. Calculate work done?**

b. Calculate Power if time taken is 5 s

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**Test 5 45 kg 12 m 7 m 10 kg 10 m 5 m a. Calculate work done?**

b. Calculate Power if time taken is 5 s

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**Test 5 45 kg 10 kg 10 m 5 m a. Calculate work done?**

b. Calculate Power if time taken is 5 s

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**Test 6 wall 45 kg Ali tolak dinding a. Calculate work done?**

b. Calculate Power if time taken is 5 s

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**Test 7 Siti panjat dinding setinggi 5 m. Kira kerja yang Dilakukan?**

wall 45 kg 5 m a. Calculate work done? b. Calculate Power if time taken is 5 s

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**The chart below shows situations when work is done and when work is not done.**

Work Done Pushing a wheelbarrow Climbing the stairs/ladder Throwing a ball upwards Brushing teeth Work not done Sitting on a chair Standing on a ladder Sleeping on a bed swimming

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**Moment of force Moment of force - turning effect of a force.**

Moment of force (Nm) = perpendicular distance from the fulcrum to the force (m) X force (N) To increase the moment of force Increase the perpendicular distance Increase the magnitude of applied force Moment of force (Nm) = perpendicular distance from the fulcrum to the force (m) x force (N) Moment either clockwise or anticlockwise Tightening a nut (clockwise ) Perpendicular distance = 15 cm = 0.15 m The moment of force = 20 N x 0.15 m = 3 Nm b. Loosening a nut (anticlockwise ) Perpendicular distance = 15 cm = 0.15 m The moment of force = 20 N x 0.15 m = 3 Nm

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Penutup 1. Baca Surah Al-`Ashr 2. Tasbih Kifarah.

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