Learning outcomes: You will learn:

Transcription

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2 Learning outcomes: You will learn: 1. To state that forces can change the state of rest or motion of a body change the size and/or shape of a body bring about turning effects in objects (e.g. levers) exert pressure on objects 2. To identify some examples of contact forces (e.g. frictional force) and non-contact forces (e.g. gravitational force and magnetic force) and predict their effects on an object (e.g. how tides are caused by gravitational interaction between the oceans and the moon). 2

3 Class Activity Let s Play Ball Observe the activity that your teacher is carrying out. Complete the table below as instructed. See Think Wonder What do you see / hear? Write down what you have seen or heard What do you think is happening? Based on what we see, what can you explain or think about? I wonder Write down any questions that you have after watching the video 3

4 What are the effects of forces? 4 What are the effects of forces that you noticed during the activity? List them down

5 How do we describe object in motion * Speed is the of distance * Speed = * SI unit is * Symbol: (or ) * Know how to convert between m/s and km/h * Average Speed = 5

6 What are the typical speeds of moving objects? 6 Shooting star: 2700km/h 100m WR: 9.58s his speed is speed is 10.43m/s Snails moving at the speed of km/h Airbus A380 flies at about 945km/h

7 What is a force? 7 * A or a * It is exerted when one thing with another. * Examples of actions involving pushing or pulling: - Lifting - - Stretching - -

8 Measuring a force * Using force-meters * Example: Spring balances * 2 types of spring balances: - Extension * The spring when a force pull on it * The greater the force, the more the spring stretches - Compression * The spring when a force push on it * The greater the force, the more the spring compresses 8

9 Units of Force 9 * SI unit: * Symbol: * Named after English scientist Sir Isaac Newton Example Strength of force (N) Pull exerted by an ant Car engine Engine of a space shuttle

10 Importance of forces around us These forces are everywhere around us * Frictional force contact force * Magnetic force non-contact force * Gravitational force non-contact force 10

11 Frictional Force 11 * Also called friction * Occurs when two surfaces are in * Effects: Slows down and/or stops a moving object Produces

12 Useful effects of friction 12 Friction enables us to walk

13 Friction can be a nuisance 13 * Makes movement more difficult * Wears away materials * Produces heat

14 Ways of reducing friction 1. Using surfaces 14

15 Ways of reducing friction 2. Using 3. 15

16 Ways of reducing friction 4. 16

17 Ways of reducing friction: some examples 17 * Can you think of more everyday examples of : - how we make use of friction? - how we reduce friction?

18 Magnetic force * Force exerted by a magnet on a material (e.g. iron and steel) or another magnet. * Bar magnet has 2 poles: - North-seeking pole (N) - South-seeking pole (S) * Forces between magnets: - Like poles : Magnets from one another - Unlike poles : Magnets each other 18

19 Forces between magnets 19

20 Uses of magnetic forces 20 Can you think of more examples in our daily lives whereby we make use of magnetic force to do things?

21 Uses of magnetic forces in transportation: Maglev Train 21 * Maglev comes from the words magnetic levitation (rising and floating) * Magnets in the track and train repel and lift the train a few centimetres above the track * As there is between the track and vehicle, friction is. * This allows the train to move very fast (up to 500 km/h)

22 Maglev Train 22 Even though there is no contact between the train and the track, there is still friction produced. Identify the sources of friction.

23 Gravitational Force * Commonly known as * Exists between any objects * This is the force that causes all objects to be attracted to the. * Example: - A coconut falling from the tree - A ball coming down after bring thrown skywards 23

24 Weight 24 * Weight: force acting on an object by Earth * Measured using a spring balance in. * The the force of gravity pulling an object, the the weight. * The force of gravity on Earth is greater than that on Mars or moon. * Therefore, we weigh more on Earth than on Mars or moon.

25 Mass Amount of matter in a body Weight Force due to pull of gravity Unit used: kilogram, grams Measured using a beam balance Unit used: Newton Measured using a spring balance 25

26 26 Weight ( ) = Mass ( ) x field strength ( ) (i.e. W = mg) Question Cindy has a mass of 45 kg. Determine her weight on Earth. Gravitational field strength on earth, g = 10 N/kg Weight on earth = mass g = = N Her weight on Earth is N. Do you know your own mass and weight?

27 27 Write down the key concepts you have learnt Write down any misconception that is corrected Write down any questions you may have about the topic