Y11 Forces

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Information about Y11 Forces
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Published on July 21, 2008

Author: dawsjonz

Source: authorstream.com

FORCE AND MOTION : FORCE AND MOTION A REVISION exercise Slide 2: The mass of an object is the measure of how much substance is in that object. Every physical object has mass: you do, the Earth does, and the Sun does. Mass is measured in kilograms. Mass: The Difference Between Weight and Mass : The Difference Between Weight and Mass Mass is the amount of matter in an object. The mass of an object will not change as you move it from place to place. Slide 4: An object's mass is constant where ever it is. Mass = 1 kg on both the Earth and the Moon. Slide 5: Weight is a force--the force with which gravity "pulls" on an object's mass. The Earth is bigger than the Moon therefore it applies a bigger pull on objects. Earth Moon Slide 6: An object's weight depends upon its location . Weight = 10 N Weight = 1.6 N Slide 7: Weight is calculated by multiplying the mass by the force due to gravity. Weight = m x g On the Earth: g is approximately 10 N kg-1 g is approximately 1.6 N kg-1 On the Moon: Slide 8: Christian Cullen on the EARTH Mass = 82 kg Weight = mass x g = 82 x 10 = 820 N Slide 9: What would Christian Cullen’s mass and weight be on the Moon? Slide 10: Christian Cullen on the MOON Mass = 82 kg Weight = mass x g = 82 x 1.6 = 131.2 N (Same as on Earth) Slide 11: Forces can be: Pushes Pulls Twists Slide 12: Forces at Work Slide 13: Forces in ACTION ! Slide 14: When two rough surfaces rub together there is more friction. Orbital sander for light sanding of wooden surfaces. Slide 15: Friction is the force between two surfacesrubbing together. When two smooth surfaces rub together there is very little friction FRICTION Slide 16: There is less friction when there is a liquid (e.g. oil) between the two surfaces. Oil is added to a car engine to lubricate the parts. Slide 17: There is more friction if the two surfaces are forced against each other. Slide 18: Friction is a good thing: Just think of walking on a wet marble floor or an icy pavement with new leather soled shoes: you might slip over! Worse still, if the tyres on your car are worn out and the road is wet and slippery, you will probably skid the car and have an accident. Slide 19: Friction is the force between two surfaces rubbing together. It is high if the surfaces are dry and rough and it is low if the surfaces are smooth and wet. Friction Force Pulling Force Slide 20: Friction is also very important for your car's brakes to work properly. When you put you foot on the brake pedal, some rough pads are squeezed tight against the brake discs. This friction slows the car down. If oil gets on the discs, the brakes will not work so well. Slide 21: Both your shoes and the car's tyres need good grip so their surfaces are rough. This increases the friction between them and the ground. Slide 22: Friction is a bad thing: Friction inside a car engine and inside the wheel axles will slow a car down and wear out the metal. To prevent this we put oil or even grease in them. This makes their surfaces more slippery and so reduces friction. Slide 23: When we push or pull on an object, we create a force. When forces are equal, they are in balance. When forces are unequal, an object moves or twists. Slide 24: At a given location on the earth and in the absence of air resistance, all objects fall with the same uniform acceleration. Slide 25: This acceleration is called the acceleration due to gravity and it is refered to commonly as g. On Earth g = 9.8 m s-2 (Approximately 10 m s-2) Slide 26: If all the forces are in balance, the object will stay as it is - stationary or moving - at a steady speed in a straight line. Slide 27: If the forces are unbalanced, it will: start to move in the direction of the force speed up (accelerate) slow down (decelerate) change direction. Slide 28: PUSH, PULL and TWIST all at the same time. Slide 29: When driving on the motorway you might travel several kilometres at the same speed. e.g. Travel 5 km at 100 km h-1 For that short period of time you are travelling at CONSTANT SPEED. Slide 30: On a normal journey you speed up and slow down. We can calculate your AVERAGE SPEED for the journey. Average Speed = Distance travelled Time taken Slide 31: e.g. Distance from Raumati Beach to Wellington is 50 km Time to travel from Raumati Beach to Wellington is 45 minutes. The journey will have fast bits and slow bits. You will travel faster in the four lane parts. You might be slowed down by the Paramata Bridge. You may have to stop for a train at McKay’s Crossing. You will not be able to travel at a constant speed. Slide 32: Now set out your work as follows to calculate the AVERAGE SPEED for the whole journey. Show all your working ! Slide 33: d V t For us Speed and Velocity are the same thing. Let:V= velocity d = distance t = time V = d t d = V x t t = d The 3 equations you need are: V Slide 34: e.g. Distance from Raumati Beach to Wellington is 50 km Time to travel from Raumati Beach to Wellington is 45 minutes. d = 50 km t = 45 minutes = 0.75 hr Average Speed = d t Average Speed = 50 0.75 66.7 km h-1 = Slide 35: We can represent a journey in graph form. What will the slope of the graph tell us about our trip? Slide 36: A B C D E A = fast constant speed B = Slower constant speed C = at rest D= medium constant speed E = at rest Slide 37: The SLOPE of the graph gives us an indication of the speed. The steeper the slope the greater the speed. A horizontal line tells us that the object is at rest (or stationary) From the graph you can calculate the speed. Slide 38: Slope = RISE RUN Slope = 30 5 RISE RUN = 6 ms-1 Time (s) Distance (m) Distance Time Graph Slide 39: The SLOPE of the graph indicates SPEED. The steeper the slope the faster the speed. Distance Time Graph Slide 40: An object that is changing its velocity is accelerating. Something that slows down or speeds up, is said to accelerate. Slide 41: All accelerations are caused by forces. A force is basically a push or a pull on an object. Slide 42: Something that is speeding up has a positive acceleration, and something that slowing down (decelerating) has a negative acceleration. Slide 43: The slope of a velocity time shows whether an object is accelerating. Accelerating at a constant rate Travelling at a constant speed Decelerating at a constant rate V V V t t t Slide 44: Acceleration Change in velocity t = = Final V - Initial V Change in time e.g. A car speeds up from rest to 24 ms-1 in 3 seconds Initial Velocity = 0 ms-1 Final Velocity = 24 ms-1 Time taken = 3 s Slide 45: Acceleration t = Final V - Initial V Initial Velocity = 0 ms-1 Final Velocity = 24 ms-1 Time taken = 3 s 3 = 24 - 0 = 8 ms-2 Slide 46: e.g. A car slows down from 35 ms-1 to 10 ms-1 in 5 seconds Acceleration t = Final V - Initial V I V = 35 ms-1 F V = 10 ms-1 t = 5 s 5 = 10 - 35 = -5 ms-2 5 - 25 = Slide 47: 1. The weightlifter is pushing up with a bigger force than the weight of the bar. 2. The forces are unbalanced so the bar accelerates up. A weightlifter is lifting up the bar. Slide 48: 1. The weightlifter is using a force equal to the weight of the bar. 2. The forces are balanced so the bar remains stationary. A weightlifter is holding the bar above his head. Slide 49: 1. The weightlifter is using a force less than the weight of the bar. 2. The forces are unbalanced so the bar accelerates down. A weightlifter is lowering the bar to the floor. Slide 50: Here are 4 types of forces: air resistance - drag: When an object moves through the air, the force of air resistance acts in the opposite direction to the motion. Air resistance depends on the shape of the object and its speed. Slide 51: contact force (support): happens when two objects are pushed together. They exert equal and opposite forces on each other. The support force from the ground pushes up on your feet as you push down to walk forwards. Slide 52: Support Force Gravitational Force Support Force Gravitational Force Slide 53: gravity: the force which pulls objects towards the Earth. We call the pull of gravity on an object its weight. The Earth pulls with a force of about 10 Newtons on every kilogram of mass. Slide 54: friction: the force which resists movement between two surfaces which are in contact. Air resistance slows the shuttle. Slide 55: What is the relationship between Force, mass and acceleration. Force = mass x acceleration Units: (Newtons) (kilograms) (ms-2) F m a m = F F = m x a a = F The 3 equations you need are: a m Slide 56: Problem: A car has a mass of 700 kg.It is accelerating at 2 m/s2.Calculate the unbalanced force causing this acceleration. (See next slide for correct working.) Slide 57: 1. Write down the equation. (learn it.) 2. Put in the values. 3. Work out the answer and write it down. (Do not forget the units.) Force = mass x acceleration Force = 700 kg x 2 m s-2 Force = 1400 N Slide 58: What force would you need to accelerate (or decelerate) a 1500 kg car at 3 m s-2? Try this question on your own paper: (See next slide for correct answer.) Slide 59: What force would you need to accelerate (or decelerate) a 1500 kg car at 3 m s-2? F = m x a = 1500 x 3 = 4500 N Answer: Slide 60: The bigger the unbalanced force the bigger the acceleration. The bigger the mass, the smaller the acceleration. Slide 61: When the mass is kept constant the acceleration is directly proportional to the force applied. When the force is kept constant the acceleration is inversely proportional to the mass of the object. Slide 62: What is POWER? Slide 63: It is not about being strong. Power is the rate of doing work. Two people do the same task. Bryce takes 30 minutes while Mariah takes 15 minutes. Who has the greatest power output? Slide 64: Who has the greatest power output? Mariah has the greater power output because she complete the same task (same amount of work) in the shortest time. Power = Work done Time taken (Watts) (Joules) (seconds) Slide 65: W P t P = W = P x t t = The 3 equations you need are: t P Power = Work done Time taken (Watts) (Joules) (seconds) W W Slide 66: A crane lifts 600 kg of concrete a height of 20 m in 15 seconds. What is the power output of the crane? (i) What is the weight of concrete? = 6000 N Weight = mass x g = 600 x 10 (ii) What is the work done by the crane? Work = F x d = 6000 x 20 = 120000 J Continue next slide Slide 67: P = W t 20 (iii) What is the power of the crane? = 120000 J = 6000 W W P t Slide 68: A Slide Height = 2 m How much work did she do to climb up to the top of the slide? Check your answer on next slide. Mass of girl = 20 kg Slide 69: A Slide Height = 2 m Mass of girl = 20 kg In climbing to the top of the slide the girl has gained gravitational potential energy. (EP) Gain in EP = Work Done in climbing up. = 400 J Slide 70: A Slide Height = 2 m Mass of girl = 20 kg = 20 x 10 = 200N W = F x d = 200 x 2 = 400J Her weight is the force she has to overcome to climb up. F = m x g Slide 71: A Slide Height = 2 m Mass of girl = 20 kg gravitational potential energy. (EP) EP = m x g x h =20 x 10 x 2 = 400 J Alternate way to calculate EP (Same answer as before)

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