FORCES AND MOTION Forces are what make things move, like a push (kicking a football), or a pull (train) THERE ARE DIFFERENT TYPES OF FORCES..... Some of the forces are more obvious than others and they can be broken down in to two different types: NON-CONTACT FORCES GRAVITY - Gravity is a force that pulls objects and people towards the Earth. It pushes things down wherever they are, whatever their size. Gravity is the force that gives us weight. MAGNETIC FORCE - We all have played with magnets and discovered the invisible force that can either attract or repel another magnet or object made of iron or steel. It is the Earth’s magnetic field that we track on a compass. ELECTRICAL FORCES - Static electricity is an example of an electrical force. You will all have noticed that when comb your hair repeatedly that the comb attracts stray strands of hair. Static electricity can cause sparks and crackles if you take off a jumper made from man-made fibres (find out what they are). [1] FORCES AND MOTION CONTACT FORCES FRICTION - This is the force that resists things rubbing against each other. It helps you walk without sliding - except when you walk on the ice, there is very little friction and therefore you slip. AIR RESISTANCE - This force slows things down when they are moving. Have you ever put your hand out of a car window when you are travelling along? If so you will have felt air resistance PULL (TENSION) - Have you ever had to compete in a tug-of-war competition or had to drag a heavy object? Pulling is a force that we come across every day PUSH (APPLIED FORCE) - Again, this is a force that we are all familiar with. Whether you are pushing a bike up a hill or simply shoving a plate across the table - you should be able to think of lots of examples. TWIST (SPRING FORCE) - Whether you are screwing the top off a fizzy drink or turning a tap on to brush your teeth, spring forces are ones that you will be familiar with on a daily basis. UPTHRUST - This is the force that pushes things up in the water. It helps boats float and keeps swimmers from sinking. (Who am I?) MASS AND WEIGHT MASS WEIGHT MASS AND WEIGHT The mass of an object is the amount of matter that is in the object, so mass is related to how much stuff there is. The weight of an object is the FORCE OF GRAVITY that acts on that object, so weight is related to the pull of the Earth. The mass of an object (measured in Kg) is the same no matter where it islocated in the universe. The weight of an object (measured in Newtons) will vary depending where in the universe it is located. [2] BALANCED OR UNBALANCED Steady Speed BALANCED AND UNBALANCED FORCES Forces can be described as being balanced or unbalanced. If the forces on an object are balanced: • an object that is not moving will remain still or • an object that is moving continues to move at the same speed and in the same direction If the forces on an object are unbalanced: • an object that is not moving starts to move or • an object that is already moving changes speed or direction UNBALANCED FORCES CAUSE CHANGES IN SPEED OR DIRECTION Activity: Have a look at the images in the panel down the side of the page. Name the forces illustrated in each image and decide whether the forces are either balanced or unbalanced. MEASURING FORCES Force is measured in Newtons. A force of one Newton will give a mass of one kilogram, and an acceleration of one metre per second. This means that each second, its speed will increase by one metre per second. When measuring a force, you must state its direction, otherwise you do not know what effect it has. We can show the forces acting on an object using a force diagram. In a force diagram, each force is shown as a force arrow. The arrow shows the size of the force (the longer the arrow the bigger the force) and also the direction in which the force acts. What is the resultant force? Standing Still [3] FORCES AND MOTION ACTIVITIES If you drop an object it will fall to the floor due to the force of gravity. If you place the same object on a a table it will not fall because the table produces an equal force upwards. However, if WRITE THE NAME OF THE FORCES IN THE BOXES: you place the object on a weak paper support, the support crumples and the object will fall to the floor because the force created by the paper platform is not as strong as the force of gravity. So, when the object is on the table, the forces are balanced and when the object is on the paper platform the forces are unbalanced (therefore the object moves/changes speed in the direction of the bigger force). Activity: Push a model car along a table, so that it slows down and eventually stops. What forces are acting on the car as it moves and stops? What are the opposing forces acting on the model car? When are the forces balanced and unbalanced? By reducing air resistance and friction forces that act on a car (i.e. stream lining), what effects will these changes have on the speed, the amount of force required to accelerate the car, and fuel consumption of the car? Design activity: Cars which create less friction use less petrol and are therefore less polluting. Design and draw a car, labeling and describing features you have changed to reduce air resistance. DRAW AND LABEL YOUR OWN ARROWS: FORCES IN ACTION Look at the images in the 4 boxes at the side of the page and label them appropriately. Draw your own pictures in these two boxes showing examples of the following forces: • Gravity • Air Resistance • Pull • Upthrust • Friction • Push SPEED Speed is calculated by the number of metres travelled per second. Speed (metres per second) = distance (metres) x time (seconds) 1. Calculate the speed of a car which travels 600m in 30 seconds 2. Calculate the distance travelled by a horse which runs 1 Kilometre in 4 minutes 3. Calculate the time it takes a man running 5m/s to run 450 m [4] SECOND LAW NEWTON’S LAWS OF MOTION THIRD LAW FIRST LAW History Apple Incident Isaac Newton (1642-1727) was an English physicist, It is said that Newton was inspired to write his theory of gravitation mathematician and astronomer who many people consider to be the after an apple fell from a tree and hit him on the head. This incident greatest scientist of all time. In his book, Principia, published in 1687, made him think about why the apple fell to Earth rather than fly off in he described his 3 laws of motion which have dominated scientific another direction. He concluded that: study for over 300 years. English poet Alexander Pope wrote ‘the earth draws it...’ and that ‘...the sum of the drawing a very famous epitaph for Newton after his death: power must be in the Earth’s centre’. “Nature and nature’s laws hid in night; God said ‘let 3 Laws of Motion Newton be’ and all was light.” FIRST LAW OF MOTION Einstein kept a picture of Newton in his study. An object continues in its state of rest or uniform Gravity on the Moon motion in a straight line unless acted upon by a net Gravity is the force that gives us weight. Everything external force SECOND LAW OF MOTION contains a certain amount of material called its mass. When you measure your weight you are really measuring the force of the When a net (or unbalanced) force acts on a body, the body is Earths gravity pulling on your mass. On the Moon your weight would be accelerated in the direction of the force about one-sixth of your weight on Earth because the Moon’s gravity is THIRD LAW OF MOTION about one-sixth of the Earth’s. For every action there is an equal and opposite reaction FIRST LAW OF MOTION GRAVITY TERMINAL VELOCITY This law is also known as the law of inertia. If forces acting on an object are balanced, then: - if it is still, it will stay still - if it is moving, it keeps moving at a constant speed and in a straight line When something falls through the air it feels friction due to air resistance. Because of this, the object will reach a terminal velocity (maximum speed). This is the velocity at which the air resistance balances its weight and so it doesn’t speed up but falls at a constant velocity. [5] 30 MINUTE STEM ACTIVITY WHY NOT TRY THIS FUN ROCKET EXPERIMENT IN YOUR SCIENCE LESSON OR STEM CLUB. Most rockets create thrust, the force that propels them forward, by an exothermic chemical reaction. Make sure you launch this rocket outside and wear safety goggles. It’s very messy and can fly a long way! TRY THIS AT SCHOOL WHAT YOU NEED: . One plastic specimen tube with a tight push fit lid . One soluble ‘Alka-Seltzer’ CAUTION . Water If it does not launch, wait at least 1 minute before examining your rocket. Often the lid might not be on tight enough and the CO2 gas has escaped. . Safety Goggles . Test tube rack HOW DOES IT WORK? The water dissolves the tablet and starts a chemical reaction which produces carbon dioxide. The carbon dioxide starts to build up, which increases the pressure inside the tube, until eventually the cap is forced off and the rocket blasts up. This principle of thrust is exactly how real rockets work, only they use rocket fuel instead of AlkaSeltzer tablets. WHAT TO DO: Set up the equipment outdoors. Wear safety goggles. 1) Break the Alka-Seltzer tablet into 8 pieces 2) Put one piece of the tablet into the plastic test tube FURTHER INVESTIGATION 3) Add 1 or 2 teaspoons of water (5-10ml) How do the following changes affect the time it takes the rocket to fire and the amount of thrust created? DO THE NEXT STEP VERY QUICKLY 4) Put the lid on the test tube Try using a greater and smaller amount of tablet. 5) Put the test tube into the test tube rack upside down(Lid on the bottom) Try breaking the tablet into smaller pieces to increase the surface area. 6) Stand back 5 METRES. Try using baking powder and vinegar rather than water and Alka- Seltzer. About 10 seconds later you will hear a large pop and your tube will launch in the air (up to 10 metres) See if you can get the rocket to go higher by adding fins to the bottom of the tube to increase stability. [6] HOW DOES A JET ENGINE WORK? Remember Newton’s Third Law of Motion? Jet engines create tremendous thrust and can propel aeroplanes with such force that they can achieve speeds in excess of 1000 MPH Jet engines (or gas turbine as they are sometimes called) all work in the same way. The engine sucks air into the front with a fan. A compressor raises the pressure and temperature of the air. The compressed air is then sprayed with fuel and an electric spark lights the mixture. The burning gases expand and blast out through the nozzle at the back of the engine, thrusting the aircraft forward. Front of a turbofan engine Not all of the air sucked in at the front of the engine goes through the engine. Some of the air flows around the core. This ‘cold’ air passes through a low pressure compressor and is mixed with the gas generator exhaust to produce a ‘hot’ jet. The objective of this sort of bypass system is to increase thrust without increasing fuel consumption. Another benefit is that it produces a much quieter jet engine. PARTS OF A TURBOFAN JET ENGINE FAN - The large fan at the front of the engine sucks in large amounts of air. The air is speeded up and split into 2 parts. One part goes through the core or centre of the engine. The second part goes through ducts which surround the core, to the back of the engine where it produces much of the force that propels the aircraft forward. This cooler air quietens the engine as well as add thrust. COMPRESSOR - This is the first part of the engine core. The fans in the compressor squeeze the air into smaller areas, resulting in an increase in the air pressure. This squashed air is forced into the combustion chamber. COMBUSTOR - The squashed air is mixed with fuel then ignited. As many as 20 nozzles spray fuel into the airstream. As the mixture catches fire, this produces a high temperature, high energy airflow. The heat can reach 2700 degrees TURBINE - The high energy airflow coming out of the combustor goes into the turbine which causes the turbine blades to rotate. These blades are linked to a shaft which drives both the fan at the front of the engine and the compressor immediately behind it. MIXER - This is where the ‘cold’ air that has flowed around the outside of the core and the ‘hot’ air from inside the core are mixed together before being ducted to the nozzle. NOZZLE - The nozzle is the exhaust duct of the engine. This is the part of the engine where the thrust is created. The combination of the ‘hot’ and ‘cold’ air are expelled and produce an exhaust, which causes forward thrust.
