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Dynamics and Space Newton’s laws Learning Outcomes • Applications of Newton’s laws and balanced forces to explain constant velocity, making reference to frictional forces. • Calculations involving the relationship between unbalanced force, mass and acceleration for situations where more than one force is acting. • Calculations involving the relationship between work done, unbalanced force and distance / displacement. • Calculations involving the relationship between weight, mass and gravitational field strength during interplanetary rocket flight. • Newton’s second law and it’s application to space travel including rocket launch and landing. • Newton’s third law and its application to explain motion resulting from a ‘reaction force’. • Use of Newton’s laws to explain free-fall and terminal velocity. Lesson 1 • Define the term friction and give examples of how to increase and decrease frictional forces. The Force of Friction • A force of friction is a force acting between two surfaces in contact which opposes the motion of the surfaces (i.e. tries to stop them moving). Reducing Friction • As friction always stops things moving it is important to reduce it if we want to move things. • We are now going to look at ways of reducing friction. Demonstrations Reducing Friction (copy out note or make your own mindmap) • 1. 2. 3. 4. There are four main ways in which we can reduce friction: Streamlining – changing the shape to reduce air resistance e.g. a car with a ‘flatter shape’ Rollers – adding rollers or wheels e.g. a trolley with wheels moves easier than one without. Lubrication – adding a layer of liquid or air between two surfaces e.g. oiling hinges to stop them ‘squeaking’. Smoothing – making a surface smoother e.g. adding wax to skis Increasing Friction • Friction can also be useful to us: 1. When a car applies its brakes it increases friction to slow down. 2. Parachutes are used to increase air resistance to slow things down. Highest Skydive EVER • 2nd Highest Parachute Jump Homework • Research a machine or animal that has been specially adapted to become more streamlined. • Present your findings in one of the following ways: – a short essay in your homework jotter – an information leaflet – A poster Lesson 2 • Define the term friction and give examples of how to increase and decrease frictional forces. Balanced Forces • Today we are going to look at the effect of two opposite but equal forces acting on an object. • Tug of war Balanced Forces • Balanced forces are equal forces acting in opposite directions. • They are equivalent to no force at all acting on an object. • When an object is stationary, or travelling at a constant velocity, all forces on it are balanced. • (HINT: this is often an exam question – they will tell you an object is travelling at constant velocity with a thrust force of say 100N. They will then ask you what the force of friction would be. This would be the same as the thrust, in this case 100N.) Balanced Forces: Motion • When a car is moving at constant speed, the forces acting on it are said to be balanced. • In the car below, force of engine = friction Other examples include: Plane • Note there are also balanced vertical forces, lift = weight. Other examples include: Boat • Upthrust = weight Newton’s First Law of Motion • Newton’s first law states that if an object is stationary or moving at a constant speed then the forces acting on it are balanced. Newton’s First Law in action • Newton’s first law shows us that when you move at constant speed it has the same effect as not moving at all. • You only feel a force if you are speeding up (accelerating) or slowing down (decelerating). In an airplane • As the plane takes off you are pushed back as the plane speeds up. • The forces are not balanced. In an airplane • As the plane lands you are pushed forward as the plane applies its brakes and slows down. • The forces are not balanced. In an airplane • When in mid-air, going at a constant speed and a constant altitude, without any turbulence you would feel exactly as you would if you were on the ground • The forces are balanced. You can do anything that you could do if you were on land. • Harlem Shake Frontier Flight 157 What about now? • Even now as you sit in the class (in the UK) you are actually travelling at around 1000 km/h (622 mph) as the Earth rotates on its axis. • You don’t feel this effect as it is a constant speed. • (Earth travels at around 1600 km/h at the Equator). Motion of a spaceship • The motion of a spaceship is consistent with Newton’s First Law. • Space is a vacuum so there are no frictional forces acting. • The ship continues moving at the same speed in the same direction until it comes under the gravitational influence of a planet. Homework - podcast • Using iTunes, search for the free podcasts called “stuff you should know” (from howstuffworks.com). • Find the episode from 21 Feb 2013 on ‘What would happen if the Earth stopped spinning’. • This will be discussed in a future lesson. Lesson 3 (usually two periods) • Investigate the relationship between unbalanced force, mass and acceleration. • Carry out calculations involving the relationship between unbalanced force, mass and acceleration for situations where more than one force is acting. Newton’s Second Law of Motion • Newton’s first law of motion looked at how balanced forces affected the velocity of an object. • We are now going to look at how unbalanced forces change the velocity of an object e.g. acceleration. Newton’s Second Law of Motion experiment 1 • • A light gate is used to measure acceleration of a trolley. Hanging masses are used for the unbalanced forces. • Note: the mass of the trolley is kept constant. Only the masses on the hanger are changed causing and unbalanced force (due to gravity acting on the masses). Results Force (N) 1 2 3 4 5 6 Acceleration (m/s2) Newton’s Second Law of Motion Conclusion: As the Force on a body increases so does the acceleration. In other words a~F Newton’s Second Law of Motion • We are now going to investigate the effect of changing the mass of an object being accelerated by an unbalanced force. Newton’s Second Law of Motion experiment 2 • • A light gate is used to measure acceleration of a trolley. The hanging mass is kept constant therefore there is an unbalanced force. • Note: only the mass of the trolley is changed. Results Mass (kg) 0.2 0.4 0.6 a (m/s2) Newton’s Second Law of Motion Conclusion: As the mass of a body increases the acceleration decreases. In other words a~ 1 m Defining Newton’s Second Law of Motion • Newton’s Second Law of Motion states that the acceleration of an object is directly proportional to the unbalanced force acting on it, • e.g. a ~ F • and inversely proportional to the mass of an object. • e.g. a ~ 1 m Newton’s Second Law of Motion • Combining the two proportions • a ~ F and a ~ 1 m • Gives us the following equation: a=F m • This is re-written as: Fun = ma • Also known as Newton’s second Law Problems involving several forces 1. In all situations apply: Fun = ma (work out the resultant unbalanced force) 2. Draw a sketch diagram for the whole system including masses and external forces. 3. Indicate the direction of acceleration on the object. Example 1 • What is the mass of an object if an unbalanced force of 20 N produces an acceleration of 4 m/s2? Example 2 • What is the acceleration of a 600 kg car, when the engine exerts a force of 1700 N, but the frictional force is 800 N? Example 3 • A 6 kg block is dragged along with a horizontal force of 16 N. • If the block accelerates at 2 m/s2, what is the force of friction acting on the block? Lesson 4 • Carry out calculations involving the relationship between work done, unbalanced force and distance / displacement. Work Done • Work done is a measure of the energy transferred. • It has the symbol Ew and is measured in Joules (J). • The work done to any object depends on the “size of the force” being applied and the “distance” it is being moved along. Work done = Force x distance Ew = F d Example 1 Ew = ? F = 600 N d=4m Example 2 Ew = 15 MJ F=? d = 50 km Example 3 Ew = 6000 J F = 25 N d=? 2010 2008 Qu: 23 Lesson 5 • Describe the difference between mass and weight. • Investigate the relationship between mass and weight. • State what is meant by gravitational field strength. • Carry out calculations involving the relationship between weight, mass and gravitational field strength during interplanetary rocket flight. Mass and Weight • The mass of an object is the quantity of matter it contains. • It is measured in kilograms (kg). • The weight of an object is a force caused by the pull of the Earth on an object. • It is measured in Newtons (N). What to do • Collect a 1-10N / 1-15N / 1-20N Newton balance and a set of slotted 100g masses. • Copy & complete the table below: Mass (kg) 0.2 0.4 0.6 Your mass = Weight (N) Ratio of weight mass Relationship Between Mass and Weight • We now have an equation linking weight, mass and g: • Weight = g mass • This can be re-written as: • weight = mass x g. Or in symbol form: W = m g What is meant by gravitational field strength? • The ratio of weight / mass is known as the gravitational field strength. • This is represented by the symbol ‘g’. • The value of g on Earth is usually taken as 10 N/kg. • g is also known as the ‘weight per unit mass’. Example • Copy and complete the table: Object on Earth (g=10N/kg) Brick Concrete block Bag of cement Tonne of sand Mass (kg) Weight (N) 3 100 500 1000 Gravitational Field Strength • The value of ‘g’ varies as we move from one body to another in our solar system. • The bigger the body, the bigger the value of g. • Our mass would remain the same and never change if we went to these different bodies, however, our weight does. • We can work out our weight (N) on different bodies using the equation W = mg as long as we have a value for ‘g’ for that body. Body g (N/kg) Mercury Venus Earth 4 9 10 Mars Jupiter Saturn 4 26 11 Uranus Neptune Pluto Moon 12 12 4 1.6 Sun 270 Your Weight in N (W = mg) Lesson 6 • Use Newton’s second law and apply it to space travel including rocket launch and landing. Newton’s 2nd Law (Fun = ma) and space travel • When using Fun = ma in space travel we follow the same rules as before: 1. In all situations apply: Fun = ma (work out the resultant unbalanced force) 2. Draw a sketch diagram for the whole system including masses and external forces. 3. Indicate the direction of acceleration on the object. However, we now must consider gravity and the force of weight. In other words, apply W = mg acting down on the object Example • What is the acceleration of a 700 kg helicopter, when the engine exerts a force of 9900 N vertically upwards, but the frictional force is 800 N? • Fun = Fup – Ffr – W • W = mg = 700 x 10 = 7000N • Fun = 9900 – 800 -7000 = 2000 N • a = Fun m = 2100 700 = 3 m/s2 2006 2003 Qu: 22 (1 of 2) 2003 Qu: 22 (2 of 2) Lesson 7 • State Newton’s third law. • Apply Newton’s third law to explain motion resulting from a ‘reaction force’. • Use of Newton’s laws to explain freefall and terminal velocity. Newton’s Third Law • • If object A exerts a force on object B then object B exerts an equal but opposite force on object A. In other words: ‘action and reaction are equal and opposite’. Lesson 7 Lesson 7 Balanced Forces: Parachute jump • When a skydiver jumps from a plane he will only accelerate for a short while. • The air friction rapidly increases until it equals the skydiver’s weight. • The skydiver will then fall with a uniform velocity called terminal velocity. Balanced Forces: Parachute jump • Terminal velocity is the constant velocity reached by an object once it is no longer accelerating. Sky diving • What not to do! 2004