Forces Newton’s Laws


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GCSE Physics double award notes

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  • An elevator or even a rollercoaster alters this support force – the effect you feel when the elevator or coaster moves up (feel heavier) is because this force is increased, the reverse is true as well. This change in applied force is what makes it so entertaining and exciting! But when you are sitting still, or at rest, these forces are balanced. It’s difficult to grasp the idea that a chair or a table could be applying a force but this is the same as the force you would need to hold up a heavy object in your hand – there is no denying that you are definitely applying a force against its weight.
  • You would expect the keeper to apply a push force when he kicks the ball that will change the direction and speed of the ball, but no extra force is applied. No net force is acting on the ball so it moves with constant speed in a straight line. Yes, there is some tiny frictional force and air resistance but on a wet flat surface we can assume it makes little difference.
  • Futurama episode when Bender takes a nap in the missile launch tube and ends up getting fired into space while the ship is already going at top speed. Bender can’t slow down, he keeps drifting at that constant speed in a straight line. Out in space what forces would be acting on Bender as he moves further and further away from the space ship? None, there is no resultant or net force yet he keeps moving. The design of cars need to consider streamline, aerodynamics to reduce the air resistance so they can get to a top space with less force from the engine, once the forces are balanced the car can’t go any faster.
  • Vancouver Winter Olympics 2010, Men’s ice hockey final – Canada 3, USA 2 after overtime. Sidney Crosby of the Pittsburgh Penguins scores to claim gold for the Canadians The Physics of the slapshot – The link between force and velocity, what were the 4 effects of a force? Change speed, direction, shape or start/stop motion. For the puck during a slapshot, which of these would you consider the force is causing to happen?
  • The Father of Classical Physics - Sir Isaac Newton FRS (4 January 1643 – 31 March 1727 [ OS : 25 December 1642 – 20 March 1726]) [1] was an English physicist , mathematician , astronomer , natural philosopher , alchemist , and theologian who is considered by many scholars and members of the general public to be one of the most influential men in human history .
  • Air track with light gate sensors and pulley (force of tension in the string = the weight, friction at a minimum) Diagram Results – Force 0.2 N, increase by 0.2 N up to 1 N Change in Velocity, Acceleration, average of three, m/s^2 The same two heading but for an object half the mass.
  • Video on left, click to begin
  • Handout – graph and results table Each reading is 0.2 N increase Draw a line of best fit for the middle acceleration line.
  • The shape of the graph indicates there is a relationship between the two variables. For a straight line graph this is a directly proportional relationship. There is a factor that links the two variables which is represented by the gradient of the graph The factor that links the resultant force acting on an object and the acceleration of the object is its mass.
  • Each reading is 0.2 N increase Calculate the gradient
  • Newton’s second Law shows the relationship between the size of the force applied, the mass of the object and the rate of change of velocity the object experiences. If the mass stays the same and the force is increased, a bigger acceleration occurs (to balance out the equation) If the mass increases and the force stays the same then the acceleration is smaller
  • Handout with Newton’s 2 nd law and these next 2 questions Negative acceleration and two forces acting
  • Draw out a force diagram displaying the conditions
  • Again a force diagram helps to visualise what is happening, extra force included
  • If an object is moving at a constant speed its motion can still be described as accelerating as long as it moves in a circle. The speed is constant but the direction of motion is always changing . How does this stay constant with Newton’s 1 st Law? Acceleration is the rate of change of velocity, a vector, if the direction is changing then the velocity must also be changing. There must then be an unbalanced force causing this change in motion Demo – bucket swung on the end of string. (The few times where a Physics teacher has the chance to show-off!) To keep the bucket moving in a circle the string has to be continually ‘pulled’ in. The force, or tension in the string causes the acceleration of the bucket. Fill the bucket up with water, move from a vertical swing to a horizontal swing. For the water inside the bucket not to move relative to the movement of the bucket, for there to be no change in its relative motion, what must the condition of the forces be on the water? Balanced, the water pushes out against the bucket to counter the reaction force caused by the tension and the bottom of the bucket. Similar to the
  • The excitement and thrill of the Round-up, fairground ride where centripetal force is in action! Sometimes called 'Meteorite' or 'Meteor‘ as a kid this was the first ride that I was instantly hooked on, I found it amazing how you could ‘stick’ to the walls, lifting your arms or legs against the force was quite an effort!
  • How will the force needed to keep an object moving in a circle vary when… object becomes more massive (the bucket on the end of the rope filled with concrete rather than water)? circular path has a smaller radius (a car trying to turn a corner tighter)? speed of the object is increased (the Moon’s motion around the Earth increases, changing direction faster, larger acceleration)?
  • All linked into F = ma, a bigger force is needed to accelerate the object faster
  • Click on the green or red boxes to see the different conditions
  • Forces Newton’s Laws

    1. 1. Forces- Newton’s Laws GCSE Year 11
    2. 2. Learning Intentions <ul><li>By the end of the lesson we will be able to… </li></ul><ul><li>Identify and label forces acting on an object </li></ul><ul><li>Recall the effect of applying balanced forces </li></ul><ul><li>Recall the effect of applying unbalanced forces </li></ul>
    3. 3. The Effect of Forces <ul><li>The effect of a force is only shown when there is an ‘unbalanced force’ acting (also known as a net force or resultant force). </li></ul>
    4. 4. Balanced forces <ul><li>If there is no net force then the forces on an object are said to be balanced </li></ul><ul><li>Ricky’s force due to weight is equal to the reaction force of the chair but they act in opposite directions </li></ul>Ricky Tomlinson is at rest… Chair’s reaction force Weight
    5. 5. Frictional Force Thrust
    6. 6. Continues at constant speed in a straight line…
    7. 7. Balanced Forces and Motion <ul><li>When an object has balanced forces acting on it will </li></ul><ul><ul><li>remain at rest </li></ul></ul><ul><ul><li>or </li></ul></ul><ul><ul><li>move with constant speed in a straight line (uniform velocity / no acceleration) </li></ul></ul>
    8. 8. Learning Intentions <ul><li>By the end of the lesson we will be able to… </li></ul><ul><li>Recall that an unbalanced force acting on an object will cause its motion to change </li></ul><ul><li>State Newton’s Laws of motion </li></ul>
    9. 10. Newton and his Laws <ul><li>Sir Isaac Newton is a dead famous physicist who studied forces. </li></ul><ul><li>Newton’s First Law of motion- </li></ul><ul><li>An object will remain at rest or travel at a constant velocity unless an unbalanced force acts on it </li></ul><ul><li>(Constant velocity means the object will not change its speed or direction ) </li></ul>Page 35
    10. 11. Investigation <ul><li>How does the change in velocity of an object vary as the unbalanced force is increased? </li></ul>
    11. 15. <ul><li>Acceleration is directly proportional to the force </li></ul>Interpreting the Graphs acceleration Force
    12. 16. Graph and Results Table for Girder One Calculate the Gradient of the middle line using m = (y2-y1) / (x2-x1) and two points on the line of best fit.
    13. 17. Learning Intentions <ul><li>By the end of the lesson we will be able to… </li></ul><ul><li>State Newton’s 2 nd Law of motion and use it to solve simple problems </li></ul><ul><li>Recognise the connection between circular motion , acceleration and an unbalanced force </li></ul>
    14. 18. Newton’s Second Law of Motion <ul><li>The size of the acceleration due to an unbalanced force acting on an object can be calculated using the formula- </li></ul><ul><li>Unbalanced Force = Mass x Acceleration </li></ul><ul><li>F = m x a </li></ul><ul><li>Newton = kg x m/s 2 </li></ul>Acceleration and change in velocity have similar meaning (increase in speed, decrease in speed or change of direction) F a m
    15. 19. <ul><li>Unbalanced Force = Mass x Acceleration </li></ul><ul><li>F = m x a </li></ul><ul><li>Newton = kg x m/s 2 </li></ul>Newton’s 2 nd Law F a m
    16. 20. Newton’s 2 nd Law- Example Question <ul><li>What is the force needed to make a rock of 2 kg accelerate at 0.25 m/s 2 ? What is the acceleration of the rock when an unbalanced force of 100 N is applied to it? </li></ul>F = m x a 100 = 2 x a 100 / 2 = a 50 m/s 2 = a F = m x a = 2 x 0.25 = 0.5 N
    17. 21. Your Turn… <ul><li>Try question 1 and 4 page 37 </li></ul>
    18. 22. Bit more complicated… <ul><li>Darren Clarke strikes a ball on a level putting green. </li></ul><ul><li>Name the force opposing the motion of the ball across the green </li></ul><ul><li>If the ball has a mass of 0.05 kg then what is the value of the opposing force causing a deceleration of 0.3 m/s 2 ? </li></ul>
    19. 23. The Answer <ul><li>FRICTION </li></ul><ul><li>F = m x a </li></ul><ul><li>F = 0.05 kg x (-0.3) m/s 2 </li></ul><ul><li>F = -0.015 N (acting against the motion of the ball) </li></ul>0.015 N Motion of the ball Causing the ball to slow down
    20. 24. Sinks it…
    21. 25. One step further… <ul><li>Padraig Harrington has an identical shot the same distance from the pin (hole). </li></ul><ul><li>A wind has started to blow from behind Padraig with a constant force of 0.01 N. If he plays the same shot as Darren, what will the acceleration of the ball be? </li></ul>
    22. 26. The Answer <ul><li>Forces Acting on the ball </li></ul><ul><li>Unbalanced force </li></ul><ul><li>0.01 N - 0.015 N = -0.005 N </li></ul><ul><li>This force causes a deceleration </li></ul><ul><li>F = m x a </li></ul><ul><li>- 0.005 = 0.05 x a </li></ul><ul><li>a = (-0.005 / 0.05) = - 0.1 m/s 2 </li></ul>0.015 N 0.01 N Motion of the ball The ball will not slow down as quickly
    23. 27. Missed it…
    24. 28. Circular Motion
    25. 30. Centripetal Force <ul><li>The unbalanced force is provided by a force acting towards the centre of the circle called the centripetal force . </li></ul><ul><li>An unbalanced force causes a change in motion , an acceleration (speed steady but direction always changing) </li></ul>F v
    26. 31. Bucket on a rope Electron around a nucleus Planetary Motion Car turning a corner Tension Electrostatic Gravity Friction Add to the diagrams the direction and name of the force causing the object’s motion +ve
    27. 32. Circular Motion Facts <ul><li>The smaller/greater the mass, the bigger the force needed for circular motion </li></ul><ul><li>The smaller/greater the radius of the circle, the bigger the force needed for circular motion </li></ul><ul><li>The slower/faster the speed of the object, the bigger the force needed for circular motion </li></ul>
    28. 33. Gravity On Gravity Off I’m so lonely right now…
    29. 34. No resultant force… <ul><li>When the centripetal force is removed the object will continue to move along the tangent to the circle </li></ul>Earth Moon If gravity was removed the moon’s motion would be in a straight line as shown
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