Gcse Forces And Motion


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Gcse Forces And Motion

  1. 1. 06/06/09 Forces and Motion
  2. 2. Moments 06/06/09 A moment is a “turning force”, e.g. trying to open or close a door or using a spanner. The size of the moment is given by: Moment (in Nm) = force (in N) x distance from pivot (in m) Calculate the following turning moments: 100 Newtons 5 metres 200 Newtons 2 metres
  3. 3. Balancing moments 06/06/09 The anti-clockwise moment is bigger so the seesaw will turn anti-clockwise 100 Newtons 200 Newtons 2 metres 2 metres Total ANTI-CLOCKWISE turning moment = 200x2 = 400Nm Total CLOCKWISE turning moment = 100x2 = 200Nm
  4. 4. Balanced or unbalanced? 06/06/09
  5. 5. Stability 06/06/09 1. Centre of mass is within the wheelbase – no problem! 2. Centre of mass is directly above the edge of the wheelbase –car is on the point of toppling 3. Car falls over
  6. 6. Centripetal force 06/06/09 Consider a ball of Pleistocene attached to some string: The ball is kept in its path by the tension in the string – an example of a CENTRIPETAL FORCE. This force also produces the change in velocity due to the direction constantly changing. This force is INCREASED if you increase the mass of the object, its speed or decrease the radius of the circle. Other examples of centripetal forces: Orbits Electrons
  7. 7. Momentum 06/06/09 Any object that has both mass and velocity has MOMENTUM. Momentum (symbol “p”) is simply given by the formula: <ul><li>What is the momentum of the following? </li></ul><ul><li>A 1kg football travelling at 10m/s </li></ul><ul><li>A 1000kg Ford Capri travelling at 30m/s </li></ul><ul><li>A 20g pen being thrown across the room at 5m/s </li></ul><ul><li>A 70kg bungi-jumper falling at 40m/s </li></ul>Momentum = Mass x Velocity (in kgm/s) (in kg) (in m/s) P V M
  8. 8. Conservation of Momentum 06/06/09 In any collision or explosion momentum is conserved (provided that there are no external forces have an effect). Example question: Two cars are racing around Teville Gate. Car A collides with the back of car B and the cars stick together. What speed do they move at after the collision? Mass = 1000kg Mass = 800kg Momentum before = momentum after… … so 1000 x 50 + 800 x 20 = 1800 x V… … V = 36.7m/s Mass = 1800kg Speed = 50m/s Speed = 20m/s Speed = ??m/s
  9. 9. More questions… 06/06/09 <ul><li>A white snooker ball moving at 5m/s strikes a red ball and pots it. Both balls have the same mass. If the white ball continued in the same direction at 2m/s what was the velocity of the red ball? </li></ul><ul><li>A car of mass 1000kg heading up the M1 at 50m/s collides with a stationary truck of mass 8000kg and sticks to it. What velocity does the wreckage move forward at? </li></ul><ul><li>A defender running away from a goalkeeper at 5m/s is hit in the back of his head by the goal kick. The ball stops dead and the player’s speed increases to 5.5m/s. If the ball had a mass of 500g and the player had a mass of 70kg how fast was the ball moving? </li></ul><ul><li>A gun has a recoil speed of 2m/s when firing. If the gun has a mass of 2kg and the bullet has a mass of 10g what speed does the bullet come out at? (Hint: total momentum before is zero and the gun has a negative velocity afterwards) </li></ul>
  10. 10. A past exam question… 06/06/09 Two lorries are travelling in the same direction along a motorway. <ul><li>Calculate the momentum of Lorry A as it travels along the motorway. </li></ul><ul><li>Calculate the momentum of Lorry B as it travels along the motorway. </li></ul><ul><ul><ul><ul><li>(3 marks) </li></ul></ul></ul></ul><ul><li>Lorry B collides with Lorry A and they stick together. Calculate the common speed of the lorries immediately after the collision. </li></ul><ul><ul><ul><ul><ul><li>(3 marks) </li></ul></ul></ul></ul></ul>June 2000 Lorry A Mass = 20,000kg Speed = 14m/s Lorry B Mass = 30,000kg Speed = 20m/s
  11. 11. Newton’s Laws of Motion 06/06/09 1) If an unbalanced force acts on an object that object will either accelerate or change direction: 2) That force is given by F=ma 3) When a force acts on an object there is an equal force acting in the opposite direction (“Action and reaction are equal and opposite”) These are my three laws of motion (summarised): F A M
  12. 12. Newton’s 2 nd Law 06/06/09 Instead of F=ma Newton actually said that the force acting on an object is that object’s rate of change of momentum. In other words… <ul><li>For example, David Beckham takes a free kick by kicking a stationary football with a force of 40N. If the ball has a mass of 0.5kg and his foot is in contact with the ball for 0.1s calculate: </li></ul><ul><li>The change in momentum of the ball, </li></ul><ul><li>The speed the ball moves away with </li></ul> mv T F Force = Change in momentum Time (in N) (in kgm/s) (in s)
  13. 13. Example questions 06/06/09 <ul><li>A golfer strikes a golf ball with a force of 80N. If the ball has a mass of 200g and the club is in contact with it for 0.2s calculate a) the change in momentum of the golf ball, b) its speed. </li></ul><ul><li>A tennis player strikes a serve with a force of 30N. If the ball has a mass of 250g and the racket is in contact with it for 0.15s calculate the ball’s change in momentum and its speed. </li></ul><ul><li>A footballer takes a goal kick by kicking a 0.4kg football away at 10m/s. If his foot was in contact with the ball for 0.1 seconds calculate the force he applied to the ball. </li></ul><ul><li>A golfer strikes a 200g golf ball away at 50m/s. If he applied a force of 50N calculate how long his club was in contact with the ball for. </li></ul>
  14. 14. Energy loss in collisions 06/06/09 We’ve also said that in a collision momentum is conserved (unless an external force acts). The same cannot usually be said for kinetic energy… For example, consider the following collision. How much kinetic energy is lost? Kinetic energy = ½ x mass x velocity squared in J in kg in m/s In the “Forces” module we looked at how to calculate an object’s kinetic energy: Before After Mass = 1000kg Mass = 800kg Speed = 50m/s Speed = 20m/s Mass = 1000kg Mass = 800kg Speed = 20m/s Speed = 30m/s
  15. 15. Energy loss in collisions 06/06/09 Consider a head-on collision where the cars stick together. How much kinetic energy is lost in this example? Where does all the energy go? In this example more kinetic energy was lost. We say it was a “less elastic collision”. An “elastic collision” is one where the kinetic energy is conserved. Before After Speed = 50m/s Speed = 30m/s Speed = 10m/s
  16. 16. 06/06/09