Motion

• Force needed to keep the person in photo A in the air


  Depends on their weight
• Weight


  A force that acts towards the centre of the Earth
• Forces


  Have both a magnitude (size) and a direction, measured in newtons (N)
• Vector quantities


  Quantities that have both size and direction, like forces
• Forces on diagrams


  Shown using arrows, with longer arrows representing larger forces
• Mass


  Measures the amount of matter in something and does not have a direction
• Scalar quantities


  Quantities that do not have a direction, like distance, speed, energy and time
• Displacement


  The distance covered in a straight line, and has a direction
• The displacement at the end of a journey is usually less than the distance travelled because of the turns or bends in the journey
• Other vector quantities


  • Acceleration - a measure of how fast velocity is changing
  • Momentum - a combination of mass and velocity
• Weight and upthrust are both vector quantities
• Displacement is not a force
• Speed


  How far an object moves in a certain time
• Velocity


  Speed in a particular direction
• Average speed


  Calculated from the total distance travelled and the total time taken
• Instantaneous speed


  The speed at a particular point in a journey
• Speed equation


  Speed (m/s) = distance (m) / time taken (s)
• To measure speed in the laboratory you need to measure a distance and a time. For fast-moving objects, using light gates to measure time will be more accurate than using a stopwatch
• Distance/time graph


  Horizontal lines mean the object is stationary, straight sloping lines mean the object is travelling at constant speed, the steeper the line the faster the object is travelling, the speed is calculated from the gradient of the line
• Gradient of a distance/time graph

  Gives the speed
• Make sure you take the starting value away from the end value each time when calculating speed from a distance/time graph
• A peregrine falcon can dive at over 300 km/h (over 80 m/s)
• Zahir starts at the park, gets a snack and has to run to catch a bus. When he starts running, he goes more slowly and then speeds up before the bus stop
• Zahir's speeds are 3 m/s for 60 seconds, 2 m/s for 90 seconds, and 5 m/s for 30 seconds
• Acceleration


  A change in velocity, a vector quantity with a size and direction, measured in m/s^2
• A fighter plane can accelerate from 0 to 80 m/s (180 mph) in 2 seconds
• Acceleration equation


  Acceleration (m/s^2) = change in velocity (m/s) / time taken (s)
• Acceleration can also cause an object to get slower, which is called deceleration and has a negative value
• Acceleration equation relating initial velocity, final velocity and distance

  v^2 = u^2 + 2ax, where v is final velocity, u is initial velocity, a is acceleration, and x is distance
• Acceleration due to gravity is 9.8 m/s^2, often rounded to 10 m/s^2
• The acceleration on an ejecting pilot is much greater than everyday accelerations
• A car travelling at 30 m/s comes to a halt in 5 seconds. What is the car's acceleration and how far does it cover while stopping?
• A train travelling at 40 m/s slows down with an acceleration of -0.5 m/s^2
• Velocity/time graph


  A horizontal line means constant velocity, a sloping line shows acceleration (steeper = greater acceleration), a negative velocity shows movement in the opposite direction
• The graph shows a simplified velocity/time graph for a dragster, with the car driving slowly to the start line, waiting, and then racing
• The area under a velocity/time graph is the distance the object has travelled
• The dragster travels at 5 m/s as it approaches the start line, and the distance travelled during the race and slowing down afterwards is the area under the curve
• Mel's graph should be called a speed/time graph, not a velocity/time graph, because velocity has a direction but speed does not
• In a fitness test, students have to run up and down the sports hall and estimate the total distance travelled