Physics

• Numbers


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• Speed


  How fast you're going, with no regard to direction
• Velocity


  How fast you're going, with direction specified
• Speed and velocity are similar, but in physics they're not quite the same
• Speed and velocity both simply say how fast you're going, and both are measured in m/s (or km/h or mph)
• You can have objects travelling at a constant speed with a changing velocity
• This happens when the object is changing direction whilst staying at the same speed
• Distance, speed, time formula

  distance = speed x time
• Example using distance, speed, time formula

  • A cat skulks 20 min 35s. Find: a) its average speed. b) how long it takes to skulk 75 m.
• Acceleration


  The rate of change of velocity
• Acceleration


  • The formulas for acceleration on this page only work when the acceleration is constant (uniform)
  • The unit of acceleration is m/s², which is velocity, but m/s per second
• Distance-time graph


  Tells you how fast an object is moving and how far it's travelled
• Distance-time graph


  • The gradient (slope) at any point gives the speed of the object
  • Flat sections are where it's stopped
  • A steeper graph means it's going faster
  • Curves represent acceleration
  • A curve getting steeper means it's speeding up (increasing gradient)
  • A levelling off curve means it's slowing down (decreasing gradient)
• To calculate the speed from a distance-time graph, just work out the gradient
• You can also calculate the average speed of an object over a period of time by dividing the total distance travelled by the time it takes to travel that distance
• Make sure you're comfortable rearranging equations
• Read the axes of any graph you get given carefully
• Velocity-time graph


  Shows you how the velocity of an object changes over time
• Velocity-time graph


  • Gradient represents acceleration
  • Flat sections represent steady speed
  • Steeper graph means greater acceleration or deceleration
  • Uphill sections are acceleration
  • Downhill sections are deceleration
  • The area under any part of the graph is equal to the distance travelled in that time interval
  • A curve means changing acceleration
• The acceleration represented by the first section of the velocity-time graph is the gradient
• The speed at any point is simply found by reading the value off the velocity axis
• The distance travelled in any time interval is equal to the area under the velocity-time graph
• Mass


  The amount of stuff in an object
• Weight


  The force of gravity pulling an object towards the centre of the Earth
• Mass is the same anywhere in the universe, but weight depends on the strength of gravity
• Weight is measured in newtons using a spring balance or newton meter, while mass is measured in kilograms using a mass balance
• Weight formula


  weight = mass x gravitational field strength
• Example using weight formula

  • What is the weight, in newtons, of a 5 kg mass, both on Earth and on the Moon?
• Weight is dependent on gravity, but mass is not