Speed (pg 1-5)

• Difference between speed and velocity:

  • Speed is how fast you’re going with no regard to the direction.
  • Velocity is how fast you’re going with the direction specified.
• 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.
• Average speed = distance moved / time taken
• Formula for average speed:$v=$$s/t$
• Acceleration is how quickly the velocity is changing. This can be a change in speed, a change in direction or both
• The unit of acceleration is $m/s²$
• Formula for acceleration:

  
  

  Acceleration = Change in velocity/ Time taken
• Formula for acceleration:$a=$$(v-u)/t$
• Formula for velocity (which includes distance):$v²=$$u²+$$2as$
• Distance-time graphs tell you how far something has travelled
• Characteristics of a distance-time graph:

  • The gradient 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)
• Formula for gradient: $gradient=$$rise/run$
• Velocity-time graphs can have a positive or negative gradient
• Characteristics of a velocity-time graph:

  • gradient = acceleration
  • Flat sections represent steady speed
  • 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
• Gravity is the force of attraction between all masses
• Characteristics of gravity:

  • On the surface of a planet, it makes all things accelerate towards the ground (all the same acceleration). In earth, it is about 10m/s²
  • It gives everything a weight
  • It keeps planets, moons and satellites in their orbits. The orbit is a balance between the forward motion of the object and the force of gravity pulling it inwards
• Weight is a force measured in Newtons. It is measured using a spring balance or a Newton meter
• Mass is not a force. It is measured in kilograms with a mass balance
• weight = mass * gravitational field strength
• Formula which relates mass, weight and gravity: $W=$$m*$$g$
• On the moon, gravitational field strength is only about 1.6N/kg
• Reaction force - acts perpendicular to a surface and away from it (so if the surface is horizontal, the reaction force acts straight upwards)
• Electrostatic force - between two charged objects. The direction depends on the type of the charge (like charges repel, opposite charges attract)
• Thrust - e.g. a push or a pull
• Drag/ Air resistance/ Friction - slows things down: opposes motion
• Friction occurs in 3 main ways:

  • Static friction - friction between solid surfaces which are gripping
  • Sliding friction - friction between solid surfaces which are sliding past each other
  • Resistance or drag from fluids (liquids or gases)
• In a fluid, friction always increases as the speed increases