Physics- p3_1

Physics- p3_1

1. Speed

   • Average Speed
     • The speed of an object tells you how fast or slow it is moving. You can find the average speed of an object if you know: The distance travelled The time taken to travel that distance
   • Increasing speed
     • The faster something moves, the greater the distance it covers in a given time
       • a car that is driven 100 km in two hours is going faster than one covering 60 km in two hours.
     • Increasing the speed reduces the time needed to cover a given distance
       • a car travelling at 30 m/s takes 200 s to complete a journey of 6 km, while a car travelling at 15 m/s takes 400 s to complete the same journey.
   • Distance-time graphs
     • Distance-time graphs are a useful way of describing the motion of an object. They show how the distance moved from a starting point changes over time.
       • When an object is stationary the line is horizontal (the distance does not change).
       • When moving at a steady speed it's a straight diagonal line
       • If the line slopes downwards the object is returning to the starting position
   • Non uniform motion
     • You should be able to use a distance-time graph to describe an object's motion when it's accelerating or decelerating
     • This graph shows acceleration
     • This graph shows decelration

2. Changing speed

   • Acceleration
     • In scientific terms, positive acceleration means an increase in speed and negative acceleration means a decrease in speed.
       • Acceleration depends on two things:
         • How much the speed changes
         • How much time the change in speed takes
     • Calculating acceleration
       • You can calculate acceleration using this equation: Acceleration = change in speed ÷ time taken
       • acceleration is measured in m/s2. You may find this triangle helpful when rearranging the equation.
       • A negative acceleration means that something has slowed down.
   • Speed time Graphs
     • A speed-time graph tells us how the speed of an object changes over time.
     • In a speed-time graph: A horizontal line means a constant speed A straight line with a positive gradient means a constant positive acceleration (speeding up) A straight line with a negative gradient means a constant negative acceleration (slowing down)
       • The greater the gradient of the line (the steeper it is) the greater the acceleration. In the graph, the blue line represents a greater acceleration than the red line next to it.
   • Distance Travelled
     • The area under the line on a speed-time graph represents the distance travelled.
   • Velocity
     • The velocity of an object is its speed in a particular direction
       • This means that two objects could be travelling at the same speed but have different velocities. For example, two cars are travelling at 30 m/s along the same road but in opposite directions: One of the cars has a velocity of +30 m/s The other car has a velocity of –30 m/s
     • Relative velocities
       • If two objects are moving in parallel their relative velocity can be calculated
     • Change in velocity
       • Acceleration is a change in velocity.

3. Forces and motion

   • A force acting on an object can cause it to accelerate. The relationship between force, mass and acceleration is given by the equation force = mass × acceleration.
   • The stopping distance of a car depends on two factors - thinking distance and braking distance. Under normal driving conditions, the thinking and braking distance depend on the speed of the car
   • Force, mass and acceleration
     • An object may have several different forces acting on it, which can have different strengths and directions. But they can be added together to give the resultant force. This is a single force that has the same effect on the object as all the individual forces acting together.
   • Thinking distance
     • It takes time for a driver to react to a situation and start to apply the brakes. The car carries on moving during this reaction time. The thinking distance is the distance travelled in this reaction time. The thinking distance increases if the reaction time increases
   • Braking distance
     • The braking distance is the distance taken to stop once the brakes are applied. The braking distance increases if: The car's brakes or tyres are in poor condition The road and weather conditions are poor (icy or wet roads, for example).

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