Speed of an object is a measure of how fast it's moving. Speed is measured in:
- Metres per second (m/s)
- Kilometres per second (km/s)
- Miles per hour (mph)
You can work out the speed if you know the distance it travels and if you know the time taken to travel that distance.
The formula to work out Speed is: Speed = Distance travelled ÷ Time taken
Average Speed = Initial Speed + Final Speed ÷ 2 (You use this equation is the speed is chaning from one speed to another)
Speed cameras take two pictures of the vehicle 0.2 seconds apart which helps get the distance and time which is then calculated to work out the speed.
Speed : Distance - Time Graphs
The steeper the gradient in a distance - time graph the greater the speed. If the line is horizontial on the graph it means the object is stationary
The accerleration or deceleration of an object is the change in speed per second.
Accerleration is only measured in m/s2
To work out the accerleration you need to know:
- The change in speed
- The time taken for the change in speed.
To calculate the acceleration or deceleration you use this formula:
Acceleration or Deceleratopm = Change in speed ÷ Time taken for change
Example of relative speed is if two cyclist are moving towards each other at the speeds of 4m/s. The first cyclist sees the other cyclist moving towards him. He appears to be moving towards him at speed 8m/s/
Force and Motion
The realtionship between force, mass and acceleration is: Resultant force = Mass x Acceleration.
Stopping distance = Thinking distance + Braking distance
Thinking distance: Distance travelled by the vehicle from the point where the driver realises they need to break to when they apply the brakes.
Braking distance: The distance it takes the vehicle to stop once they have applied the brakes.
Factors that affect Thinking Distance:
- Driving vehicle fast
- If the driver is ill, tired or under the influence of alcohol or drugs
- If driver is distracted or isn't concentrating
Force and Motion (Continued)
The factors that affect Braking distance:
- The vehicle is travelling faster
- If the weather is poor / bad road conditions
- If the vehicle is in a poor condition, e.g. worn brakes or under-inflated tyres
Road safety regulations
- Obey speed limits
- Keep distance from car in front
- Allow extra room between cars in bad weather or poor road conditions
Braking distance of a vehicle is increased if:
- Mass of vehicle is increased - has greater kinetic energy
- Friction between tyres and road is decreased
- The braking force applied is decreased
Double speed = Double the distance. Dould Speed = Quadruple the braking distance
Work and Power
Weight = Mass x Gravitational Field (N/Kg)
Works is done whenever a force moves an object. Energy is transferred to the object, these are examples:
Work done is equal to the energy transferred.
The amount of work done depends on the:
- Size of the force
- Distance the object is moved
Work done = Force applied x Distance moved in direction
Work and Power (Continued)
Power is the meaure of how quickly work is done.
Some cars have higher power ratings than others and may use more fuel.
The equation for power is: Power = Work done ÷ Time
However is speed of an object is known power can be calculated from:
Power = Force x Velocity
Energy on the Move
Kinetic energy is the energy an object has because of its movement. Kinetic energy depends on it's mass and its speed.
If a car moves with a greater speed and greater mass it has more kinetic energy.
Formula for kinetic energy is: Kinetic energy = 1/2 x Mass x Speed (squared)
Car fuel consumption depends on the:
- Energy required to increase the kinetic energy
- Energy required to work against friction
- Driving style, speed and road conditions
Frictional forces such as drag, friction and air resistance are reduced by:
- Changing the shape of the object
Greater drag can lead to energy loss and inefficiency by vehicles
- Seatbelts- prevent people being propelled forward
- Air bags - cushion the impact for the driver
- Crumple Zone - part of car designed to crumple during collision, They increase the time taken to change the momentum in a crash, which reduces the force involved.
- Collapsible steering column - absorbs energy and breaks.
These features change shape during impact to absorb energy.
Safety cage is a metal cage which strengthens. Prevents vehicle
from collapsing when upside down or rolling however it doesn't absorb energy.
ABS - prevents tyres from skidding. Pump brakes on and off automatically. Increases area of the tyres that is in contact with the road.
Traction Control - prevents car from skidding when accelerating
Crumple Zone (Continued)
During a collision a quantity called momentum is conserved.
When a car slows down during impact its momentum decreases this leads to whiplash.
Momentum = Mass x Velocity
The quicker the change in momentum the greater the force experienced.
Force = change in momentum / time taken
Stopping forces experienced in a collision can be reduced by:
Increasing the stopping or collision time
Increasing the stopping or collision distance
- Before the parachute opens - when the skydiver is in free-fall
- After the parachute opens - when air resistance is greatly increased
1. When the skydiver jumps he accelertaes due to force of gravity
2. As he falls he feels frictional force of air resistance. Weight is greaters than air resistance so he contiues to accelerates.
3. As speed increases so does air resistance
4. Air Resistance increases until it's equal to Weight. His falling speed becomes constant as focres are balanced. He has reached terminal speed
After parachute opens
5. Air resistance is bigger than Weight
6. The increase of air resistance decreases his speed.
7. Resistance decreases til equal with weight. Falls ata steady speed. New Terminal speed
Falling Safely (continued)
At higher speeds, falling objects experience more drag. If you increase the area of the object you increase the drag.
When an object is at terminal speed the speed isn't changing so the kinetic energy doesn't increase.
The GPE decreases as the object does work against friction.
Weight is due to the force of gravity on an object. The mass of an object is the amount of matter that it contains.
Weight = mass x gravitational field strength
Weight = mass x Acceleration of free fall.
The energy of Games and Theme Rides
Gravitational Pontential Energy = Mass x Gravitaional Field strenght x height
KE = 1/2 x mass x acceleration