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Understanding how and why things move.
When an objects moves, we can measure its speed (or velocity), and its acceleration.
If there are forces acting on it then we can predict how this affects its motion.
Or how much energy is needed to move it.
Speed (or Velocity)
Speed (or Velocity)
Speed measures how far is travelled in a given time.
The SI unit of speed is metres per second (m/s).
When we see ‘per' it means ‘for every’ or ‘divided by’.
Speed is calculated as distance divided by time.
Speed is also called velocity, especially when it is defined with a direction.
Acceleration
Acceleration
Acceleration is an increase in speed.
Acceleration measures how fast the speed is changing.
It is calculated as the change in speed divided by time.
The SI unit of acceleration is metres per sec per sec (m/s/s or m/s2).
At 1 m/s/s, the speed increases by 1 m/s every second.
Acceleration a = (v - u)/t, where v is final speed, u is initial speed and t is time.
A negative value of acceleration means that the speed is reducing (also known as deceleration).
If we drop an object it will fall to ground with an acceleration of 9.8 m/s/s, known as g, the acceleration due to gravity.
Force
Force
We make an object accelerate by applying a force.
Force is a measure of the strength of a push or pull.
The SI unit of force is Newtons (or kg.m/s2).
Force is calculated as mass times acceleration: F = ma.
A force of 1 Newton accelerates 1 kg by 1 metre/sec every second.
So after 100 secs the speed will be 100m/s.
Energy (or Work)
Energy (or Work)
Energy measures how much work has been done, or how much can be done by stored energy.
There are many forms of energy - kinetic, potential, thermal, electrical etc.
Energy can be converted from one form to another, but is never lost.
When we apply a force to an object the work done (or energy used) depends on the distance moved: E = F.s
Also, the energy of a moving object (it's kinetic energy) is E= ½ mv2
The SI unit of energy is Joules (or kg.m2/s2).
Power
Power
Power is how much energy is used per second.
It is calculated by energy divided by time.
The SI unit of power is the Watt (W).
Questions
Questions
A car travels 100metres in 10seconds, what is it's speed?
Answer: Speed (or velocity) = distance/time = 100/10 = 10metres/sec.
The car's speed increases from 10m/s to 20m/s in 5secs, what is it's acceleration?
Answer: Acceleration = speed change/time = (20-10)/5 = 10/5 = 2metres/sec/sec.
If the car weighs 1000kg, what force is needed to give this acceleration?
Answer: Force = mass x acceleration = 1000 x 2 = 2000 Newtons.
What Energy did the car have at the start?
Answer: Energy = ½ mv2 = ½ x 1000 x 100 = 50,000 Joules (or 50kJ) at start.
What Energy does the car have at the end?
Answer: Energy = ½ mv2 = ½ x 1000 x 400 = 200,000 Joules (or 200kJ) at end.
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