Speed and Velocity
Speed and Velocity are BOTH: HOW FAST YOU'RE GOING
SPEED is how fast you're going (eg 20 mph or 10m/s) with no regard to the direction
VELOCITY must have the direction specified (eg 30 mph north or 10 m/s 60degrees) The distance in a particular direction is called the DISPLACEMENT
How do you work out velocity?
Average velocity= displacement / time
REMEMBER: S= V x T
Example: a) Find the velocity of a mouse which walks 3.2 metres in 35 seconds. b) Find how far he would go in 25 minutes
Answer: a)0.91m/s b)137m
Speed cameras measure the speed of cars
- speed camera can be used to catch speeding motorists at dangerous accident spots
- Lines are painted on the road a certain distance apart to measure the distance travelled by the car
- A photo of the car is taken as it passes the first line and a second photo is taken a certain time later
- These photos can then be used to measure the distance travelled by the car in this time
How do you work out speed?
Average speed= distance / time
Example: Two photos are taken 0.5s apart and from the marked lines on the road the distance is 5m. What is the average speed?
Answer: 5 / 0.5= 10 m/s
Acceleration and Velocity- Time Graphs
Acceleration is how quickly velocity is changing
This change in velocity can be a CHANGE IN SPEED or a CHANGE IN DIRECTION or both.
How do you work out acceleration?
Acceleration= change in velocity / time taken
(v-u) / a x t --> 'v' is the final velocity and 'u' is the initial velocity
'(v-u)' means working out the change in velocity
Example: A cat accelerates from 2m/s to 6m/s in 5.6s. What is the acceleration?
Answer: a= (v-u) / t = (6-2) / 5.6 = 4 / 5.6 = 0.71 m/s2
Velocity- Time Graphs
VERY IMPORTANT TO REMEMBER:
- Flat sections represent steady speed
- The steeper the graph, the greater the acceleration or deceleration
- Uphill sections (/) are acceleration
- Downhill sections (\) are deceleration
- The area under any section 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 a graph is:
acceleration= gradient= vertical change / horizontal change= eg. 30 / 20= 1.5m/s2
The velocity at any point is simply found by reading the value off the velocity axis
The distance travelled in any time interval is equal to t are under the graph.
Friction Forces and Terminal Velocity
Friction is always there to slow things down
- If an object has no force propelling it along, it will always slow down and stop because of friction
- Friction always acts in the opposite direction to movement
- To travel as a steady speed, the driving force needs to balance the frictional forces
- You get friction between two surfaces in contact, or when an object passes through a fluid (drag)
Drag increases as the speed increases
Resistance from fluids always increases with speed. A car has much more friction to work against when travelling at 70mph than at 30 mph. So at 70 mph the engine has to work more to maintain a steady speed
When cars and free-falling objects first set off they have much more force accelerating them than resistance slowing them down.
As the speed increases the resistance builds up
This gradually reduces the acceleration until eventually the resistance force is equal to the accelerating force and then it wont be able to accelerate any more.
It would have reached it's terminal velocity.
REMEMBER: The accelerating force acting on all falling objects is gravity and it would make them all fall at the same rate if it wasnt for air resistance.
Forces and Acceleration
Things only accelerate or change direction if you give them a push.
A balanced force means steady speed and direction
If the forces on an object are all BALANCED then it'll keep moving at the SAME SPEED in the SAME DIRECTION
- To keep going at a steady speed, there must be zero resultant force
A resultant force means acceleration
If there is an UNBALANCED FORCE then the object will ACCELERATE in the direction of the force.
- An unbalanced force will always produce acceleration or deceleration
- Acceleration can take 5 different forms: starting, stopping, speeding up, slowing down and changing direction
The overall unbalanced force if often called the resultant force
How do you work out resultant force?
F = ma
m = mass, a = acceleration, F = resultant force
- The bigger the force, the greater the acceleration/deceleration
- The bigger the mass the smaller the acceleration
- To get a big mass to accelerate as fast as a small mass, it needs a bigger force
1) What force is needed to accelerate a mass of 12kg at 5 m/s2?
Answer= F= ma = F=12 x 5 = 60N
2) The same force acts on another mass and it accelerates at 6 m/s2. What is its mass?
Answer= m= F / a = m= 60 / 6= 10kg
If object A EXERTS A FORCE on object B then object B exerts THE EXACT OPPOSITE FORCE on object A
- If you push against a wall, the wall will push back against you, just as hard
- As soon as you stop pushing, so does the wall
- There must be an opposing force when you lean on a wall- otherwise you and the wall would both fall over
- If you put a book on a table, the weight of the book acts downwards on the table- and the table exerts an equal and opposite force upwards on the book
Whenever an object is on a horizontal surface, there'll always be a REACTION FORCE pushing UPWARDS, supporting the object. The total REACTION FORCE will be EQUAL AND OPPOSITE to the weight.
The stopping distance of a car is the distance covered in the time between the driver first spotting a hazard and the car coming to a complete stop.
Many factors affect your total stopping distance:
1) Thinking distance: the distance the car travels after seeing a hazard and applying the brakes
- How FAST you are going- the faster you're going, the further you'll go
- How DOPEY you are- tiredness, drugs, alcohol, old age
- How BAD the VISIBILITY is- lashing rain, oncoming lights
2) Braking distance: the distance the car travels when it decelerates whilst the brakes are applies
- How FAST you're going- the faster you're going, the further it takes to stop
- How good your BRAKES are- worn or faulty brakes can let you down
- How HEAVILY LOADED the vehicle is- a heavily laden vehicle will take longer to stop
- How good the GRIP is- road surface, weather conditions, tyres
Momentum= Mass x Velocity
1) The greater the mass of an object and the greater its velocity, the more momentum the object has.
2) It has size and direction but not speed
Moment (kg m/s) = Mass (kg) x Velocity (m/s)
Forces cause change in momentum
- When a force acts on an object, it causes a change in momentum
- Force acting (N) = change in momentum (kg m/s) / time taken (s)
- A large force means a faster change of momentum (greater acceleration)
- If someone's momentum changes very quickly (like in a car crash), the forces on the body will be very large (and more likely to cause injury)
- This is why cars are designed to slow people down over a longer time when they have a crash- the longer the change in momentum, the smaller the force
- CRUMPLE ZONES crumple on impact, increasing the time taken for the car to stop
- SEAT BELTS stretch slightly, increasing the time taken for the wearer to stop. The reduces the forces acting on the chest
- AIR BAGS also slow you down more slowly
Car safety features save lives
Your willingness to take a risk can be influenced by man factors:
- Familiarity- people drive in cars all the time so don't feel at risk to do so
- Whether you are forced to take a risk or choose to
- If you are in control of the situation- driving a car is safer than flying cos you are in control
- Personal experience- if someone you know had a nasty bungee jumping accident, you're probably not going to be too keen to try it yourself
- Age or personality type- some people like to take risks and some dont