Speed = distance travelled / time taken
or v = d / t
Speed is usually in metres per second (m/s)
Other units include: km/h, cm/s, mph (this is not metric, exam questions will use metric units)
Average Speed = total distance travelled / time taken for journey
A speedometer of a car shows the instantaneous speed of the car because it changes from instant to instant.
Speed guns use microprocessors to produce an instant reading.
Distance-time graphs show us how the car is traveling.
If the car is travelling in equal time then it is travelling at constant speed. This is shown when the graph is a straight line.
The gradient tells us the speed!!!!
Some distance time graphs have negative gradients - distance from starting point is decreasing.
Displacement - distance travelled in a particular direction
Displacement is a vector because it has both magnitude and direction.
Velocity and Acceleration
Velocity is a vector because it it speed in a particular direction.
Velocity = increase in displacement / time taken
Acceleration - the rate at which objects change their velocity
Acceleration = change in velocity / time taken
or a = (v - u) / t u = initial velocity v = final velocity
Acceleration is a vector because direction matters!
Deceleration - slowing down
A decelerating object will have a smaller final velocity than starting velocity. A negative sign when calculating acceleration means deceleration.
A straight line on a velocity-time graph indicates uniform acceleration
The gradient of a velocity-time graph is the acceleration
The area under a velocity-time graph is equal to the distance travelled
A Ticker Tape is produced by a ticker timer
It produces 50 dots per second on a tape moving through the machine. Therefore each gap between dots is equal to the distance travelled in 0.02 seconds
The tape is often cut to lengths that represent equal time
Gravitational force or weight acts on everything with mass
An upward force is sometimes called a normal force because it is at 90 degrees to a surface.
Force is a VECTOR
Unit of force: Newton (N) named after Sir Isaac Newton
A force of 1 Newton will make a mass of one kilogram accelerate at one metre per second squared
Other examples of forces: air resistance or drag (similar to friction), upthrust (e.g. in hot air balloons and liquids), magnetic force, electrostatic force.
When more than one force is acting on an object, we must find the resultant/unbalanced force
If the resultant force is zero then we say that the force is balanced because the forces cancel each other out
Friction opposes the motion
Machines work more efficiently if friction is reduced
Swimmers wear special fabrics to reduce fluid friction but it is useful when someone uses a parachute after jumping out of a plane
Friction can be tested by adding weights to a pulley which drags a test block along the surface which is to be tested
Forces can change the shape of an object
The shape change can be permanent or temporary
All materials will stretch a little when you put them under tension or shorten when you compress them
Some materials e.g. glass are brittle and break rather than stretch noticeably
Resilient or elastic materials do not break and tend to return to their original shape when the force is removed
Other materials like putty and plasticine are not resilient but plastic and they change shape permanently when even a small force is applied to them
Springs and Wires
Springs are coiled lengths of certain metals which can be stretched and compressed
Springs change their length when a force acts on them and return to their original length when the force if removed
This is true provided you do not overstretch them
If springs are stretched beyond a certain point, they do not spring back to their original length
Robert Hooke discovered an important property of springs
extension of spring is directly proportional to the force applied to spring
Hooke's Law only applies if you do not stretch a spring too far
Elastic Limit - the point where the spring will stretch more for each successive increase in load force. After this point, the spring will change shape permanently
Hooke's Law also applies to wires
Elastic bands are made of rubber. The graph of load against extension is not a straight line until it's elastic limit therefore elastic bands do not obey Hooke's Law
Force, Mass and Acceleration
The acceleration of an object is affected by both its mass and the force applied to it
Force is directly proportional to acceleration
acceleration is inversely proportional to mass
Force (in Newtons) = mass (in kg) x acceleration (in m/s squared)
F = m x a
One newton is the force needed to make a mass of one kilogram accelerate at one metre per second squared
Deceleration in a collision
A negative acceleration is a deceleration!
If a large deceleration is needed then the force causing the deceleration must me large too
Usually, a car is stopped by using the brakes in a controlled was so that the deceleration is not excessive but in an accident, the car may collide with another vehicle or obstacle, causing a very rapid deceleration
Friction and Braking
Brakes work by increasing the friction between the wheels and the body of the car
The friction force between the tyres and the road depends on: the condition of tyres (e.g. tread & inflation), surface of road and weight of vehicle
If the brakes are applied too hard then the tyres will not grip the road surface and the car will skid
A driver does not have control over a skidding car - it will take longer to stop
ABS (anti-lock braking system) - a computer-controlled brake system that senses when the car is about to skid and momentarily releases the brakes
Safe Stopping Distance
The stopping distance is the sum of the thinking distance and the braking distance
The thinking distance can also be called the reaction time. It will depend on:
- If the driver is tired, under the influence of alcohol or drugs
If the distance between two cars is not at least the thinking distance then a violent collision is inevitable even with an emergency stop.
In an emergency, you brake as hard as you can with ABS braking. The braking force will be a maximum.
a = F / m
Vehicles with large masses will have smaller rates of deceleration for a given braking force. They will therefore travel further.
In poor conditions, the braking force will be lower
Acceleration due to Gravity & Weight
Accel due to gravity = 9.8 m/s(squared)
The symbol g is used to represent acceleration due to gravity
Although this equation presumes no other forces such as air resistance are acting on the object
Weight (in N) = mass of object (in kg) x accel due to gravity (in m/s(squared))
W = m x g
The value of g depends on how strong a planet's gravity is. It is sometimes called gravitational field strength and is measured in N/kg : the gravitational force per unit mass.