P4 – Explaining motion
P4.1 How can we describe motion?
The speed of a moving object can be calculated if the distance travelled and the time taken is known.
Speed is just the travelled distance in a certain time
When an object moves in a straight line at a steady speed, you can calculate its average speed.
The instantaneous speed of an object is the speed of an object at a particular instant – the average speed of an object over a very short period of time.
A distance-time graph shows how the distance travelled by an object changes with time – the slope or gradient of a distant time graph is a measure of the speed of the object.
The steeper the slope, the greater the speed.
Distance-time graphs can also be drawn as displacement-time graphs, where the displacement of an object is its net distance from its starting point together with an indication of direction – when distance is given with a particular indication of direction –it’s called displacement.
It is possible to calculate a speed from the gradient of a straight section of a distance-time graph.
This is done by picking any point on the gradient and read off the distance travelled at the is point and the time taken to get there then use the formula:
A speed-time graph tells us how the speed of an object changes over time.
A horizontal line indicates a steady speed.
If a line has a slope then the speed is changing – the steeper the gradient of line, the greater the acceleration.
The slope of a speed-time graph represents the acceleration of the object
The acceleration of an object is the rate at which its velocity changes – it is the measure of how quickly an object speeds up or slows down.
In many everyday situations, acceleration is used to mean the change in speed of an object in a given time interval.
The velocity of an object is its speed in a particular direction – e.g. positive or negative velocity depending on the direction.
A velocity-time graph shows how the velocity at which an object is moving changes with time. Velocity has a direction, so if moving in a straight line in one direction is a positive velocity, then moving away in a straight line in the opposite direction will be a negative velocity.
The instantaneous velocity of an object is its instantaneous speed together with an indication of the direction
To calculate the acceleration from a velocity-time graph you use the formula:
P4.2 What are forces?
Forces occur when there is an interaction between two objects. These forces always happen in pairs – when one object exerts a force on another, it always experiences a force in return. These two forces become an interaction pair. They are in equal in size and opposite in direction.
Some forces such as friction and reaction (of a surface) only occur as a response to another force.
A force occurs when an object is resting on a surface – the object is being pulled down to the surface by gravity and the surface pushes up with an equal for called the reaction of the surface.
For example:
A book on a table has a downwards force (its weight) due to gravity – this downward force, pushing on the table produces an upwards force called reaction.
The weight and the reaction of the surface are the same size, and in opposite directions.
However they are NOT an interaction pair, because the weight of the book is caused by the Earth’s gravity not by the table.
When two surfaces slide past one another – both objects experience a force that tries to stop them moving – this interaction is called friction.
For example:
A book is moving to the right across the table.
The blue and green arrows show the interaction pair of friction forces.
The book experiences a backwards force – this will tend to slow it down
The table experiences a forwards force – this will tend to move it forwards with the book.
Friction can also be seen in walking and driving.
Rockets and jet engines produce a driving force through a pair of equal and opposite forces – the rocket’s engines push gas backwards (action) and the gas pushes the rocket forwards (reaction), thrusting it through the atmosphere.
P4.3 What is the connection between forces and motion?
In many real situations, the forces acting on an object are not all the same size – they’re unbalanced
The resultant force is the overall force acting on an object – the force you get when you take into account (add up) all the individual forces and their directions.
If a resultant force acts on an object, it causes a change of momentum in the direction of the force – this is because it is the force that decides the motion of the object – whether it will accelerate, decelerate or stay at a steady speed.
Momentum is a measure of the motion of an object. The momentum of an object is calculated using the formula:
The greater the mass of an object or the greater its velocity the more momentum the object it has.
The change in momentum depends on the force.
When a resultant force acts on an object, it causes a change in momentum in the direction of the force – the change of momentum it causes is proportional to the size of the force and to the time for which it acts:
The horizontal motion of objects (like cars and bicycles) – a car or bicycle has a driving force pushing it forwards. There are always counter forces of air resistance and friction pushing backwards.
For an object moving in a straight line, if the driving force is:
- Greater than the counter force – the vehicle will speed up
- Equal to the counter force – the vehicle will move at constant speed in a straight line
- Smaller than the counter force – the vehicle will slow down
In situations involving a change of momentum (such as collision), the longer the duration of the impact, the smaller the average force for a given change in momentum – this means the greater the time for a change in momentum the smaller the force.
In a collision, you can’t really affect the change in momentum – however the average force on an object can be lowered by slowing the object down over a long period of time. Safety features in a car increase the collision time to reduce the forces on the passengers:
- CRUMPLE ZONES – crumple on impact, increasing the time taken for the car to stop
- AIR BAGS – slow you down more gradually
- SEAT BELTS – stretch slightly, increasing the time taken for the wearer to stop – this reduces the forces acting on the chest
- CYCLE AND MOTORCYCLE HELMETS – provide padding that increases the time taken for your head to come to a stop if it hits something hard
Vertical motion – if a ball is released from a height then forces begin to act on it – it will start to fall due to the force of gravity acting on it and the ball will begin to accelerate
If an object is dropped that is light relative to its size (like a feather) it will speed up when it is first released at first but then fall at a steady speed – this is due to air resistance
The faster an object moves, the greater the force of air resistance on it becomes – the light objects will reach a steady speed when the force of air resistance balances out the force of gravity.
If the resultant force acting on an object is zero its momentum will not change. If the object
- Is stationary – it will remain stationary
- Is already moving – it will continue moving in a straight line at a steady speed
P4.4 How can we describe motion in terms of energy changes?
When a force moves an object, it does work and energy is transferred to the object – whenever something moves, something else is providing some sort of effort to move it.
When work is done ON an object, energy is transferred TO the object – gains energy
When work is done BY an object, energy is transferred FROM the object to something else – loses energy according to the relationship:
The energy of a moving object is called kinetic energy. The amount of kinetic energy an object has depends on the mass of the object and the velocity of the object.
The greater the mass and velocity of an object – the more kinetic energy it has.
When a force acting on an object makes its velocity increase, the force does work on the object and this results in an increase in its kinetic energy.
Gravitational potential energy is the energy stored in an object when you raise it to a height against the force of gravity.
As an object is raised, its gravitational potential energy increases, and as it falls the gravitational potential energy decreases.
When an object is lifted to a higher position above the ground, work is done by lifting the force – this increases the G.P.E.
If we ignore the effects of air resistance and friction the increase in kinetic energy will be equal to the amount of work done. However, in reality some of the energy will be lost, as heat and the increase in kinetic energy will therefore be less than the work done.
Energy is always conserved – the total amount of energy present stays the same before and after any changes.
P5 – Electric currents
P5.1 Electric current – a flow of what?
Some insulating materials can become electrically charged when they rub against each other – the electrical charge then stays on the material i.e. it does not move (the charge is static)
When two insulating materials are rubbed together, electrons are scraped off one and dumped on the other.
Electrons are negatively charged – the material receiving the electrons becomes negatively charged and the one giving up electrons becomes positively charged.
When two charged materials are bought together, they exert a force on each other so they are attracted or repelled.
Two materials with the same type of charge repel each other – two materials with different charges attract each other.
An electric current is a flow of charge – it is measured in amperes (amps)
In an electric circuit the metal conductors (the components and the wires) are full of charges that are free to move. When a circuit is made, the battery causes these charges to move in a continuous loop – the charges are not used up.
In metal conductors there are lots of charges free to move. Insulators, on the other hand have few charges that are free to move. Metals contain free electrons in their structure – the movement of these electrons create the flow of charge (electric current).
P5.2 What determines the size of the current in an electric current in an electric circuit and the energy it transfers?
Current will only flow through a component if there’s a voltage across the component – the amount of current flowing in a circuit depends on the voltage (potential difference) of the battery and the resistance of the compounds in the circuit.
Components such as resistors, lamps and motors resist the flow of charge through them i.e. they have resistance.
Resistance is caused by things in the circuit (such as compounds e.g. lamps) that resist the flow of charge (slows down the charge down) – units ohms Ω
The greater the resistance of a compound or components, the smaller the current that flows for a particular voltage or the greater the voltage needed to maintain a particular current.
Even the connecting wires in the circuit have some resistance, but it is such a small amount that it is usually ignored.
Anything that supplies electricity is also supplying energy – so power supplies all transfer energy to the charge which then transfers it to the components (and sometimes their surroundings).
When electric charge flows through a component (or device) work is done by the power supply and energy is transferred from it to the component and/or its surroundings.
Power is the rate at which an electrical power supply transfers energy to an appliance – power is usually measured in watts, W or kilowatts kW (1kW = 1000W)
When an electric current flows through a component (resistor) it causes the component to heat up. As the current flows, the moving charges collide with the vibrating ions in the wire giving them energy – this increase in energy causes the component to become hot.
In a filament lamp this heating effect is large enough to make the filament in the lamp glow.
The resistance of some materials depends on the environmental conditions:
Adding resistors in series increases the resistance because the battery has to push charges through all of the resistors.
Adding resistors in parallel reduces the total resistance and increases the total current because this provides more paths for the charges to flow along
Voltage – current graphs show us how the current in a circuit varies as you change the voltage.
The current through a component is proportional to the voltage across it when the resistance stays constant.
P5.3 How do parallel and series circuits work?
The potential difference (voltage) across a component in a circuit is measured in volts (V) using a voltmeter connected in a parallel across the component – a voltmeter can be used to measure the potential difference between any two chosen points.