forces - physics chapter 5

• scalar and vector quantities -

• scalar quantities only have magnitude (size) - distance, time, speed, mass

• vector quantities have magnitude and direction - velocity, displacement, momentum, acceleration

• these can be shown on arrows, with the length of it representing the magnitude and the direction shows the direction

• a force is a push or a pull on an object due to the interaction with another object

• all forces have a magnitude and a direction

• measured in newtons (N)

• contact forces - 2 objects are physically touching e.g tension on a rope, friction, air resistance and normal contact force - when something on something exerts a downward force and the thing below it makes an upward force - they have to be in direct contact

• non contact forces - a force that doesn’t require things to be touching e.g magnetic, gravitational and electrostatic (force between 2 charged objects) can be attraction and repulsion

mass and weight -

• the mass of an object tells us how much matter it has in it - kg

• gravity doesn’t change mass

• the weight of an object is the amount of force acting on it due to gravity - N

• something that weighs 1kg on earth will experience 9.8N of gravity

• for every kg of mass on earth the object experiences a force of 9.8N

• gravitational field strength is a measure of the force of gravity in a location - N/kg

• the weight of an object is directly proportional to the mass of an object

• you determine weight using a calibrate spring balance or newtonmeter

• you can think of weight as acting on a single point on the object called its center of mass

resultant forces -

• the resultant force is the overall force on a point or an object

• to work it out we subtract the smaller force from the larger force

• if two or more arrows are pointing one way, add them together to get the total force acting in that direction

• if both arrows have the same amount of force, the resultant force is 0

• in a free body diagram, the object is shown as a point and the forces shown as arrows from the point

• when a force moves an object through a distance, work is done on the object or energy is transferred

• for an aeroplane moving at a constant velocity, it will experience lift which is equal to weight, keeping it in the air, along with thrust to propel it forward and air resistance or drag, these are also balanced as the velocity is constant

vector diagrams -

• usually forces act paralel to eachother but to find the resultant force when forces are acting at angles you use a vector diagram

• make a scale - usually 1cm equals 1N

• draw out the forces using a ruler e.g a 10N force would be 10cm

• draw any angles using a protractor

• 

• draw a line across the shape and measure it - this is the resultant force

resolving forces

• 

• first put a point on your page

• the draw a line going up and a line going across to represent your horizontal and verticle componets

• draw your angle from the horizontal line or the verticle line depending on the question

• from that angle, draw your force line using a ruler

• use a scale like 1N equals 1cm

• then add dotted lines from the top of your force line to the bottom

• measure the lengths of the horizontal and verticle lines to the dotted bits

• put your measurements into the scale to give you the force

work done and energy transferred

• work done can be measured in J and Nm

• when a box is pushed along a carpet, energy is transferred from the chemical store of the person’s muscles to the kinetic store of the box but friction is also working against it, causing energy to be transferred into the box’s thermals store

• a force does work to move an object, and this causes energy to be transferred

• 1J of work is done when 1N of force is used to move an object 1m

• work done = force x distance

forces and elasticity

• applying a force to an object may compress it, bend or stretch it

• to do this you need more than one force acting on the object, otherwise it’ll just move

• elastically deformed - an object can return to its original shape and length after the force has been removed - these are called ELASTIC OBJECTS

• inelastically deformed - an object cannot return to its original shape and length after the force has been removed

• work is done when an object is compressed, bent or stretched and this causes energy to be transferred into the elastic potential energy store of the object

• extension is directly proportional to force, applying lots of force means the object is stretched more and ect

• force = spring constant x extension

• spring constant (k) tells us how much force is needed to compress a spring by 1m, therefore a stiffer spring has a higher spring constant

• you can find extension by subtracting the natural and compressed lengths

• when a the line begins to curve on a force - extension graph, this tells you when the spring’s limit of proportionality has been reached

• after the limit of proportionality has been reached - force is dp to extension

• after it has been reached - force is no longer dp to extension

• this means that the limit of proportionality is when a spring stops obeying hooke’s law - force is dp to extension - meaning it will get stretched too far and not be able to return to its origional shape and length (inelastically deformed) or may even snap

method for investigating springs -

• we are going to investigate to see how adding masses to a spring causes it to stretch - force and extension - the force used here is weight

• set up a clamp stand, 2 clamps, 2 bosses and a heavy weight to stop it from falling over

• attach a meter ruler to one clamp, and a spring to the other

• the top of the spring must be at the zero point on the ruler

• ensure the meter ruler is verticle or the readings will not be accurate

• stick a pointer from the bottom of the spring and read off where it is on the ruler, this tells you the og length off the spring - the pointer must be horizontal to ensure its an accurate reading

• hang a 1N weight on the spring and record the new length off of the ruler

• continue adding weights onto the spring - make sure to measure the mass of your weights using a mass balance before you begin to make sure they’re all the same

• subtract the og length from the new length to get the extension for each weight

• plot a force extension graph for each weight

• the graph is a straight line going through the origin - directly proportional

• we can see the spring is elastic because if we remove the weight or force the extension returns to 0

• use the linear (straight) part of the graph to calculate the spring constant - it will be the same at every point - k = f / e

speed

• distance is how far an object has moved - it’s a scalar quantity

• displacement is how far an object has moved and in what direction - it’s a vector quantity - it measures the distance and direction from an objects starting point to its finishing point - if you walked 10m south then 10m north your distance would me 20m but your displacement would be 0m

• speed is how fast you’re going - scalar

• velocity is how fast you’re going in a certain direction - vector

• you can have objects changing speed but at a constant velocity, when an object is changing direction but staying at the same speed

• you can find the speed of an object that moving at a constant speed by timing how long it takes to travel a certain distance then using speed = distance x time

• walking - 1.5m/s

• running - 3m/s

• cycling - 6m/s

• car - 25m/s

• train - 30m/s

• plane - 250m/s

• sound - 330m/s

• for people, speed can be effected by age, fitness, distance and terrain

• for sound speed - what the sound waves are travelling through

• for wind speed - buildings, atmospheric pressure, temperature

• if an object is accelerating at a constant rate we can use v2 - u2 = 2 x acceleration x distance

• v = final vel u = initial vel

terminal velocity

• friction acts in the opposite direction to movement and slows things down - it can be limited by streamlining and lubricating

• to move at a steady speed, the driving force has to equal the frictional forces

• drag increases as speed increases, its a resistance you get in fluids

• streamlining reduces drag as it allows particles to flow over it easily

• objects falling through fluids reach a terminal velocity - at the beginging gravity is stronger than drag but as speed increases so does drag and eventually the acceleration reduces until the accelerating force is equal to the drag force - it will now fall at a steady speed

• terminal velocity is the maximum speed

• the less streamlined an object is the lower its terminal velocity as the drag force will be higher than the accelerating force because particles will hit it head on instead of glide over it

• objects with a larger surface area will have a lower terminal velocity as there is more area for air resistance to occur

newtons first law - law of inertia - an object will stay at rest or at a constant speed in a straight line until a resultant force acts on it

newtons second law - force = mass x acceleration - acceleration is proportional to the resultant force

newtons third law - when 2 objects interact the forces they exert on eachother are equal and opposite

intertia - the tendancy for motion to remain unchanged

intertial mass - how difficult it is to change the velocity of an object

method for investigating momentum -

• this practical investigates newton’s second force = mass x acceleration

• set up an air track (to reduce friction) along with a trolley of a known mass, attatched to string which is attatched to a oulley on the end of the track, on the end of the pulley there should be a weight of a known mass, along with light gates at the start line and end line to measure the initial and final velocity, you should also use a stopwatch to find the time, as you have to use acceleration = change in velocity / time

• release the car from a fixed point on a slope

• start the stopwatch and stop it at the end point, note this down and ote down the velocity from both the light gates

• put your values into the equation

• add another mass to the end of the string and repete the method

• the total mass of the system has to be constant so each time you add a mass to the end of the string you have to take it off of the trolley

• draw a force acceleration graph and you should get a dp graph

• if it asks about how changing mass changes accelation - same method but begin with all masses on string and then add them one by one to the trolley to increase its mass, should get a graph that shows when mass increases acceleration halves - inversely proportional

reaction time - same method as bio but use the acceleration equation rearranged to find time - time = change in velocity / acceleration

stopping distance

• stopping distance is the distance it takes to stop a car in an emergency

• thinking distance is the time between the driver seeing a hazard and applying the brakes

• braking distance is the time it takes for the car to stop under the force of the brakes

• stopping distance = thinking distance + braking distance

• thinking distance is affected by - your speed and reaction time

• braking distance is affected by - speed, the weather and road surface, tyre condition and how good your brakes are

• braking relies on friction - when a car brakes the brake exerts a contact force on the wheel, this causes friction as the car begins to decelerate, eventually the force of friction will be higher than the kinetic energy of the car, causing it to stop completely and transferring some of the friction to the thermal store of the brake, causing it to heat up

• a larger breaking force means a larger deceleration - this can be dangerous as lots of thermal energy can be made so the vehicle may overheat or skid

• average reaction time is between 0.2s and 0.9s

• can be affected by tiredness, drugs and alcohol

• you can use time = change in velocity / acceleration to calculate reaction time when doing the ruler test - same method applies in biology

• you use the acceleration equation because acceleration due to gravity is constant

• momentum is a vector quantity

• momentum = mass x velocity

• in a closed system, momentum before an event = momentum after

• this is the conservation of momentum

• if the momentum before an event is zero, it will be zero after

• in an open system, external forces such as friction can act on the objects, adding or removing momentum

• 2 balls in snooker have the same mass, the red ball isn’t moving, therefore it has 0 momentum, when a white ball (which is moving with velocity, meaning it has a momentum) hits the red ball, it causes it move move, meaning it now also has a momentum

• the white ball continues to move, but at a smaller velocity, so the combined momentum if the 2 balls is equal