Forces Flashcards
Vector
Magnitude and direction
Scalar
Only have magnitude
Examples of vector quantities
Force, velocity, displacement, acceleration, momentum
Examples of scalar quantities
Speed, distance, mass, temperature, time
How are vectors represented
- arrow
- length of arrow= magnitude
- direction of arrow= direction of quantity
What is a contact force an examples of it
- when 2 objects are touching for a force to act
- eg. Friction, air resistance, tension in ropes, normal contact force
What is non contact force and examples of it
- if objects don’t need to be touching for a force to act
- magnetic force, gravitational force, electrostatic force
Sun and earth example of non contact force
- they’re both attracted to eachother by the gravitational force
- this is a non contact force
- an equal but opposite force of attraction is felt by both the sun and the earth
Chair ad floor example of contact force
- a chair exerts a force on the ground, whilst the ground pushes back at the chair with the same force (the normal contact force)
- equal but opposite forces are felt by both the chair and the ground
2 important effects of gravity
- on the surface of a planet, it makes all things fall towards the ground
- it gives everything a weight
What is gravity
- gravitational force is the force of attraction between masses
- gravity attracts all masses, but you only notice it when one of the masses is really really big, eg. A planet
What is mass
- just the amount of stuff in a object
- for any given object, this will have the same value anywhere in the universe
What is weight
- the force acting on an object due to the pull of the gravitational force on the object)
- close to earth, this force is caused by the gravitational field around earth
Gravitational field strength
- it varies with location
- its stronger the closer you are to the mass causing the field
What is the weigh of an object dependent of
- the strength of the gravitational field at the location of the object
- this means that the weight of an object changes with its location
What is mass’s unit and how is it measured
- kg
- mass balance
Unit of weight and how is it measured
- force measured in newtons, N
- the force is acting from a single point on the object, called its centre of mass. For a uniform object (one thats the same density throughout and is a regular shape) this will be at the centre of the object
- measured with calibrated spring balance or newton meter
Relationship between mass and weight
Directly proportional
What is a free body diagram
- diagram showing all the forces acting on an object using arrows
What happens when a resultant force moves an object
- when a force moves an object through a distance, energy is transferred and work is done on the object
- to mark something move, a force must be applied. The thing applying the force needs a source of energy. The force does work to move the object and energy is transferred from one store to another
How to use scale drawings to find resultant forces
- Draw all the forces acting on an object, to scale
- Then draw a straight line from the start of the first force to the end of the last force - thats the resultant force
- Measure length of the resultant force on the diagram to find the magnitude and the angle to find the direction of the force.
It should look like a triangle
How to calculate force if object is in equilibrium
- If all the forces acting on an object combine to give a resultant force of 0, the object is in equilibrium
- On a scale diagram, this means that the tip of the last force you draw should end where the tail of the first force you drew begins (for three forces, the diagram will form a triangle
- If told to find a missing force, draw on the forces you know, and then draw a line to fill the gap and work out the force
Calculating a force by splitting a force into components
- some forces are at awkward angles
- to make it easier, it can be split into two components at right angles to eachother (usually horizontal or vertical)
- actin together, the components have the same effect as the single force
- draw the force to scale, then add the horizontal and vertical components along the same grid lines (should form a right angle triangle). The jus tmeasure them
Elastic deformation
-an object has been elastically deformed if it can go back to its original shape and length after the force has been removed
- objects that can be elastically deformed are elastic objects
- an object has been inelastically deformed if it doesn’t return to its original shape and length after the force has been removed
What happens when work is done to stretch or compress an object
- work is done when a force stretches or compresses an object and causes energy to be transferred to the elastically potential energy store of the object
- if its elastically deformed, ALL this energy is transferred to the objects elastic potential energy store
Relationship between force and extension
- directly proportional
What does spring constant depend on
- depends on the material
- a stiffer spring has a greater spring constant
What is the limit of proportionality
- ## when a maximum force occurs, making the graph curve showing that extension is no longer proportional to force
What is momentum
- the greater the mass of an object, or the greater its velocity, the more momentum the object has
- its a vector quantity, with size and direction
- p =mv
What is the conservation of momentum
- in a closed system, the total momentum before an event is the same as after the event
Conservation of momentum in snooker balls
- each collision is an event where the momentum of each ball changes, but the overall momentum stays the same:
- Ball A is stationary so it has zero momentum. Ball B is moving with a velocity, so it has momentum
- Ball B hits ball A causing it to move, so it now has momentum. The white ball continues moving but at a smaller velocity, so smaller momentum
- The combined momentum of both balls is equal to the original momentum of ball B
What is the unit of momentum
Kg m/s
What happens to momentum in an explosion
The momentum before is zero. After the explosion, the pieces will fly off i different directions so that the total momentum cancels out to zero
Newton’s first law
If the resultant force on a stationary object is zero, the object will remain stationary. If the resultant force on a moving object is zero, it’ll just carry on moving at the same velocity
5 different forms of acceleration
- starting, stopping, speeding up, slowing down and changing direction
- on a free body diagram, the arrows will be unequal
Newton’s second law
- f=ma
- force and acceleration are directly proportional
- acceleration is inversely proportional to the mass of the object
Newtons 3rd law
- when two objects interact, the forces they exert on each other are equal and opposite
What happens when 2 people of different masses push against eachother
- the force are the same size an in opposite directions, (newtons 3rd law) but the person with the smaller mass will have accelerated more due to newton 2nd law because acceleration is inversely proportional to mass so as mass decreases, the acceleration ill increase)
What is inertia
- its the tendency for motion to remain unchanged
- until acted upon by a resultant force, object stay at rest and objects moving at a steady speed will stay moving at that speed
- an object’s inertial mass measures how difficult it is to change the velocity of an object IMPORTANT
- its defined by using ratio of m=f/a
Newtons 3rd law in equilibrium
- a man pushing against a wall
- as the man pushes the wall, there is a normal contact force acting back on him
- these 2 forces are the same size
- as the man applies a force and pushes the wall, the wall ‘pushes back’ on him with an equal force
Experiment to link force and extension
- Measure the natural length of the spring with a millimetre ruler clamped to the stand. Make sure t take reading at eye level and add a marker to the bottom of the spring to make the reading more accurate
- Add a mass to the spring and allow it to come to rest. Record the mass and measure the new length of the spring. The extension is the change in length.
- Repeat this process until you have enough measurements
- Plot a force-extension graph of you results. It will only start to curve if you exceed the limit of proportionality
When can E=1/2ke^2 be used
- as long as spring isn’t stretched beyond limit of proportionality
What does it mean if an object has been elastically deformed
- if it can go back to its original shape and length after the force has been removed
What does it mean if an object has been inelastically deformed
- if it DOES NOT return to its original shape and length after the force has been removed
Difference between distance and displacement
- distance is scalar so it doesn’t involve direction, only how much an object has moved
- displacement is a vector. It measures distance and direction in a straight line from an object starting point to finishing point. Like the plane flew 10m north
- if you walk 5m north then 5m south, your displacement is 0m,but the distance travelled would be 10m
Difference between speed and velocity
- speed is how fast you’re going (20m/s)
- velocity is speed in a given direction ( 20m/s north)
- if a car is travelling in a circle at a constant speed, the speed stays the same the whole time whereas the velocity is constantly changing
Speed of person walking
1.5 m/s
Speed of person running
3 m/s
Speed of person cycling
6 m/s
Speed of car
25 m/s
Speed of train
30 m/s
Speed of plane
250 m/s
Speed of sound in air
330m/s
Factors that can change the speed of a person travelling
- fitness
- age
- distance travelled
- terrain
Factors that can change wind speed
- temperature
- atmospheric pressure
- large buildings or structures
What is acceleration
- the change of in velocity in a certain amount of time
- unit: m/s^2
What is deceleration
- negative acceleration
- if one thing slows down,the change in velocity is negative
What is uniform acceleration
- constant acceleration
- v^2 - u^2 = 2as
Uniform speed
- constant speed
- d = st
Non - uniform speed equation
- use average speed because speed is constantly changing
Distance time graph
- gradient = speed
- flat section = stationary
- straight uphill section = travelling at steady speed
- curve=acceleration (steepening curve) or deceleration (curve levels off)
- speed at a point on curve= gradient of tangent
Velocity time graph
- gradient= acceleration
- flat section= steady speed
- up hill / = acceleration
- down hill \ = deceleration
- steep= greater acc/deceleration
- curve= changing acceleration
- area under graph= distance travelled in that time interval
Friction
- always acts in opposite direction to movement
- to travel at a steady speed the driving force needs to balance the frictional forces
- you can reduce friction between surfaces using a lubricant
Drag
- the resistance you get in a fluid
- air resistance is a type of drag
- to reduce drag: keep object streamlined
- frictional forces from fluids increase with speed (a car has more friction to work against when travelling at 70 mph than at 30 mph, so at 70 mph the engine has to work harder at just to maintain a steady speed)
How does an object falling through fluid reach terminal velocity
- at start, force of gravity is much more than the frictional force, so it accelerates
- as speed increases, friction builds up
- the gradually reduces acceleration and eventually the frictional force is equal to the accelerating force (resultant force is 0)
- it will have reached its maximum speed, or terminal velocity and will fall at a steady speed
What does terminal velocity depend on
- less streamlined object= lower terminal velocity
- large surface area = lower terminal velocity
Why does a marble have a higher terminal velocity than a beach ball
- more air resistance acting on the beach ball at any given speed, so the beach ball spends less time accelerating before the air resistance is large enough to equal the accelerating force
How to work out stopping distance
Stopping distance= thinking + braking distance
What is thinking distance
- how far the car travels during the driver’s reaction time (the time between the driver seeing the hazard and applying the brakes)
What is braking distance
- the distance taken to stop under the braking force (once the breaks are applied)
What are typical car braking distances
- 14m at 30mph
- 55m at 60mph
- 75m a 70 mph
What is thinking distance affected by
- speed: the faster the speed, the further you’ll travel during the time you take to react
- reaction time: the longer the reaction time, the longer your thinking distance
Factors affecting braking distance
- speed: for a given braking force, the faster the vehicles travels, he longer it takes to stop
- weather or road surface: if its wet or icy, there is less friction between vehicles tyres and the road, which can cause tyres to skid
- conditions of tyres: if the tyres are bald, then they can’t get rid of water in wet conditions. This leads to them sliding on top of water
- how good brakes are: if they’re worn or faulty, they wont be able to apply as much force as well- maintained brakes
Friction between brakes and wheels
- when brake pedal is pushed, it causes the brake pads to be pressed onto the wheels. This contact causes friction, which causes work to be done between the brakes and the wheels transfers energy from kinetic store of wheels to the thermal store of the brakes. The brakes increase in temperature
- the faster a vehicle is going, the more energy it has in its kinetic store, so the more work needs to be one to stop it. This means a greater braking force is needed t make it stop within a certain distance
- a larger braking force means a larger deceleration
Why can large declarations be dangerous
- can cause brakes to overheat so they don’t work as well or it could cause vehicle to skid
Typical reaction times
Between 0.2-0.9 s
Factors that affect reaction time
- tiredness
- drugs
- alcohol
- distractions
Ruler drop test
- Sit with arm resting on edge of a table (to stop arm moving up or down during test)
- Someone else should hold ruler to that the 0 is lined up with thumb an forefinger
- 3rd person needs to be at eye level with ruler yo check that its lined up
- Without giving warning , the person holding ruler should drop it. Close thumb and finger to try and catch ruler as quickly as possible
- Calculation time using t= d/s (speed is 9.8m/s^2 due to gravity is constant)
