Forces Flashcards
Scalar quantities
Have magnitude only
Vector quantities
Have magnitude and direction
How to represent vector quantities
- arrow
- length represents magnitude
- direction represents direction
Force
Push/pull on object, caused by it interacting with something
Force quantity
Vector
Types of force
- contact
- non-contact
Contact force
When 2 objects have to touch for a force to act
What are the contact forces
- friction
- air resistance
- tension
- normal contact force
Non-contact force
When objects don’t need to be touching for force to act
Non-contact forces
- gravitational force
- electrostatic force
- magnetic force
What happens when 2 objects interact
A force is produced on both objects
Interaction pair
Pair of forces equal and opposite and act on two interacting objects
Gravitational force
Force of attraction between masses
Weight
Force acting on object due to gravity
Why is there a force of gravity
Due to gravitational field strength around Earth
Effects of gravity
- gives everything weight
- on surface of planet, makes all things fall to ground
Mass
amount of ‘stuff’ in an object
What does weight depends on
Gravitational field strength at location of object
Unit of weight
Newtons (N)
Centre of mass
Single point on object where weight force acts on it
Relationship between object’s weight and mass
Directly proportinal
How is weight measured
With a newtonmeter - calibrated spring balance
Resultant force
Single force that has same effect as all original forces acting together
What happens when a force moves an object through a distance
Energy is transferred + work is done on object
What must happen to make something move
A force must be applied
What does thing applying force need
Source of energy
When has 1 joule of work been done
When a force of 1N causes an object to move a distance of 1 metre
What is 1 Nm equal to
1J
How to use scale drawings to find resultant forces
- draw all forces acting on object ‘tip-to-tail’
- draw straight line from start of 1st force to end of last force - this is resultant force
- measure length of resultant force + angle measured clockwise from north
- ‘resultant force is N on a bearing of °’
When is object in equallibrium
If all forces acting on object combine to give resultant force of 0
How to know object is in equilibrium on scale drawing
Tip of last force ends where tail of first force begins
How to resolve a force
- done if object is not acting horizontally/vertically
- split force into 2 components - horizontal + vertical
- acting together, components have same effect as same force
Elastic deformation
Object has been elastically deformed if it can go back to original shape + length after force removed
Elastic objects
Objects that can be elastically deformed
Inelastic deformation
Object has been elastically deformed if it doesn’t return to its original shape + length after force has been removed
What kind of deformation has all energy go to elastic potential store
When an object is elastically deformed
How does spring’s stiffness affect spring constant
Stiffer springs have greater spring constant
Limit of proportionality
Maximum force, above which extension is no longer proportional to force (graph will curve upwards)
Link between force/extension PRACTICAL
- apparatus - spring, ruler, clamp, tape, weighted stand, hanging mass, extra masses
- measure mass of each mass and find weight (w=mg)
- measure natural length of spring with no load applied with ruler clamped to stand, add marker (tape)
- add mass to spring and let it come to rest, record mass + new length of spring - extension is change is length
- repeat until there are at least 6 measurements
- plot force-extension graph
- when graph is straight line, gradient is spring constant
- when graph curves - relationship is non-linear - reached limit of proportionality
Distance
How far an object has moved
Distance quantity
Scalar
Displacement
Distance and direction in straight line from object’s starting to finishing point
Displacement quantity
Vector
Speed
How fast you’re going with no regard to direction
Velocity
Speed in a given direction
Speed/velocity of object moving in circle
- constant speed
- constantly changing velocity
- e.g- car going round roundabout
Typical person walking speed
1.5 m/s
Typical person running speed
3 m/s
Typical person cycling speed
6 m/s
Typical car speed
25 m/s
Typical train speed
30 m/s
Typical plane speed
250 m/s
Speed of sound
330 m/s
Factors affecting speed people move at
- fitness
- age
- terrain
Factors affecting speed of sound
What sound waves are travelling through
Factors affecting speed of wind
- temperature
- atmospheric pressure
- if there are large structures nearby
Uniform acceleration
Speeding up or at a constant rate (constant acceleration)
Deceleration
- negative acceleration
- object slowing down
What does gradient represent on distance-time graph
Speed
What does flat section represent of distance-time graph
Stationary
What to straight uphill sections represent on distance-time graphs
Travelling at steady speed
What do steepening curves represent of distance-time graphs
Acceleration
What do levelling off curves represent on distance-time graphs
Deceleration
How to find speed at a point during acceleration on distance-time graphs
- draw tangent to point you want
- find gradient of tangent
What does gradient represent on velocity-time graphs
Acceleration
What do flat sections represent on velocity-time graphs
Travelling at steady speed
What do uphill sections represent on velocity-time graphs
Constant acceleration
What do downhill sections represent on velocity-time graphs
Constant deceleration
What does a curve represent on velocity-time graphs
Changing acceleration
How to find distance travelled on velocity-time graph
Area under a section of graph (or all of it) is distance travelled for that time interval
What direction does friction act in
Opposite direction to movement
How to reduce friction
Lubricant
When does friction occur
- between 2 forces in contact
- when object passes through fluid
Streamlining
Designing object to allow fluid to flow easily across it, reducing drag resistance
How does streamlining affect terminal velocity
Less streamlined = lower terminal velocity
What happens to object moving through fluid
- initially accelerates due to gravitational force being greater than frictional force slowing it down
- speed increases - friction builds up
- friction reduces acceleration until frictional force equal to accelerating force - resultant force is 0
- terminal velocity has been reached - object will fall at steady speed
What can affect terminal velocity
- shape (streamlining)
- area
How does area affect terminal velocity
Objects with large surface area tend to have lower terminal velocity
Newton’s first law
- if resultant force on stationary object is 0, object will remain stationary
- if resultant force on moving object is 0, it will continue moving at same velocity
Inertia
Tendency of objects to continue in their state of rest or of uniform motion until acted upon by resultant force
Inertial mass
- measure of how difficult it is to change the velocity of an object
- f/a
Newton’s second law
Acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object
Newton’s third law
When 2 objects interact, the forces they exert on each other are equal and opposite
Mass/force/acceleration PRACTICAL
- clamp pulley to end of ramp, add mass + attach string to trolley, calc force acting on trolley (w=mg)
- set up light gates at top + bottom, measure distance between with ruler
- put 10cm card in trolley
- let trolley go - light gates record initial/final V + time taken
- calc change in V (final-initial)
- calc acceleration - a=ΔV÷t
- repeat trolley journey removing 1kg from trolley and adding to pulley for 5 values
- plot graph of f against a
Stopping distance
Sum of distance vehicle travels during driver’s reaction time (thinking distance) + distance it travels under braking force (braking distance)
Emergency stop
Maximum force is applied to brakes in order to stop car in shortest possible distance
Factors affecting thinking distance
- speed
- reaction time
Factors affecting breaking distance
- speed
- weather / road surface
- tyre condition
- brakes quality
How can weather / road surface affect braking distance
Wet/icy/oily road means less friction between tyres and road, can cause tyres to skid
How can tyre condition affect braking distance
Bald tryes (have no tread left) - can’t get rid of water in wet conditions, can skid on water
How can brakes quality affect braking distance
Worn/faulty brakes won’t apply as much force as well-maintained brakes
How do brakes work
- brake pedal pushed - brake pads pressed onto wheels
- contact causes friction, causing work to be done
- work done between brakes/wheels transfers energy from wheels’ kinetic store to brakes’ thermal store
When is greater braking force needed
When a vehicle is going faster and there is more energy in it’s kinetic store - more work needs to be done to stop it
Why can large braking forces be dangerous
Large braking force means large deceleration - can cause brakes to overheat of vehicle to skid
Typical range of human reaction times
0.2 s - 0.9 s
Factors affecting reaction time
- tiredness
- alcohol
- distractions
How to measure reaction time
- do ruler drop test multiple times and take average distance
- calc change in velocity of ruler with v² - u² = 2as
- calc reaction time with a=Δv ÷ t
Momentum quantity
Vector
Conservation of momentum
In closed system, total momentum before event is equal to total momentum after event
Ways of deforming an object
- stretching
- crompressing
- bending
