P2 - Forces Flashcards
What are the differences between a scalar and vector quantity?
- Scalar only has magnitude (size)
- Vector has magnitude and direction, eg forces (can be represented by arrows)
What are the differences between contact and non contact forces?
- Contact: objects are touching ( friction, drag, tension, up thrust)
- Non-contact: objects aren’t touching (gravity, electrostatic, magnetic)
What is gravity?
- force of attraction between all masses
- force of gravity close to earth is due to gravitational field around planet
What is the difference between mass and weight?
- Mass is the amount of matter an object contains and is constant
- Weight is the force acting on an object due to gravity, changes in different gravitational fields
- weigh and mass are directly proportional
What is a resultant force?
When more than one force acts on an object, it is a single force that is the overall result of all the other forces acting on the object
How is energy transferred when work is done on an object?
When work is done in a system, energy transfers take place eg work done to overcome friction increases heat energy
Describe what happens in terms of work done when overcoming forces
Overcoming forces requires energy
When a force moves an object, work is done on the object
This movement is called displacement
What is the difference between elastic and inelastic deformation?
Elastic deformation - will return to original shape once forces of compression, stretching or bending are removed (this requires more than one forces and must be balanced)
Inelastic deformation - do not return to original shape once forces are removed
What is a moment?
turning effect of a force, must be perpendicular
Describe a balanced moment (eg a seesaw)
Anti clockwise moment = clockwise moment
force x distance = force x distance
Why do some objects topple?
- Weight acts directly down from centre of mass due to gravity
- When the surface or object is tilted, the line of action of the weight (pointing down from centre of mass) will lie outside of the base of the object
- unbalanced / resultant moment acting on block so it topples
Explain how levers and gears transmits the turning effects of a force
- transmit turning effect (moment) of a force
- magnify size of force or distance force moves over
Describe pressure in a fluid
- fluid = liquid or gas
- particles move in fluid and collide with objects in fluid or surface of container
- create a force normal (right angle) to surface
- creates pressure
Explain why atmospheric pressure decreases as altitude increases
- greater the altitude, less dense the atmosphere and lower the atmospheric pressure
- at high altitudes, there is less air above a surface than at lower altitudes
- less weight of air acting on the same surface (pressure = force / area would result in lower pressure outcome)
What does pressure in a column of liquid depend on?
- height of column above the point
- density of liquid
The higher the column and more dense the liquid…
weight, force, pressure
- greater weight above the point
- greater force on the surface at that point
- greater pressure
How does density effect pressure exerted?
The deeper an object is submerged the greater the pressure
So a more dense liquid exerts a greater pressure
Define upthrust
upwards force exerted by a fluid on a submerged object
greater height of liquid above bottom surface than top surface
What happens in terms of upthrust when an object floats?
weight = upthrust (not effected by density)
What happens when an object is less dense than a liquid?
- displace a volume of liquid greater than its own weight
- rises to surface
- floats with some object remaining below surface
- liquid displaced = weight of object
- the lower the density the more of the object stays above surface
What happens when an object sinks?
- denser than surrounding liquid
- can’t shaped enough liquid to equal own weight
What is the difference between distance and displacement?
- distance is scalar
how far an object moves, doesn’t take in to account direction of object or if it ends up back where it started - displacement is vector
it has magnitude, which describes how far object has travelled from origin
it has direction (direction of the straight line between start and end)
What variables effect speed?
age, fitness, terrain, distance
When travelling in a straight line, what is an objects velocity?
in a straight line, an object has constant speed and velocity
What happens to speed and velocity when an object isn’t travelling in a straight line (eg round a corner)
- speed can still be constant as it is scalar
- velocity changes because direction has changed (its is a vector that takes direction in to account)
What happens to velocity and acceleration when an object is travelling in a circle?
- constantly changing direction so constantly changing velocity
- it is accelerating even if speed is constant
(eg orbiting planets where gravity makes the object accelerate)
What is Newton’s first law?
an object will remain in the same state of motion unless acted on by an external force
When resultant force acting on an object is 0, what happens?
- if object is stationary, it will remain stationary
- if object is moving it will continue to move at the same speed and same direction (constant velocity)
What is the tendency for objects to continue in the same state of motion called?
inertia
What causes velocity to change? (speed or direction)
external force acting
When a car is travelling at a steady speed, what balances the driving forces?
resistive forces (eg friction)
How is speed found on a distance-time graph?
gradient
How is a stationary line show on a distance time graph?
Horizontal line
In a d-t graph, the faster the speed…
steeper the gradient
What does a distance time graph looking like when an object is accelerating?
curved line
How is speed of an accelerating curve found?
at a particular time, a tangent can be drawn and gradient calculated
Define acceleration
how quickly an object speeds up, slows down or changes direction
What does the gradient of a velocity time graph show?
acceleration
What is Newton’s second law?
acceleration of an object is proportional to resultant force acting on an object
and inversely proportional to mass of object
Define mass
- measure of inertia
- describes how difficult it is to change velocity (m=f/a)
- larger mass, bigger force needed to change velocity
Explain terminal velocity in a fluid
- object accelerates due to force of gravity
- as it speeds up, resistance increases
- result force reaches 0 when resistance and gravity are balanced
- object falling at steady speed = terminal velocity
What is the accelerating due to gravity near Earth’s surface?
10m/s2
Explain the forces acting on a skydiver as it falls through the air
- skydiver accelerates due to force of gravity
- skydiver experiences frictional forces
- weight is greater than resistance so skydiver continues to accelerate
- speed and resistance increase as acceleration decreases
- when resistance = weight and resultant forces =0, skydiver falls at terminal velocity
How is terminal velocity represented in a velocity time graph?
speed gradient
curves as resistance increases but still accelerating
levels off at terminal velocity before parachute opens
Explain Newton’s third law
for every action there is an equal and opposite reaction
- when one object exerts a force on another, the other object exerts a force back
- reaction force is of same type and equal in size, but opposite direction
When does a change in momentum occur?
- All moving objects have momentum
- when an unbalanced force acts on a object that is moving or unable to move, momentum changes
What is momentum?
product of mass and velocity
force is the rate of change of momentum
Explain why momentum should be considered in safety devices?
- safety devices reduce force by increasing time over which change of momentum takes place
e. g - gym crash mats cushion fall impacts increasing time it takes for someone to come to rest when they fall on to the floor
How is momentum conserved in a closed system?
total momentum before an event = total momentum after event
can be seen in collisions
What does a vehicles stopping distance depend on?
- thinking distance (distance travelled during reaction time)
- braking distance (distance travelled under braking force)
What is the typical human reaction time?
- 2-0.9s
- a car travelling at 30m/s will travel 12-27m before braking starts
Which factors can effect reaction time?
tiredness, drugs, alcohol, distraction (eg phone)
How can reaction time be measured?
- use light or sound signal and time how long it takes for someone to react
- in a classroom - drop ruler vertically and catch as it falls - distance ruler drops can be used to calculate reaction time
What factors can effect braking distance?
road conditions, vehicle conditions (worn breaks and tyres, over and under inflated tyres) and weather (wet, icy, snowy roads)
How to brakes stop a vehicle?
apply force to the wheels
The greater the braking force…
greater the deceleration of the vehicle
Why does brake temperature increases during braking?
- work done by frictional force
- transfers kinetic energy of vehicle in to heat energy
What happens when braking force is too large?
- brakes overheat
- tyres loose traction which leads to skidding
- more likely if brakes or tyres are in poor condition
For a given braking distance, what happens when
a) mass is doubled
b) speed is doubled
- doubling speed doubles force required
- doubling speed quadruples force required
