Forces

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Created by
Hafsa Farouk

Cards (40)

  • • Scalar – magnitude only
    o E.g. mass, distance, speed, temperature
    • Vector – magnitude & direction
    o E.g. velocity, displacement, acceleration
    o Represented as an arrow – length represents magnitude & direction represents the direction
  • • Contact – objects physically touching
    o E.g. friction, air, resistance, tension (e.g. in rope), normal contact force
    • Non-contact – objects are physically separated
    o E.g. magnetic force, gravitational force (weight), electrostatic force
  • o One joule of work is done when a force of one newton causes a displacement of one metre
  • • Elastically deformation – returns to its original shape & length once force removed
    o Objects that can be elastically reformed are called elastic objects
    o All the energy is transferred to the elastic potential energy store
    • Inelastic deformation – the object doesn’t return to its original shape & length once force removed
  • • Hooke’s Law: In an elastic object (e.g. a spring): F ∝ e (extension is directly proportional to force applied)
    o Extension = extended/compressed length – natural length
    o Force (N) = spring constant (N/m) × extension (m)
    o F = K e
  • • F ∝ e until the limit of proportionality
    • For elastic deformation, this is the amount of energy transferred to spring as it is deformed/compressed
    • The energy in the elastic potential energy store of a stretched spring is equal to the total area under a force-extension graph up to that point
  • RPA 6: investigate the relationship between force & extension for a spring
    Method:
    1. Setup the equipment as shown to the right – a retort stand holding a spring, weights on the end of the spring & a ruler on the side of the stand
    2. Measure the natural length of the spring (no load applied) – mark this on the ruler
    3. Add a mass to the spring, record the mass & measure the extension
    4. Repeat the process until you have at least 6 readings
    5. Plot a force-extension graph, it will start straight & then it may curve if the limit of proportionality is reached
  • This turning effect of force is called a moment
    • Example of use of moments
    o The force on the spanner causes a turning effect/moment on the nut (the pivot) – a larger force or a longer spanner would cause a larger moment
    o To achieve the maximum moment, you need to push perpendicular to the spanner – pushing at any other angle decreases the distance & means a smaller moment
  • • Levers – make doing work easier
    o Levers increase the distance from the pivot at which force is applied
    o Therefore, less force required to get the same moment
    o Making it easier to do work e.g. lift a load
  • • Gears – transmit rotational effect of force
    o Gears – circular disks with ‘teeth’ around the edges
    o Teeth interlock so that turning one causes another to turn (in opposite direction)
    o Transmitting the rotational effect of force
    o Different sized gears can be used to change the moment of force
    ▪ A larger gear transmits a larger moment as the distance to the pivot is greater
    o The larger gear will turn slower
  • • Fluid - liquid or gas, substances that can flow are particles are able to move
    • Pressure in fluids cause a force normal to any surface (at a right angle) – calculated with the equation
  • o Density
    ▪ Higher density means more particles in a smaller area, increasing the frequency of collisions
    ▪ More collisions means higher pressure
    o Height
    ▪ As the depth of a liquid increases, the number of particles above a point increase
    ▪ The weight of the particles adds to the pressure felt by the point
  • • Upthrust
    o When an object is submerged in a fluid, pressure of the fluid exerts a force on it from every direction
    o Pressure increases with depth, so the force exerted at the bottom of the object is greater than the force
    acting on the top of the object
    o This causes a resultant force upwards called up thrust
    o The upthrust is equal to the weight of the displaced fluid
    • If upthrust on an object is equal to the object’s weight, the forces balance – the object floats
    • If an object’s weight is more than the upthrust – the object sinks
  • • Floating
    o A less dense object than the fluid weights less than the equivalent volume of fluid
    o So, it displaces a volume of fluid larger than the upthrust before being fully submerged
    o At this point, the forces balance & the object floats
  • • Sinking
    o An object denser than the fluid it is placed in is unable to displace enough fluid to equal its weight
    o This means its weight is always larger than the upthrust & it sinks
  • • Submarines
    o To sink, tanks fill with water to increase the weight – so it is more than the upthrust
    o To rise, tanks fill with compressed air to decrease the weight – so it is less than the upthrust
  • The apple floats as it is less dense than the water
    The apple has a displaced volume of water equal to its weight so it floats
    The potato sinks as it is denser than the water
    The potato can never displace a volume of water equal to its weight so it sinks
  • Atmospheric pressure
    • The atmosphere is a thin layer (relative to Earth’s size) of air around the Earth
    • Its density decreases as altitude increases
    o Air molecules colliding with a surface create atmospheric pressure
    o As altitude increases the atmosphere’s density decreases, so there are fewer air molecules to collide
    o So as height increases there are fewer air molecules above – so the weight of the air above it decreases
  • • Distance – scalar
    o How far an object moves
    o Doesn’t involve direction, only magnitude
    • Displacement – vector
    o Magnitude - how far the object moved measured in a straight line from the start to finish point
    o Direction – bearing of the straight line
  • • Speed – scalar
    o Speed of a moving object is rarely constant e.g. objects moving, speed of sound, wind speed
    o Speed at which someone can walk/run/cycle depend on many factors
  • • Velocity – vector
    o Speed in a given direction
    o e.g. 30m/s at 060° or 30mph north
    • Motion in a circle
    o Speed remains constant
    o Velocity changing (so accelerating
    o Resultant force is towards the centre
  • • You can find speed at a specific point during acceleration/
    deceleration by drawing a tangent & measuring the gradient of the
    tangent
  • o The area under any section of graph is the distance travelled or displacement
    ▪ If irregular (e.g. a curve) you can count squares & multiply by area of squares
  • • Friction
    o If an object isn’t constantly supplied with force it will slow down due to friction
    o Friction acts in the opposite direction of movement
    o To travel at a steady speed friction & the driving force must be balanced
    o Friction is caused by
    ▪ Two surfaces in contact
    ▪ An object passing through a fluid (drag)
  • • Drag – resistance to movement in a fluid (air/water resistance)
    o Frictional forces increase with speed, so as speed increases much more force is required to maintain a
    steady speed
    o Reducing drag involves streamlining, allowing the fluid to flow easily across the object, reducing drag
    o Parachutes do the opposite – they have a huge surface area to create huge amounts of air resistance
  • • Terminal velocity
    o An object falling through a fluid initially accelerates due to the force of gravity (weight)
    o Eventually, the resultant force will be zero (between friction & weight) & the object will move at its terminal velocity
  • • Sky diver with a parachute
    o Accelerating force of gravity on object would be equal for all objects if there was no air resistance
    o Air resistance causes things to fall at different speeds, terminal velocity is determined by the drag in comparison to its weight (drag depends on shape & area)
    o A skydiver without a parachute has as small area & a force of W = mg pulling him down whereas with a parachute there is a lot more air resistance with the same amount of driving force
    o The terminal velocities are 120 mph (without) vs 15mph (with)
  • Newton’s First Law
    • If the resultant force acting on an object is zero and ...
    o The object is stationary – the object remains stationary
    o The object is moving – the object continues to move at the velocity (speed & direction)
    • When a vehicle travels at a steady speed the frictional forces are balanced with the driving force
    • The velocity (speed and/or direction) of an object will only change if a resultant force is acting on an object
    o On a free body diagram, the arrows will be of unequal size
    • Inertia – the tendency of objects to continue in their state of rest or uniform motion
  • Newton’s Second Law
    • Acceleration of an object is...
    o Proportional to the resultant force acting on an object (a ∝ F)
    o Inversely proportional to the mass of the object (a ∝ 1m)
    o As an equation
    ▪ Resultant force (N) = mass (kg) × acceleration (m/s2)
    ▪ F = m a
    • Inertial mass – measure of how difficult it is to change the velocity of an object
    o Derived from F = m a
    o Inertial mass is the ratio of force over acceleration
  • Newton’s Third Law
    • Whenever two objects interact, the forces they exert on each other are equal & opposite
    • Examples
    o Car tyre on a road - contact forces
    ▪ Tyre pushes the road backwards
    ▪ Road pushes the type forwards
    o Satellite in Earth orbit – non-contact gravitation forces
    ▪ Earth pulls the satellite
    ▪ The satellite pulls the Earth
  • • Newton’s 3rd Law & equilibrium
    o E.g. a man pushing a wall, as the man pushes the wall there is the normal contact force acting back on him (equal size)
    o But a book on a table is in equilibrium (between the weight of the book & normal contact force)
    ▪ This is not Newton’s 3rd Law though because they are both acting on the book (& are different types – non-contact & contact)
  • • Thinking distance - distance the vehicle travels during the driver’s reaction time
    • Breaking distance - distance the vehicle travels under the braking force
    o Stopping distance = thinking distance + breaking distance
    o For a given breaking force, the greater the speed, the greater the stopping distance
  • • Reaction times vary from person to person between 0.2s & 0.9s
    • Factors affecting reaction time
    o Tiredness, drugs, alcohol & distractions
    • Thinking distance is affected by
    o Speed - faster you are going, the further you will travel during the time it takes to react
    o Reaction time – the longer it takes you to react, the further you will travel
    • Measuring reaction time
    o Dropping a ruler, the larger the measurement on the ruler the slower your reaction time
    o A computer, very accurate – e.g. pressing a button as the screen changes colour
  • Factors affecting breaking distance
    o Weather conditions & adverse road conditions, i.e. wet or icy conditions
    ▪ This leads to less grip & less friction between the types & road, can lead to skidding
    o Poor condition of tyres
    o Poor condition of brakes
    ▪ If brakes are worn or faulty, they won’t be able to apply as much force – so harder to brake quickly in emergencies
    o Speed
    ▪ the faster a vehicle travels, the longer it takes to stop, at a given braking force
  • • Forces involved in breaking
    o Energy transfers
    ▪ when the brake pedal is pushed the brake pads press onto the wheels – causing friction, which causes work to be done
    ▪ this work done transfers energy from the kinetic energy stores of the wheels to the thermal energy stores of the brakes
    ▪ so the kinetic energy the vehicle (its speed) decreases & the temperature of the brakes increases
    o the greater the kinetic energy store of the vehicle the greater the breaking force required to stop the vehicle (more work needs to be done to reduce the kinetic energy store of the vehicle)
  • • Large decelerations
    o The greater the breaking force, the greater the deceleration of the vehicle
    o These are very dangerous because they lead to
    ▪ Brakes overheating (damaging & worsening their effectiveness)
    ▪ Loss of control (e.g. skidding)
  • • Momentum is a vector quantity
    o The greater the mass of an object & the greater its velocity the more momentum the object has
  • • conservation of momentum means...
    o in a closed system (no external forces) the total momentum before an event is equal to the total momentum after the event
  • • If momentum changes very quickly e.g. a car crash, the body experiences huge force (leading to injury)
    • Cars – many safety features:
    o Crumple zones, crumple on impact increasing the time taken for the car to stop
    o Seat belts, they stretch, increasing the time taken for the wearer to stop (more time means less force)
    o Air bags, inflate before you hit the dashboard, compressing air inside it slows you down more if you just hit the dashboard
  • • Bike helmets
    o Contain a crushable layer of foam, this lengthens the time it takes for your head to stop in a crash, reduces impact on your brain
    • Crash mats & cushioned surfaces for playgrounds
    o Increase the time taken for you to stop if you fall, this is because the material is compressible
    • These are all designed to slow people down over a longer period of time
    o The longer it takes for a change in momentum, the smaller the rate of change of momentum – so a smaller force is felt by the body – less chance of injury