forces

• elasticity:

  to change the shape of an object, more than one force must be applied

  if the object returns to its original shape, it has been elastically deformed, if it does not return to its original shape, it has been inelastically deformed

  the extension of an object is directly proportional to the applied to the applied force

  one the limit of proportionality has been exceeded, the relationship becomes non-linear and will no longer plot a straight line on a graph

  
• moment (NM) = force (N) x distance (M)

  levers and gears can be used to transmit the rotational effects of forces and to magnify either the size of the applied force or the distance it moves over
• atmospheric pressure:

  the atmosphere is a layer of air around the earth

  a greater altitude = less dense atmosphere = lower atmospheric pressure

  higher altitude = less air above a surface than at a lower altitude, so there is a smaller weight of air acting on the surface

  pressure = force/area
• upthrust - when an object is submerged in a liquid, there is a greater height of liquid above the bottom of the surface than the top surface. the bottom surface experiences a greater pressure than the top surface, creating a resultant force upwards (upthrust). an object floats when its weight is equal to the upthrust and sinks when its weight is greater than it. an object less dense than the liquid displaces a volume of liquid greater than its own weight, rising to the surface. an object denser than the surrounding liquid cannot displace enough liquid equal to its own weight, so sinks.
• scalar quantities only have a magnitude, eg mass, volume, distance, speed. it is only how far an object moves, not the direction or if it ends up where it started.
• vector quantities have a magnitude and direction, eg displacement, forces, movement, time, velocity. they describe the distance an object has travelled from its origin, measured in a straight line, and the direction it has travelled in.
• speed is a measure of how fast an object is moving, measured in metres per second
• velocity is the speed of an object in a given direction.
• when travelling in a straight line, an object with a constant speed also has a constant velocity. if it isn't, eg turning a corner, the speed will be constant but the velocity will change. an object moving in a circle is constantly changing velocity, so it is accelerating even if at a constant speed, eg orbiting planets
• newton's first law states that an object will remain in the same state of motion unless acted on by an external force.
• inertia is the tendency for objects to continue in the same state of motion.
• newton's second law states that the acceleration of an object is proportional to the resultant force acting on it, and is inversely proportional to the mass of the object. if the resultant force is doubled, the acceleration will be doubled, if the mass is doubled, the acceleration will be halved.

  force = mass x acceleration
• newton's third law states that for every action there is an equal and opposite reaction. this means that when one object exerts force on another, the other object exerts a force back, of the same type and equal in size, but opposite in direction.
• distance-time graphs are used to show the motion of an object travelling in a straight line. the speed of the object is found from the gradient of the line.

  A) greater constant speed

  B) constant speed

  C) stationary

  D) accelerating

  E) returning to start

  
• the gradient of velocity-time graphs can be used to find an object's acceleration. the total distance is equal to the area under the graph.
• terminal velocity:

  when an object falls through a fluid, it accelerates at first due to the force of gravity, but as it speeds up, the resistive forces increase

  the resultant force equals zero when the resistive forces balance the force of gravity - the object now falls at a steady speed called its terminal velocity
• terminal velocity for a skydiver - example:

  the skydiver accelerates due to the force of gravity

  the skydiver experiences frictional force due to air resistance, but weight is still greater than the resistive forces so they continue to accelerate

  speed and resistance increase and acceleration decreases

  resistance increases until it is the same as weight

  the resultant force is now 0 and the skydiver falls at terminal velocity
• stopping distance relies on thinking distance (the distance travelled during the driver's reaction time) and braking distance (the distance travelled under the breaking force). the greater the speed of the vehicle, the longer the stopping distance.

  stopping distance = braking distance + thinking distance
• the typical reaction time of a human is 0.3 - 0.9 seconds. this can be affected by tiredness, drugs, alcohol, or distractions
• the braking distance can be affected by the condition of the vehicle (condition of tires or brakes) and the weather, eg wet or icy roads
• pressure = force/area

  in liquids, pressure increases as the depth increases, and acts in all directions.
• using SUVAT:

  S=displacement (m)

  U=initial velocity (m/s)

  V=final velocity (m/s)

  A=acceleration (m/s²)

  T=time (s)

  V=s/t

  a=change in V/t
• hooke's law:

  force (N) = spring constant (N/m) x extension (m)

  elastic potential energy (J) = 1/2 x spring constant x extension² (m)
• momentum (a vector quantity) is the product of an object's mass and velocity.

  momentum (kgm/s) = mass (kg) x velocity(m/s)

  f=mv-mu/t

  (this shows the force involved equals the rate of change of momentum)
• investigating force on a spring practical:

  set up the equipment as shown

  measure the initial length of the spring

  add 100g (1N) to the mass holder

  measure the extension of the spring and record the result

  repeat this process for a range of masses from 1N to 10N

  record results in a table and plot a graph of force against extension and draw a line of best fit.

  hazards/risks - masses falling on the experimenter's feet. keep masses to a minimum and apparatus on a stable surface to minimise the risk.

  A) ruler

  B) spring

  C) mass holder

  D) masses

  E) clamp and stand

  
• investigating mass, force and acceleration practical:

  set up the equipment as shown

  release the trolley and use light gates to take the measurements needed to calculate acceleration

  move 100g from the trolley onto the mass holder

  repeat this process until all masses have been moved

  A) pulley

  B) trolley

  C) string

  D) bench

  E) mass holder