Physics P5

• Have a magnitude and a direction
• Include - force, velocity, displacement, acceleration, momentum
• Represented by an arrow

• Only have a magnitude and no direction
• Include - speed, distance, mass, temperature, time

• When two objects have to be touching for a force to act
• Contact forces = friction, air, resistance, tension in ropes, normal contact force, etc

• If the objects do not need to be touching for the force to act
• Non-contact forces = magnetic forces, gravitational force, electrostatic force, etc

• A pair of forces that are equal and opposite and act on two interacting objects (basically Newton's Third Law)

• Attracts all masses
• Important effects - makes all things fall towards the ground and gives everything a weight

• Mass = amount of 'stuff' in an object, same mass whether on earth or the moon and it is not a force (measured in kg with a mass balance)
• Weight = force acting on an object due to gravity depends on the gravitational field at the location and it is a force measured in Newtons by a calibrated spring (newton metre)

Weight (N) = Mass (kg) x Gravitational field strength (N/kg)

The gravitational field strength of the earth = 9.8N/kg

• Mass and weight are directly proportional

• When a force moves an object through a distance, energy is transferred and work is done on the object

Work done (J) = force (N) x distance (moved along the line of the force) (m)

• Use scale drawings to find resultant forces
• An object is in equilibrium if the forces on it are balanced
• You can split a force into components

• Applying a force may cause it to stretch, compress or bend
• Work done = force stretches or compresses an object - if it is elastically deformed - all energy transferred to the elastic potential energy store
• elastically deformed = the object can go back to its original shape

• The extension of a stretched spring (or another elastic object) is directly proportional to the load or force applied

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

• There's a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionality
• When the force is great enough, the extension and force are not directly proportional

• As long as a spring is not stretched past its limit of proportionality, the work done in stretching (or compressing) a spring can be found using

elastic potential energy (J) = 1/2 x spring constant (m) x extension squared (N/m)

• Distance = how far an object has moved and it is a scalar quantity that doesn't involve direction
• Displacement = vector quantity and measures the distance and direction in a straight line (starting point)

• Speed and velocity both measure how fast you're going, but speed is a scalar quantity and velocity is a vector quantity
• Speed is just how fast you're going with no regard to the direction. Velocity is speed in a given direction.

distance travelled (m) = speed (m/s) x time (s)

s = v x t

• A person walking = 1.5 m/s
• A person running = 3 m/s
• A person cycling = 6 m/s
• A car = 25 m/s
• A train = 30 m/s
• A plane = 250 m/s

• Not the same as velocity or speed
• Change in velocity in a certain amount of time

acceleration = change in velocity / time

deceleration = negative acceleration

final velocity - initial velocity = 2 x acceleration x distance

• Friction is always there to slow things down
• Drag increases as speed increases
• Objects falling through fluids reach a terminal velocity
• Terminal velocity depends on shape and area (streamline)

• 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 (same speed and direction)

• The larger the resultant force acting on an object, the more the object accelerates. The force and the acceleration are directly proportional
• Acceleration is also inversely proportional to the mass of the object 
• Newton's second law = resultant force = mass x acceleration

• Inertia is the tendency for the motion to remain unchanged
• An object's inertial mass measures how difficult it is to change the velocity of an object
• Inertial mass can be found using Newton's Second Law rearranged into m = F / a

• Newton's Third Law says - When two objects interact, the forces they exert on each other are equal and opposite

stopping distance = thinking distance + braking distance

typical car braking distances - 14m at 30mph, 55m at 60mph and 75m at 70 mph

• Your speed
• Your reaction time

• Your speed
• The weather or road surface
• The conditions of your tyres
• How good your brakes are

• Everyone's reaction time is different, but a typical reaction time is between 0.2 and 0.9 seconds. This can be affected by tiredness, drugs or alcohol
• Distractions can also affect your ability to react

• Momentum is mainly about how much 'oomph' an object has (it is a vector quantity)
• greater the mass/velocity = more momentum
• momentum = mass x velocity
• momentum before = momentum after