physics - forces & their effects (9.1 - 9.10)

9.1 why do some objects interact at distance without contact?

some forces act at distance with no contact due to a field

force field

space around an object where it can affect other objects

9.1 how objects interact - at distance without contact: gravitational fields

gravity occurs between 2 objects with mass

moon & earth affect each other - moon in earth’s gravitational field & vice versa

9.1 how objects interact - at distance without contact: electrostatic fields

object charged with static electricity has electric/electrostatic field around it - affects objects in it

2 close same charge objects produce pair of forces - same size, act in opposite directions

9.1 how objects interact - at distance without contact: magnetic fields

magnetic field = space around magnet where it affects other materials

magnet attracts objects made from magnetic materials

magnet can attract/repel another magnet

9.1 why do some objects interact by contact?

objects touching = contact forces between them

9.1 how objects interact - by contact: normal contact force

e.g. standing on floor:

• upwards force from floor on you = normal contact force

9.1 how objects interact - by contact: friction

e.g. boat on water:

• needs engine to keep moving - water resistance (form of friction) slows it down

• force from engine & water resistance = contact forces

9.1 how objects interact - producing pairs of forces

e.g. gravitational forces between 2 objects with mass:

• can be represented as vectors (arrows showing direction & magnitude)

• these 2 forces = action-reaction forces

action-reaction forces

pairs of forces acting on diff. objects in opposite directions

9.2 vector vs scalar quantities

vector: magnitude & direction - e.g. weight, displacement

scalar: magnitude - e.g. mass, distance

9.3 vector diagrams - finding resultant force

1. draw force arrows to scale at correct angles

2. draw lines to make parallelogram

3. resultant force = diagonal of parallelogram - measure arrow to find size of resultant force

9.3 vector diagrams - resolving forces

1. draw force arrow to scale at correct angle

2. draw rectangle with sides in directions needed (e.g. horizontal & vertical)

3. sides of rectangle = resolved forces - measure arrows to find size of forces

9.4 free body force diagrams

direction of arrow = direction of force

length of arrow = size of force

9.5 forces acting on isolated solid object/system

several forces lead to resultant force on object

9.5 balanced forces

forces equal in size & opposite in direction:

• resultant force = 0

• forces on object balanced

moment definition

turning force

what does moment of force depend on?

size of force

where force applied

9.6 situations where forces can cause rotation

e.g. pivot:

• fixed point object rotates around

9.7 moment of a force equation

moment of a force (N m) = force (N) x distance normal to direction of force (m)

M = fd

9.8 principle of moments - rotational forces in equilibrium

sum of clockwise moments = sum of anti-clockwise moments

9.9 how do levers transmit rotational effects of forces?

bar that pivots about a point

transfers force

9.9 how do gears transmit rotational effects of forces?

rotation passed from one gear → next by interlocking teeth

9.10 ways of reducing unwanted energy transfer - lubrication

reduces friction between moving parts - less heat lost