Lecture 2: Force & Newton's laws of motion

Question 1

What are Newton’s three laws?

Answer 1

1. any object continues at rest or constant velocity unless acted upon by an external force
2. external forces cause acceleration proportional to force applied & proportionality of mass of object
   (F = ma)
3. every force has an equal and opposite reaction force

Question 2

define net force

Answer 2

the vector sum of all individual forces acting upon an object (i.e. the sum of each action-reaction pair or third-law force pairs)

Question 3

define system of interest

Answer 3

objects whose behaviour we are interested in & net forces acting on them

Question 4

3 fundamental forces

Answer 4

1. electromagnetic force - force holding atoms & molecules together; responsible for physicals properties of objects; results in chemical behaviour
2. gravitational force - acts upon everything on earth
3. strong & weak nuclear forces - maintain structure of atomic nucleus & release radiation

Question 5

2 derived forces

Answer 5

1. tension - exist in the body of a flexible cable/line; no lateral forces are supported; inelastic; exists when forces are applied in opposite directions at each end
2. normal force & friction - electrical forces between atoms of surface & object; contact forces; normal = perpendicular to surface; friction = parallel to surface

Question 6

formulae for friction and normal force when stationary & at critical angle

Answer 6

If stationary - N = mg cosθ & f = mg sinθ
f = N tanθ
critical angle - fmax = μmaxN → μmax = tanθ

Question 7

Coefficient of kinetic friction vs coefficient of static friction

Answer 7

f = μN
coefficient of kinetic friction < coefficient of static friction

Question 8

Define drag in fluid

Answer 8

force opposing motion of an object through a fluid
magnitude depends on viscosity of the fluid

Question 9

explain and give the formula for drag in air

Answer 9

f = kv^2
constant of proportionality, k, is influenced by the shape of the object & density of air

Question 10

Explain Newtonian gravity

Answer 10

gravity exists between two bodies due to their mass & is proportional to the separation between the COM of these bodies
F = Gm1m2/r^2
G = 6.67 x 10^-11 Nm^2kg^-2

Question 11

Define and give an example of fictitious forces

Answer 11

an apparent force that acts on all masses whose motion is described using a non-inertial frame of reference
e.g centrifugal force - force felt pushing back into seat when turning a corner in a car

Question 12

what is a inertial reference frame?

Answer 12

a frame of reference which is not undergoing any acceleration and F = ma only holds true in an inertial reference frame