A.2 Forces and momentum

Question 1

Forces

Answer 1

Push or pull vectors, represented by arrows

Question 2

Resultant force (unbalanced/net force)

Answer 2

Sum of all the forces acting on an object
Found using pythagoras/trig

Question 3

Resolving forces

Answer 3

When single forces are broken down into component forces
e.g. F => Fsin# and Fcos#

Question 4

Newton’s Law 1

Answer 4

“An object continues in uniform motion in a straight line or at rest unless a resultant external force acts on it”
*resultant force = acceleration

Question 5

Translational equilibrium

Answer 5

If resultant force = 0, no acceleration

Question 6

Newton’s 2nd Law

Answer 6

F = ma
“The resultant force on an object is proportional to the acceleration providing the mass of the object remains constant”

Question 7

Newton’s 3rd Law

Answer 7

“For every action on one object there is an equal but opposite reaction on another object”
i.e. forces come in pairs
e.g. collision of balls, planets orbiting

Question 8

Contact forces

Answer 8

Require objects to be in physical contact
e.g. Fn, Ff, Fd, Fb, Ft, Fh

Question 9

Field forces

Answer 9

Can be at a distance, no contact needed
e.g. Fg, Fe, Fm

Question 10

Normal Force (Fn)

Answer 10

Force perpendicular to the surface an object is resting on that pushes back on the force of an object pushing on the surface

Question 11

Surface Frictional Force (Ff)

Answer 11

Opposes the relative motion between the surfaces of two solid objects
Produced because no surface is perfectly smooth

Question 12

Static Frictional Force (us)

Answer 12

Equal to magnitude of applied force in opp. direction

Occurs when no relative motion between surfaces
i.e. when force applied to an object resting on a surface but not large enough to move the object

Max. depends on nature of surface and magnitude of normal force actng on object

Question 13

Static Friction Coefficient (us)

Answer 13

= tan#

Question 14

Dynamic Frictional Force (ud)

Answer 14

When force applied to object exceeds max. static friction value and object begins to slide

Magnitude of dynamic friction force < max. static value

Independent of relative speed, constant value

Question 15

Viscous Drag Force (Fd)

Answer 15

Drag force acting on a moving object due to the viscosity of the fluid through which it is moving

Size depends on…
- objects size, shape, cross-sectional area, nature of surface, and speed
- fluid’s nature

Question 16

Viscosity (n)

Answer 16

Resistance of a fluid to movement through it, affected by temperature and units of Pas (pascal seconds)

Question 17

Stoke’s Law

Answer 17

Fd = 6(pi)nrv
n = viscosity
r = radius (m)
v = velocity of object

If object is a small, smooth, sphere it has laminar flow

Question 18

Buoyancy (Fb)

Answer 18

Ability of a fluid to provide a vertical upwards force on an object placed in/on it

As density of fluid increases, buoyancy force increases

Fb = pVg

Question 19

Buoyancy scenarios

Answer 19

Size = weight of fluid displaced
Object will sink until it displaces its own weight
If floating => Fg = Fb

Question 20

Terminal velocity

Answer 20

Fall at a constant speed
Fg = Fb + Fd
*Fd and Fg constant, Fb increases as velocity increases

Question 21

Tension Force (Ft)

Answer 21

Pulling force which causes objects to stretch

Question 22

Elastic Restoring Force

Answer 22

Restoring force exerted by object to return object to its equilibrium position when it has been stretched

Elastic if object returns to its original shape after tension removed

Question 23

Hooke’s Law (Fh)

Answer 23

Magnitude of restoring force is proportional to extension (increase in length)

Fh = -kx
k = spring constant (Nm-1), as k increases, the “stiffness” of object increases
x = extension (m)
*negative sign because restoring force in opp. direction to extension

Question 24

Elastic limit

Answer 24

Max. extension beyond which an object becomes permanently deformed

Question 25

Field

Answer 25

Region of space where an object experiences force due to property e.g. mass/charge Acts across space

Question 26

Gravitational Force (Fg)

Answer 26

Fg = mg Region of space where an object experiences gravitational force due to its mass

Question 27

Normal force in a lift

Answer 27

Stationary/constant speed: Fnet = a = 0 N = W = mg => Normal Accelerating up: N > W Fnet incr.=a incr. =ma incr. N - W = ma N = m(g + a) => Heavier Accelerating down: W > N Fnet decr.=a decr.=mg decr. W - N = ma N = m(g - a) => Lighter Cable cut, free falling: W - N = mg N = 0 => Weightless

Question 28

Joined masses calculations

Answer 28

a = mg / M + m (due to same tension force and mg downward, Ma right) *Frictionless table

Question 29

Linear momentum (p)

Answer 29

Product of mass and velocity of an object (vector) p = mv

Question 30

Momentum when external force applied

Answer 30

Linear momentum remains constant unless resultant external force (impulse) acts on it, changing momentum Impulse: J = F x change in t J = Impulse (Ns) F = Resultant external force (N) change in t = time for whuch force is applied (s) Newton's 2nd Law: F = ma = change in p / change in t *2nd law assumes mass is constant whereas p / t assumes mass is changing *If force changing, use average value

Question 31

Alternative calculations of impulse

Answer 31

Area below force-time graph

Question 32

Applications of impulse

Answer 32

- Large force, short period - Small force, long period

Question 33

Conservation of momentum of a system

Answer 33

"The total linear momentum of a system remains constant provided no resultant external force acts" => total Pbefore = total Pafter *add total momentum when calculating

Question 34

Energy and Collisions

Answer 34

Total energy is conserved in collisions, but may be converted from one form to another Ek = 1/2mv^2 OR Ek = p^2/2m

Question 35

Elastic collision

Answer 35

No Ek lost

Question 36

Inelastic collision

Answer 36

Ek lost because some converted to heat and sound. However, total amount of energy remains constant.

Question 37

Explosions

Answer 37

Can be treated as a collision, stationary system breaks apart. Total p is conserved = 0. Kinetic energy increases in explosions, coming from source of the explosion (e.g. chemical/elastic potential energy)

Question 38

Fuel

Answer 38

Source of energy e.g. coal, oil

Question 39

Circular Motion Linear speed formulae

Answer 39

v = 2(pi)r / T constant speed, radius r and take time/T period to complete one loop *one revolution = 2pi

Question 40

Angular velocity and displacement

Answer 40

w = # / t v = wr Calculated using angle through which object moves Angular velocity (v) (rads-1) Angular displacement (#) in degrees or radians

Question 41

Why does an object moving along a circular path at a constant speed accelerate?

Answer 41

- Direction of motion constantly changing => velocity constantly changing - Acceleration is the rate of change of velocity *acceleration towards centre of circle

Question 42

Centripetal force (Fc)

Answer 42

Unbalanced force that acts to create circular motion, always acts towards the centre F = mv^2 / r = mw^2r

Question 43

Horizontal centripetal force comes from...

Answer 43

Friction force between e.g. tyres and road (unbalanced horizontal frictional force towards centre, balanced vertical forces)

Question 44

If object undergoing horizontal circular motion exceeds max. frictional force...

Answer 44

Object continues at straight line tangent to the curve/bend

Question 45

Banked corners centripetal force from...

Answer 45

Horizontal component of the normal force Travels faster because sideways friction doesn't contribute to Fc

Question 46

Wall of Death

Answer 46

Steeper banking, increased angle, increases horizontal component => travel at higher speeds *must travel at high enough speed so Fn vertical component is balanced with Fg (diagram of extrapolated Fg up and Fn increasing to match)

Question 47

Banked corners alternative examples

Answer 47

Conical pendulum, plane turning

Question 48

Vertical circular motion Fn

Answer 48

Size of Fn varies throughout, speed not always constant. As speed increases, Fn increases At top, Fc = Fn + Fg => feel light At bottom, Fc = Fn - Fg => feel heavier

Question 49

Vertical circular motion examples

Answer 49

Mass on string Ft instead of Fn

Question 50

Calculations vertical circular motion

Answer 50

Minimum speed at which object still loops: v = sq root(rg) => Feel weightless at top if at minimum speed *independant of mass Energy conservation: Ek (bottom) = Ek (top) + Ep (top)