Topic 5 - Forces, Density and Pressure

Question 1

Forces

Answer 1

actions upon an object that results in an interaction between objects.

Question 2

Common type of forces. ( 6 )

Answer 2

• Gravitational forces
• Electrival forces
• Magnetic forces
• Frictional forces
• Drag forces / viscous drag
• Upthrust force / Buoyant force

Question 3

Gravitational Force

Answer 3

Region where mass experience a force in gravitational field.

• acts on all masses
• weakest force
• field lines are directed towards centre of m

Question 4

Electrical Force

Answer 4

attractive and repulsive interactions that acts on all charged particles

Question 5

Magnetic Forces

Answer 5

acts on magnetic materials / moving charges

Question 6

Frictional Forces

Answer 6

forces which opposes motion that are in contact with each other

Question 7

Drag force / Viscous drag

Answer 7

forces that oppose / prevent motion

• found in fluids
• directly proportional with speed
• also known as air resistance

Question 8

Upthrust / Buoyant Force

Answer 8

vertically acting upwards force on an object partially or fully immersed in fluid due to difference in pressure of fluid

• bottom pressure - top pressure

Question 9

Terminal velocity

Answer 9

Maximum constant velocity when an object is in motion in a fluid / resisting medium and net force is zero.

Question 10

How different masses varies the terminal velocity?

Answer 10

• higher mass - higher terminal velocity
• higher mass requires a larger air resistance to balance out the weight of the object
• higher velocity is needed to gain a larger air resistance
• hence results in a higher terminal velocity

Question 11

How energy is conserved in free fall at terminal velocity?

Answer 11

• Object experiences loss in GPE
• No gain in kinetic energy (constant velocity)
• Energy is used to do work against air resistance
• Heat increases - thermal vibration also increases
• Energy converted to internal energy, increase in internal energy

Question 12

Centre of gravity

Answer 12

(def.) a point where the whole weight / gravity is considered to act, regardless of its orientation.

• uniform shape
- at the geometrical centre

• non-uniform shape
- towards higher mass

Question 13

Pivot point

Answer 13

• stationary point

* if centre of g > pivot point, object will topples.

Question 14

Moment of a force

Answer 14

(def.) Turning effect of a force

Product of force and the perpendicular distance of pivot to the line action of force

• M = F x d
• clockwise & anti clockwise rotation

Question 15

System in equilibrium

Answer 15

TRANSLATIONAL EQUILIBRIUM
• Sum of forces in any direction = 0

ROTATIONAL EQUILIBRIUM
• Sum of moments about any point = 0

Question 16

Principle of Moments

Answer 16

(def.) for an object to be in equilibrium, the sum of moments about any point must be zero

enables us to :
• check whether object will rotate / remains balanced

• calculate unknown force or distance if the object is balanced

Question 17

Common point

Answer 17

Intersection points of all forces so that the sum of moments at this point is zero

Question 18

Couples

Answer 18

(def.) a pair of equal and oppositely directed paralled forces whose line of action does not coincide.

Question 19

Torque

Answer 19

(def. ) turning effect or moment of a couple
(def. ) product of one of the forces with the perpendicular distance between two forces

Torque = F (d1 + d2)

Question 20

Difference in moment and torque

Answer 20

Moment
• turning effect of ONE force
• affected by the perpendicular distance of force from PIVOT

Torque
• turning effect of a COUPLE
• affected by the perpendicular distance BETWEEN TWO FORCES

Question 21

Density

Answer 21

(def.) mass per unit volume

• constant for :

• solid
• liquid that is not compressed

• not constant if
- there’s change in T / V

Question 22

Pressure

Answer 22

(scalar quantity)

(def.) Force acting normally per unit surface area

Question 23

Pressure in liquid

Answer 23

• increases the with depth below its surface
• equal in all directions at the same depth
• proportional to the density
• depends on height of liquid only

P = pgh

Question 24

Pressure in gas

Answer 24

due to collision between particles and the wall of container
• change in momentum (N2L)
• impulses
• impulsive force is exerted on the wall (N3L)
• gas pressure produced

Question 25

Type of meters

• Barometer
• Manometer

Answer 25

Barometer : determines Patm

Manometer : determines Pgas
also known as U-tube