4.1 Forces, Density, Pressure

Question 1

Force

Answer 1

Rate of change of momentum

Question 2

Density

Answer 2

Mass per unit of volume of a substance

Question 3

Pressure

Answer 3

Force per unit area

Question 4

Finding resultant (nose to tail)

Answer 4

• By accurate scale drawing
• Using trigonometry

Question 5

Forces on masses in gravitational fields

Answer 5

A region of space in which a mass experiences an (attractive) force due to the presence of another mass.

Question 6

Forces on charge in electric fields

Answer 6

A region of space where a charge experiences an (attractive or repulsive) force due to the presence of another charge

Question 7

Upthrust

Answer 7

An upward force exerted by a fluid on a submerged or floating object

Question 8

Origin of Upthrust

Answer 8

• Pressure on Bottom Surface > Pressure on Top Surface
• ∴ Force on Bottom Surface > Force on Top Surface
• ⇒ Resultant force upwards

Question 9

Frictional force

Answer 9

• Force that arises when two surfaces rub
• Always opposes relative or attempted motion
• Always acts along a surface
• Value varies up to a maximum value

Question 10

Viscous forces

Answer 10

• A force that opposes the motion of an object in a fluid
• Only exists when there is motion.
• Its magnitude increases with the speed of the object

Question 11

Centre of gravity

Answer 11

Point through which the entire weight of the object may be considered to act

Question 12

Couple

Answer 12

A pair of forces which produce rotation only

Question 13

To form a couple

Answer 13

• Equal in magnitude
• Parallel but in opposite directions
• Separated by a distance dd

Question 14

Moment of a Force

Answer 14

• Product of the force and the perpendicular distance of its line of action to the pivot
• Moment = Force × ⊥Distance from Pivot

Question 15

Torque of a Couple

Answer 15

• The product of one of the forces of the couple and the perpendicular distance between the lines of action of the forces.
• Torque = Force × ⊥Distance between Forces

Question 16

Conditions for Equilibrium

Answer 16

• Resultant force acting on it in any direction equals zero
• Resultant torque about any point is zero.

Question 17

Principle of Moments

Answer 17

For a body to be in equilibrium, the sum of all the anticlockwise moments about any point must be equal to the sum of all the clockwise moments about that same point.