Forces and moments

Question 1

Define force

Answer 1

Force is the rate of change of momentum of an object which is free to move

Question 2

When is a force conservative

Answer 2

A force is conservative if the work it does on an object moving between 2 points is independent of the path the object takes

Question 2

When is a force non-conservative

Answer 2

A force is non-conservative if the work it does on an object depends on the path taken by the object between its initial and final points

Question 2

Examples of conservative forces

Answer 2

• Gravitational force
• Spring force

Question 2

Examples of non non-conservative forces

Answer 2

• Friction
• Drag force

Question 3

Examples of dissipative forces

Answer 3

• Friction
• Drag force

Question 4

Define frictional force

Answer 4

Frictional force is the force exerted by one body on another body when both bodies slide over one another

Question 5

Define drag force

Answer 5

Drag force is the frictional force experienced either by an object moving through a fluid or by a fluid moving over a surface

Question 6

Define centre of gravity

Answer 6

Centre of gravity of an object is the single point where the entire weight of the body is seen to be acting on

Question 7

State Hooke’s Law

Answer 7

Hooke’s Law states that force is directly proportional to extension, provided that the elastic limit has not been exceeded

Question 8

How to find k(eff) of springs in parallel

Answer 8

k(eff) = k(1) + k(2)

Question 9

How to find k(eff) of springs in series

Answer 9

1/k(eff) = 1/k(1) + 1/k(2)

Question 10

How to find elastic potential energy

Answer 10

Area under force-extension graph

Question 11

Define moment of a force

Answer 11

Moment of a force about a pivot is the product of magnitude of force and perpendicular distance between line of action of force and pivot

Question 12

Define a couple

Answer 12

A couple is a pair of forces of equal magnitude but acting in opposite direction, who’s lines are parallel but separate

Question 13

Define torque of a couple

Answer 13

Torque of a couple is the product of one of the forces and the perpendicular distance between the forces

Question 14

What are the conditions for equilibrium

Answer 14

• Resultant force on object is zero in any direction
• Resultant torque on object is zero about any axis of rotation

Question 15

What does equilibrium consist of

Answer 15

• Translational equilibrium
• Rotational equilibrium

Question 16

State the Principle of Moments

Answer 16

Principle of Moments states that for a system in equilibrium, the sum of clockwise moments about any axis is equal to the sum of anticlockwise moments about the same axis

Question 17

How to tell if system is in equilibrium based on forces

Answer 17

• Lines of action of non-parallel forces pass through a common point