2.2 Forces

Question 1

Newton’s first law of motion

Answer 1

An object continues to remain stationary or to move at a constant velocity unless an external force acts on it

Question 2

F = ma

Answer 2

Newton’s second law: Force (N) = mass (kg) x accelaration (ms^-2)

Question 3

Newton’s third law of motion

Answer 3

Every action force has an equal and opposite reaction force.

● Action-reaction pair must be of the same type

Question 4

The rules for a free-body force diagram

Answer 4

● The diagram is for one body only and the force vectors are represented as arrows.
● Only the forces acting on the body are considered.
● The force (vector) arrows are drawn to scale originating at a point that represents the centre of mass of the body.
● All forces have a clear label

Question 5

Procedure for drawing and using a free-body diagram

Answer 5

● Begin by sketching the general situation with all the bodies that interact in the situation.
● Select the body of interest and draw it again removed from the situation.
● Draw, to scale, and label all the forces that act on this body due to the other bodies and forces.
● Add the force vectors together (either by drawing or calculation) to give the net force acting on the body. The sum can be used later to draw other conclusions about the motion of the object

Question 6

Transnational equilibrium

Answer 6

(of an object) either at rest or moving at a constant velocity

Question 7

Rotational equilibrium

Answer 7

(of an object) either at rest or rotating at a constant angular speed

Question 8

A triangle of forces

Answer 8

A right-angled triangle formed by the scaled lengths and directions of three force vectors
https://i.imgur.com/8q53kJM.png

Question 9

Static friction

Answer 9

A frictional force - when there is no relative movement between the surfaces

Question 10

Dynamic friction

Answer 10

A frictional force - when there is relative movement

Question 11

F_f ≤ µ_sR

Answer 11

Static friction force:
F_f: static frictional force exerted by the surface on the block
R: normal reaction of the surface on the block, = weight of the block without vertical acceleration.
µ_s: coefficient of static friction

Question 12

F_f = µ_dR

Answer 12

µ_d: the coefficient of dynamic friction

Question 13

Contact forces

Answer 13

Surface force, friction, drag

Question 14

‘Distance force’

Answer 14

Gravitational pull, electric, magnetic