3.2 Forces in Action

Question 1

Centre of Gravity

Definition

Answer 1

The point through which the object’s weight appears to act

Question 2

Principle of Moments

Answer 2

For object to be in equilibrium: sum clockwise moments = sum anticlockwise moments exerted on it

Question 3

Couple

Definition

Answer 3

A pair of forces that together cause a resultant moment (Torque), but not a resultant force

(Opposite & of equal magnitude)

Question 4

Centre of Mass

Definition

Answer 4

avg. pos. of that object’s mass

In uniform gravitational field, this = centre of gravity

Question 5

Density Units

{and eq}

Answer 5

kgm^(-3)

p = m/v

Question 6

Torque

Definition

Answer 6

A resultant moment
(due to a couple): Torque = Fd

F = size of 1 of the forces | d = perp. distance between the forces

Question 7

What are coplanar forces?

Answer 7

multiple forces acting on an object in the same plane

so you can present them in 2D

Question 8

If an object is floating, what is it’s upthrust?

And Why?

Answer 8

U = W

Because resultant Force = 0, so the weight acting downwards must be bala

Question 9

Calculating Upthrust Eq

(Not Given)

Answer 9

U = pVg
(p being density)

Same as Pressure eq. (Where did U=phg come from?)

Question 10

Newton in Standard Units

Answer 10

kgms^(-1)

Question 11

What ^ drag?

Answer 11

> SA ^
speed ^
fluid viscous ^
shape less aerodynamic

Question 12

Ratio for fraction of volume submerged

Answer 12

P0/Pf = fraction of volume submerged

P0 = object’s density | Pf = fluid’s density

Question 13

1 Object pulls another object, what is the eq linking a, W, and the masses?

Not Given

Answer 13

a = W/(M+m)

Question 14

What does the h in P = hpg mean?

Answer 14

the vertical height of the liquid

(even if container is holding liquid at an angle)

Question 15

State the 2 conditions for an object to be in equilibrium

Answer 15

1. Resultant Force = 0
2. Resultant moment = 0

Question 16

Ratio for fraction of height submerged

(If object of height h is submerged to depth d)

Answer 16

P0/Pf = d/h
P0 = Object’s density | Pf = fluid’s density

! NOTE: Does not have to be fully submerged

Question 17

Pressure in liquid Units

Answer 17

Pa

P = hpg where h is always vertical

Question 18

{What is the motion of an object with resultant force 0?}

{Wording q.}

Answer 18

acceleration = 0

Question 19

Moment eq

Answer 19

Moment = F x perp. distance

moment = Fxsin(@)

Question 20

What is the principle relating to Upthrust?

Want Name & Meaning

Answer 20

Archimedes Principle:
Upthrust exerted by fluid = weight of displaced fluid

Question 21

Pressure Units

Answer 21

Pa

(1 Pa = 1 Nm^(-2)) bcs P = F/A

Question 22

Newton’s 2nd Law Eq

Answer 22

F = ma

When m constant

Question 23

{Maybe work on those coplanar force diagram things & add any problems here}

Question 24

Define 1N

Answer 24

Force required to accelerate a 1kg object at 1ms^(-2)

Question 25

{PRACTICALS} TODO

Question 26

Archimides Principle

Answer 26

Upthrust exerted by fluid = weight of displaced fluid