4. Forces, density and pressure

Question 1

Define center of gravity

Answer 1

The point where the entire weight of an object appears to act

Question 2

Define moment

Answer 2

The moment is the turning effect of force.
N m is the SI unit.

Question 3

Calculating perpendicular force
and non-perpendicular force.

Answer 3

1. perpendicular force- moment= force* perpendicular distance from the pivot.
2. Non-perpendicular force= f*d cos(theta)

Question 4

what is the principal of moments.

Answer 4

For a system to be in equilibrium, the sum of clockwise moments about a point must be equal to the sum of the anticlockwise moments (about the same point).
also;
clockwise moments = anticlockwise moments

Question 5

Define a couple and torque

Answer 5

couple: a pair of equal and opposite forces that act on an object in different points producing rotation only.
torque: the turning effect of a couple is known as torque.

Question 6

What are the conditions for a couple

Answer 6

two forces must be:
1. equal in magnitude
2. parallel, but in opposite direction
3. separated by a distance d

Question 7

How do you calculate torque-

Answer 7

torque(N m) = one of the forces (N) * perpendicular distance between the forces (m)

Question 8

What are the conditions for a system to be in equilibrium-

Answer 8

1. there is no resultant force
2. there is no resultant torque

Question 9

What are the conditions of forces being in equilibrium-

Answer 9

1. at rest
2. moving at constant velocity

Question 10

How are coplanar forces represented in equilibrium-

Answer 10

in closed vector triangles.

Question 11

Define density

Answer 11

Density is mass per unit volume
density= mass/ volume
units= g/cm3 and kg/m3

Question 12

Define pressure

Answer 12

Pressure if force per unit area
pressure= force/ area
UNITS- N/cm2 , N/m2 and pascals where;
1 P= 1N / m2

Question 13

what is u-tube manometer:

Answer 13

U-tube manometer is an instrument used to measure and density of two liquids.

Question 14

Define hydrostatic pressure-

Answer 14

Hydrostatic pressure is the pressure that is exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity.

Question 15

What is the equation for hydrostatic pressure-

Answer 15

change in pressure= density x acceleration due to gravity x change in height
Δp=density x g x Δh

Question 16

How to calculate the total pressure-

Answer 16

total pressure= hydrostatic pressure + atmospheric pressure
atmospheric / barometric pressure= 101325 pa

Question 17

Deine upthrust-

Answer 17

Upthrust is a force which pushes upwards when an object is submerged in a fluid- i.e- gases or liquids

Question 18

What causes upthrust-

Answer 18

Upthrust is caused by the difference in hydrostatic pressure at the top and the bottom of an object immersed in the fluid.
The force of upthrust is significantly larger in liquids than in gases, this is because liquids are much denser than gases.
NOTE- upthrust is influenced by pressure and not by the density of the object.

Question 19

What does Archimedes’ principle state: elaborate

Answer 19

Archimedes’ principle states that an object submerged in a fluid at rest has an upward buoyancy force (upthrust) equal to the weight of the fluid displaced by the object.
The object sinks until the weight of the fluid displaced is equal to its own weight.
Therefore the object floats when the magnitude of the upthrust equals the weight of the object.

Question 20

How to calculate the magnitude of the upthrust?

Answer 20

force=density x acceleration due to gravity x volume displaced