Chapter 4. Forces, Density and Pressure

Question 1

What is pressure?

Answer 1

Force per unit area
p = F/A
F is the contact force at 90 degrees to the surface
It is a scalar quantity
pressure = height(m) x density(kgm^-3) x gravitational field strength(Nkg^-1)
NB: formula above only applies to incompressible fluids as density is assumed not to change with depth

Question 2

What is density?

Answer 2

The mass per unit volume(m/v)

Question 3

What causes upthrust?

Answer 3

Pressure in fluids inreases with depth so the pressure at the base of an object is greater than that at the top so there is a resultant force called upthrust

Question 4

state Archimede’s principle

Answer 4

When an object is completely or partly immersed in a fluid, it experiences an upthrust equal to the weight of the fluid displaced.
This is true for all fluids

Question 5

What does upthrust depend on?

Answer 5

The volume of the object submerged and the density of the fluid

Question 6

Note

Answer 6

An object floats when upthrust is equal to it’s weight.(Principle of flotation)
An object will float when it’s density is less than that of the fluid

Question 7

What conditions must be true for an object to be in equilibrium?

Answer 7

-no net force must act in any direction
-there must be no net turning effect about any point

Question 8

What is a moment?

Answer 8

A measure of the turning effect of a force.
moment of a force about a point(Nm) = force(N) x perpendicular distance from the force to that point(m)

Question 9

What is a couple?

Answer 9

Two forces that are equal in size and opposite in direction but do not act along the same traight line.
They do not create a resultant force but a torque(turning effect of a couple)

Question 10

What is the formula for torque?

Answer 10

torque = magnitude of one force(N) x perpendicular distance between the two forces forming the couple(m)

Question 11

What is the principle of moments?

Answer 11

When an object is in equilibrium, the sum of clockwise moments about any point is equal to the sum of anti-clockwise moments about the same point

Question 12

What is the difference between centre of gravity and centre of mass?

Answer 12

The centre of gravity of an object is the point at which we can take its entire weight to act whilst the centre of mass of an object is the point at which we can take its entire mass to be concentrated.
In places where the gravitational strength is uniform the centre of gravity and the centre of mass are at the same point.

Question 13

How do you locate the centre of gravity?

Answer 13

For uniform, symetrical objects the centre of gravity is at the geometric centre.
If an object is hung so that it can swing, it will always come to rest with its centre of gravity directly below the point of support so we can use a plumbline to find the centre of gravity of irregular laminas.
A lamina is a thin flat shape

Question 14

Note

Answer 14

When, solving equilibrium problems draw a free-body force diagram for the object. Don’t forget its weight acting downwards at its centre of gravity and any other forces like contact forces.
Make sure to resolve forces so that they act in a convenient direction

Question 15

Note

Answer 15

1gcm^-3 = 1000kgm^-3
1m³ = 10⁶cm³

Question 16

Note

Answer 16

The separation of molecules in solids or liquids are nearly equal as the densities of the substances in a compound add up to give the total density

Question 17

What is static friction and dynamic(sliding friction)?

Answer 17

Static friction is the friction between surfaces which are not moving (but gripping). Sliding or dynamic friction is the friction between surfaces which are sliding past each other. Sliding friction is less than static friction.

Question 18

What cause friction?

Answer 18

(i) Uneven surfaces, the surfaces of objects have humps and hollows which catch each other.
(ii) Molecules in the two surfaces which tend to stick together when pressed.

Question 19

Note

Answer 19

During streamline flow, drag is proportional to velocity but at higher speeds the flow becomes turbulent and drag becomes proportional to the square of the velocity

Question 20

Similarities and differences between frictional aand viscous forces

Answer 20

Similarities
1. Oppose motion.
2. Occur due to attractive forces between molecules.
3. Dissipative in nature.
Differences
1. Frictional forces act along the surfaces in contact between two solid objects, whereas viscous forces act between a solid and a fluid or between a fluid and a fluid/or solid.
2. Viscous forces increase with relative speed between two surfaces whereas frictional forces are not affected by relative speed between two surfaces.
3. Viscous forces do not exist when there is no relative motion whereas frictional forces exist even when the object is at rest.