Chapter 4 Forces, Density & Pressure Flashcards

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1
Q

a couple has a pure turning effect. Why?

A
  • because a single force will tend to make an object accelerate (unless another force balances it)
  • where a couple is a pair of equal, opposite forces = will not make object accelerate.
  • The turning effect, well force is determined by its torque
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2
Q

balanced or unbalanced

A
  • check whether an object remains balanced or start to rotate - calulate an unknown force or distance if we know the object is balanced
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3
Q

centre of gravity

A

the point where the entire weight of an object appear to act

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4
Q

finding moment of force (method 1)

A

-draw perpendicular line from the pivot to the line of the force - find the distance from pivot. -find force -calculate moment of force (F1 x x2) -we see that: x2 = d sin(angle) hense: moment = F2 x d sin(angle) x2 = distance from pivot to 90 degree of F2 line(figure 4.17)

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5
Q

find the moment (method 2)

A

calculate the component of F2 that is at 90 to the lever.

-this is F2 sin(angle).multiply by d moment = F2 sin(angle) x d

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6
Q

For an object to be in equilibrium two conditions must be met?

A

-resultant force acting on the object is zero -the resultant moment is zero

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7
Q

how to resolve a force F into its horizontal and vertical components

A

horizontal component of F, Fx = F cos(angle) vertical component of F, Fy = F sin(angle)

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8
Q

moment of a force Def

A

of a force about a point is the product of a force and perpendicular distance from the line of action of the force to the point.

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9
Q

moment of force =

A

force x distance from pivot depends on two forces: -magnitude of force -perpendicular distance of the force from pivot Torque=FD Torque= Nm Force = F Distance= the perpendicular force from pivot (meters)

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10
Q

principle of moments

A

the sum of clockwise moments about a point is equal to the sum of the anticlockwise moments about the same point provided the body is in equilibrium

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11
Q

resolving

A

to split a vector (such as force) into two vectors at right angles

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12
Q

to form a couple, the two forces must be:

A

-equal in magnitude -parrel, but opposite in direction -separated by a distance

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13
Q

torque def

A

the product of one of the forces and the perpendicular distance between the forces. (the turning effect or moment of a couple)

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14
Q

torque of a couple =

A

one of the forces x separation of forces Torque about O = (F x OA) + (F x OB) O = FD

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15
Q

two forces at a right angle (drawn) (Combining vectors)

A

-first draw a horizontal arrow to represent the Force -Next, starting from the end of this arrow, draw the second arrow in the irection its effecting -now, draw a line from the start to the end of the second arrow ( represent resultant force ) -resulatant force = scaled diagram or by calculation (phytagorus theorm)

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16
Q

what is a couple

A

a pair of equal and opposite forces that act on an object at different points and produce rotation only Can not produce translation motion

17
Q

what is a triangle of forces (Combining vectors)

A

a closed triangle drawn for an object in equilibrium. The sides of the triangle represent the forces in both magnitude and dircetion

18
Q

what is equilibrium mean (Combining vectors)

A

an object in equilibrium is either at rest or traveling with constant velocity because the resultant force on it is zero

19
Q

Electric force

A

The force acting on an electric charge due to the electric field F= EQ E- Energy field N per coloum Q- Charge

20
Q

Gravitational force

A

The force acting on a ,ass due to the gravitational field F= MG

21
Q

Types of forces

A

-gravitational force -electrical force -upthrust and buoyancy -frictional and viscous forces

22
Q

Upthrust or buoyancy

A

The reactions force acting against the weight of a body in a liquid

23
Q

Frictional and viscous forces

A

Friction - is the force that resists the motion of a body (mew) Viscous- resistive force to a liquid flow or gas (gamma)

24
Q

Density

A
  • Density is the mass per unit volume of an object
  • Objects made from low-density materials typically have lower mass. For example, a balloon is less dense than a small bar of lead despite occupying a larger volume
  • The units of density depend on what units are used for mass and volume: –If the mass is measured in g and volume in cm3, then the density will be in g/cm3 –If the mass is measured in kg and volume in m3, then the density will be in kg/m3
25
Q

The volume of an object may not always be given directly, but can be calculated with the appropriate equation depending on the object’s shape

A
26
Q

Pressure

A

Pressure tells us how concentrated a force is, it is defined as the force per unit area p= F/A This equation tells us —If a force is spread over a large area it will result in a small pressure —If it is spread over a small area it will result in a large pressure

27
Q

The units of pressure depend on the units of area

A
  • If the area is measured in cm2 (and the force in N), then the pressure will be in N/cm2
  • If the area is measured in m2 (and the force in N), then the pressure will be in N/m2
  • Pressure can also be measured in pascals, Pa where 1 Pa is the same as 1 N/m2 -Pressure, unlike force, is a scalar. Therefore pressure does not have a specific direction
28
Q

U-tube manometer

A

A manometer is an instrument to measure pressure and density of two liquids

29
Q

Hydrostatic pressure

A
  • is the pressure that is exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity
  • This is when an object is immersed in a liquid, the liquid will exert a pressure, squeezing the object
  • The size of this pressure depends upon the density (ρ) of the liquid, the depth (h) of the object and the gravitational field strength (g): ∆p = ρg∆h
30
Q

When asked about the total pressure remember to also add the atmospheric pressure

A

-Total pressure = Hydrostatic pressure + Atmospheric pressure -Atmospheric pressure (also known as barometric pressure) is 101 325 Pa

31
Q

Upthrust

A

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

  • Also known as buoyancy force, upthrust is due to the difference in hydrostatic pressure at the top and bottom of the immersed object
  • The force of upthrust is significantly larger in liquids than in gases, this is because liquids are much denser than gases
  • Recall that hydrostatic pressure depends on the height (h) or depth that an object is submerged in from P = ρgh -Therefore, the water pressure at the bottom of an object is greater than the water pressure at the top
  • Upthrust is a force and is directly proportional to the pressure. The force on the bottom of the can will be greater than the force on top of the can -This resultant pressure causes a resultant upward force on the can known as upthrust
  • Upthrust is why objects appear to weigh less when immersed in a liquid. If the upthrust is greater than the weight of the object, the object will rise up
  • For an object to float, it must have a density less than the density of the fluid its immersed in
32
Q

Archimedes’ Principle

A

-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
  • The magnitude of upthrust can be calculated by F= pgV F= upthrust (bouyancy force) (N) p= density (kgm^-3) g=gravitional (ms^-1) v= volume displaced(m^3)
33
Q

Stability

A

The position of centre of gravity affect its stability -an object is stable when it’s centre of gravity lays above its base -wider the base = lower centre of gravity = more stable -narrower the base = higher centre of gravity = less stable

34
Q

Centre of gravity vs centre of mass

A

-centre of mass does not depend on the gravitational field -since weight= mass x gravitational potential energy means that the centre of gravity depends on the gravitational field