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OCR A-Level Physics > 3.2 Forces in Action > Flashcards

3.2 Forces in Action Flashcards

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1
Q

define resultant force

A

is a single force which has the same effect as the sum of all the forces acting on a body

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2
Q

what is the formula for resultant (or net) force? and what law is this?

A 
f = ma
where f = resultant force
m = mass of object
a = acceleration
form of Newton's second law when mass is constant
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3
Q

when can you use f = ma?

A

when mass is constant (e.g not a rocket)

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4
Q

what is the unit of force and define it?

A

newtons, N

1 newton is the force required to accelerate a mass of 1kg by 1ms^-2 in the direction of the force

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5
Q

if the forces on an object are balanced the object is said to be in…?

A

equilibrium

no resultant force, no acceleration

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6
Q

what is the formula for weight force?

A

w = mg
where w = weight force
m = mass of object
g = acceleration due to gravity

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7
Q

what are the four fundamental forces of nature?

A

-the gravitational force
the force between two masses, has infinite range
-the electromagnetic force
holds atoms and molecules together, has infinite ranfe
-the weak force
responsible for radioactive decay, acts over very short ranges
-the strong force
-responsible for holding subatomic particle together in the nucleus and is the strongest force of all nature

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8
Q

define tension

A

is the force experienced by any rope, string, cable or wire that is being pulled, hung, rotated or supported

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9
Q

define normal contact force (or reaction force)

A

is the force that acts perpendicularly (at right angles) to the point of contact of a body and the surface with which it is in contact

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10
Q

define upthrust

A

the upwards force that a liquid or gas exerts on a body floating in it due to the fluid displaced

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11
Q

define friction

A

the force of friction occurs between two surfaces in contact with one another and resists the motion at the point of contact

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12
Q

what are free body force diagrams and what should they contain?

A

simplified labelled drawing where the object is modelled as a point, it should contain…

  • the body on which the forces act
  • the direction of application of each force
  • the type of each force
  • the size of each force
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13
Q

if you drop two objects of different masses which will hit the ground first? (ignoring air resistance)

A

both hit the ground at the same time because acceleration is independent of mass, objects fall at the same rate

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14
Q

what is drag?

A

the frictional force experienced by an object travelling through a fluid (can be liquid or gas)

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15
Q

what factors affect drag for an object travelling through air?

A
  • cross sectional area of the object

- the density of the fluid

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16
Q

what is the relationship between drag caused by air resistance and the cross sectional surface area of the object?

A

drag ∝ cross sectional area

directly proportional

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17
Q

what is the relationship between drag caused by air resistance and velocity of the object?

A

drag ∝ velocity^2

directly proportional to square of velocity

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18
Q

what is terminal velocity?

A

the velocity at the point during freefall when the object stops accelerating as the weight force is balanced by the air resistance (becomes equal), object falls at constant velocity

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19
Q

outline an investigation to determine the terminal velocity in fluids

A
  • set up a clamped cylinder containing a viscous liquid e.g glycerol and place elastic bands at fixed distances down the tube
  • drop a steel ball bearing into the tube at the top and use a timer to record the time at which the ball reaches each band, repeat to reduce error and record results in table (a strong magnet can be used to remove ball bearing)
  • measure the distances between each consecutive bands and use average times to calculate average velocities between each pair of elastic bands
  • plot a graph with v on the y axis against cumulative time t on the x axis, draw a smooth curve and identify the time at which the ball reaches its terminal velocity (lines become constant/flat)
20
Q

what does it mean for an object to be in equilibrium?

A

objects are in equilibrium when all the forces acting on them in the same plane (coplanar forces) are balanced - there is zero net or resultant force, in terms of motion, the object is either stationary or is travelling at constant velocity

21
Q

what is a moment?

A

a turning effect of a force

22
Q

what is the definition/equation of a moment?

A
  • the moment of force (or turning moment) is the product of a force and the perpendicular distance of its line of action or axis
  • moment = force x perpendicular distance from pivot
  • Moment = F x d
23
Q

what is the principle of moments?

A

the principle of moments states that for an object to be in rotational equilibrium, the sum of the clockwise moments must be equal to the sum of the anticlockwise moments
(moments must be balanced or the object will turn)

24
Q

what is a couple?

A

a couple occurs when two equal, anti-parallel forces act to produce a rotation (no linear motion occurs)

25
Q

what is the definition/equation of a torque of a couple?

A
  • the turning moment (torque) due to a couple is the product of one of the forces and the perpendicular distance between them
  • torque of couple = size of one of the forces x perpendicular distance between them
  • T = F x d
26
Q

what are examples of objects that experience moments?

A

muscles, bones and joints act as LEVERS, an effort force acts against a load force by means of a rigid object rotating around a pivot

27
Q

define the centre of mass

A

the centre of mass of an object is the single point at which all of the mass of the object can be assumed to be situated

28
Q

define the centre of gravity

A

the centre of gravity of an object is the single point through which the entire weight of the object can be thought to act

29
Q

for a symmetrical body of constant density where is the centre of mass?

A

at the centre of the body

30
Q

outline an investigation to determine the centre of mass or centre of gravity of an object

A
  • make three holes in random places on the edge of the shape
  • hang the shape from first hole by placing through pin, let it swing until comes to rest
  • use a plumb line to draw a vertical line down from the point of suspension to the bottom of the shape
  • repeat these steps but with the different holes
  • where the lines intersect is the centre of mass
31
Q

what does a wide base and a low centre of mass mean for an object?

A

it has more STABILITY

32
Q

how does an object become unstable?

A

if an object is titled so that the centre of mass lies outside the width of the base of the object it will become unstable and will topple and fall

33
Q

what is neutral stability and give an example?

A

neutral stability is when the centre of mass can never fall outside the with of the base of the object (that is touching the ground), for example objects that can roll like a football

34
Q

what does resolving a force mean?

A

splitting it into its horizontal and vertical components using trig

35
Q

what does coplanar forces mean?

A

forces that act in the same plane

36
Q

what does a closed loop mean in vector triangles?

A

there is no resultant force, acceleration is zero

37
Q

what does a velocity-time graph look like for terminal velocity?

A

line starts increasing with high gradient and slowly flattens out (curves to gradient of zero)

38
Q

what does a velocity-time graph look like for terminal velocity of a skydiver with a parachute being opened?

A

line starts increasing with high gradient and slowly flattens out (curves to gradient of zero)
then once parachute opened steep drop to lower terminal velocity, continues straight with zero gradient

39
Q

what is the formula for density?

A

ρ = m / v

the mass per unit volume

40
Q

what is the symbol of density?

A

ρ , rho

41
Q

how do you calculate the density of an irregular shape? (E.g not a cuboid, sphere or something you can calculate the volume of)

A
  • measure the object’s mass using scales
  • then calculate the volume by putting the object in a cylinder of water of known volume (cm^3) and work out the volume of water displaced (this will be equal to the volume of the object) - can use a displacement can
42
Q

what is Archimedes’ principle?

A

Archimedes’ principle states that the upward buoyant force (upthrust) exerted on an object immersed in a fluid, is equal to the weight of the fluid the object displaces

43
Q

what is the formula for pressure exerted by a fluid at any depth?

A 
p = ρhg
where p = pressure
ρ = density of fluid
h = depth of fluid
g = acceleration due to gravity
44
Q

how do you derive p = ρhg?

A

density = m / v
therefore m = ρ X v
for a cylinder/column shape volume is equal to h X A (where h is the height and A is the cross sectional area)
therefore m = ρhA where m is the mass of the fluid
the weight of this mass is equal to mg, so weight force = ρhAg
pressure = force / area so ρhAg / A gives you pressure = ρhg

45
Q

what happens when a force is applied through the centre of a mass of a body?

A

the body will accelerate in the direction of the applied force in a STRAIGHT LINE WITH NO ROTATION

46
Q

what is the formula for pressure for solids, liquids and gases?

A
  • pressure = force / area
  • measured in pascals
  • the force is perpendicular to the area

OCR A-Level Physics (24 decks)

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