Chapter 4 Forces

Question 1

Newton 1st law

Answer 1

A body will remain at rest or continue at uniform velocity unless acted on upon by a RESULTANT FORCE

Question 2

Newton 2nd law ?
Equation
Full version?

Answer 2

Acceleration experienced by a body is DIRECTLY PROPORTIONAL to the resultant force acting upon it

• F=MA
• F = change in momentum

Question 3

Newton 3rd law

Answer 3

When two bodies interact they exert forces upon each other which are equal in magnitude but opposite in direction

And act on OPPOSITE BODIES , so it doesn’t cancel out

Question 4

Resultant force give rise to what?

What does resultant force up mean

Answer 4

Only acceleration , without it motion can still happen also resultant force up
Means deceleration not flying up remember

Question 5

What is acceleration under free fall?

Answer 5

Normally just g

But with drag g- D/m

Question 6

What is mass

Link to weight ?

How measure weight ?

Answer 6

A measure of a body’s inertia - its resistance to a change in velocity

Mass will always be constant at normal speeds however weight Is product of gravitational force times mass

3) use Newton meter which shows 9.8 n when 1 kg attached

Question 7

what happens in elevator

Answer 7

come back

Question 8

what is the centre of mass definition proper both

Answer 8

a point through where any externally applied force produces straight line motion but no rotation and

where the body’s mass can be thought to be concentrated, weight vertically down

Question 9

define where the centre of mass is for a uniform mass distribution object

Answer 9

the point at which all lines of symmetry cross

Question 10

how to find the centre of mass Sarai answer

Answer 10

1) suspend a body using a clamp or something near its edge with holes

ALLOW IT TO COME TO REST, so its in equilibrium and weight acts through COM

2) suspend a plumb line which will always be straight due to gravity
3) draw a line vertically down using the plumb line
4) now repeat for a different edge until lines of symmetry intersect.

Question 11

how to trial and error to find com with ruler

what about in space?

Answer 11

balance card on ruler until balanced and draw line, repeat until lines intersect

2) in space as no gravity have to apply force through which will make STRAIGHT LINE motion but no rotational , imagine line and repeat another orientation where they intersect minor

Question 12

what is a drag force

what are the two factors most important in calculating drag

Answer 12

a force that opposes the motion of the object in fluids

• speed and cross sectional area (viscosity and density of fluids)

drag is proportional to v^2
and drag is proportional to cross sectional are

Question 13

what is freefall and what is equation that links everything together

Answer 13

freefall is the motion of a body through a drag producing medium without a drag producing device

here a =f/m +g-d/m = g -d/m

here an increase of mass for the same drag will thus increase the maximum acceleration and thus final velocity too

Question 13

what is freefall and what is equation that links everything together

Answer 13

freefall is the motion of a body through a drag producing medium without a drag producing device

here a =f/m +g-d/m = g -d/m

here an increase of mass for the same drag will thus increase the maximum acceleration and thus final velocity too

Question 14

how do you reach terminal velocity and how explain how something with more mass but same drag reaches faster

Answer 14

1) at point of drop there is no drag so resultant force is just weight and accelerate down at 9.8
2) as speed increase, so does drag, therefore acceleration decreases as resultant force decreases
3) at one point the drag = weight to there is no resultant force, and as no speed increase drag is constant. this means constant velocity- terminal

4) if parachute open then drag > weight so resultant force and declaration, but this cause drop in speed which cause drop in drag until they are same again, so terminal velocity

for someone with less drag in same mass or more mass in same drag, they will have a greater acceleration and so go at a faster terminal velocity

Question 15

when can dragn be ignored and when not

Answer 15

• can be because its not comparable to weight several mag lower

however

• if very fast then drag increase
• or mass very small to surface area in which drag to mass ratio insane and max acceleration low

Question 16

What is moment and principle of moment

Answer 16

Force time perpendicular distance

For a body in rotational equilibrium the sum of the anti-clockwise moments = clockwise about any point

Question 17

What conditions needed for equilibrium

Answer 17

Net force acting on it is 0, net moment about any point is also 0

Question 18

Where does support force act

Answer 18

Always at the pivot or point of contact with a surface

Question 19

How to make an object ROTATE WITHOUT TRANSLATION, what’s is this
Learn

Answer 19

Here you apply a pair of equal forces but in the SAME PLANE but opposing directions and parallel

This is a couple

Question 20

What is a couple

Torque

Answer 20

A pair of EQUAL forces that give rise to resultant MOMENT but no RESULTANT FORCE , thus no translational movement ( along a distance)

Moment of the couple is known as torque

Question 21

Coplanar

Answer 21

Act in same plane and meet up

Can never be greater than the addition of three or less than the subtraction of them

Question 22

Triangle of forces

When can use

Answer 22

Only equilibrium

All three head to toe

Question 23

How to find density

Answer 23

Mass

Displacment can let it drop to zero and measure

Question 24

Pressure

Answer 24

Force /area NORMAL FORCE

We normally add up all normal force to give you one

Question 25

Density of mixtures ?, what assumptions you need to make here .

Answer 25

That they don’t react when combining which don’t affect there density of packed molecules

Question 26

Pressure in a liquid =

Answer 26

Pressure = depth * density of fluid * gravational pull

Question 27

How to derive the pressure in fluids equation

Answer 27

At the bottom of a column Pressure will still be force / area

Force = mass * g
Mass = density * volume
Volume = height * cross a
As a result mass = density * height * cross a

So force = g * density * height * cross a

And if you cancel a you get g * density * height

Question 28

What is the pressure in the same fluid in the same depth no matter what the cross sectional

Answer 28

As g constant and h contestant and p constant = the same

Question 29

How to do change in pressure in same fluid

Answer 29

Pressure change = change in height * density * gravity

Question 30

How to do total pressure at depth

Answer 30

REMEMBER TO ADD ATMOSPHERIC

Question 31

How is upthrust about from change in pressure and how is it equal to weight of fluid displaced ???? Important

Answer 31

Upthrust is a force and due to the pressure differences at top and bottom of a object , as p=f/a so f= p*a

• different pressures due to depth means different forces and resultant upward is upthrust

Heres if cross section Sam’s then
Upthrust = change in pressure * area
= change in height g density * area

2) change in height * area = volume
Volume * density = mass
Mass * g = weight of fluid
= upthrust = weight of fluid displaced

Question 32

This what is equation for upthrust

Answer 32

Change in height * density * gravity * cross sectional area too

Question 33

And what principe have you described and what is it ? (Arch)

Answer 33

(Archimedes principle ) = the upthrust experienced by an objects immersed in a fluid whether fully or partially submerged is EQUAL to the WEIGHT of the fluid dispersed

Question 34

So what determines if object floats or sinks

Answer 34

For an object to float its upthrust must = it’s weight
Which means it’s weight must = the weight of fluid displaced

If it can’t displace a weight = to its weight , then it will sink until it can

If it still can’t it will sink all the way ( this because of densities and how that relates to weight )

Question 35

How to work out how much somethint will be displaced if they give you densities

Answer 35

Divide little by big trust

Question 36

A few things to note about weight and displacement , is volume constant ?

Answer 36

Weight of somethint displaced won’t necessarily mean same weight but volume will always be constant

This if you displace weight 0.2 and that is = to whole object inside then that is = to all of its volume !

Question 37

How to show fraction of cube under water = d1/d2

What important

Answer 37

Basically say volume is x3 and then you know weight as it’s volume* density * gravity

This has to be equal to weight of water displaced , same volume x3 but actually a fraction of volume so fx3 multiplied by d2 and g

And rearrange for the fraction which is d1/d2!!!

Important = find the weights !!