3.2: Dynamics

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66 Terms

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Net Force

The vector sum of all forces acting on an object (Newtons, N)

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Net Force Equation

F=ma

net force= mass x acceleration


Equation only works for net force

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Net Force Equation Explanation

The amount of force is directly proportional to acceleration, with mass as the constant

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Weight

The weight of an object is the gravitational force acting upon it (Newtons, N)

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Weight equation

W=mg

Weight=mass x acceleration of freefall

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Tension

The force within a stretched cable

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Normal Contact Force

The reaction force between an object and surface, acting at a 90o angle to plane of contact

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Upthrust

The upwards (buoyancy) force that a fluid applies on an object

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Friction

The resistive force produced when there is a (relative) movement between the two surfaces

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Drag

A frictional force experienced by an object moving through a fluid that opposes the motion of an object

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What happens when a constant force acts on an object

Object experiences a constant resultant acceleration, determined with F=ma

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One dimensional motion under a constant force

  • Involves up and down or left and right

  • On the ground

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Two dimensional motion under a constant force

  • Involves up and down and left and right

  • On a slope

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What is Drag

The frictional force experienced by an object moving through a fluid that opposes the motion of the object

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Factors Affecting Drag

  1. Speed

  2. Surface area (less surface area= less drag)

  3. Density of fluid (type of fluid)

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Most crucial factor affecting drag

Speed (d∝v2)

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Journey of Drag: Part 1

  • Initial velocity is zero.

  • Net force is the weight.

  • Acceleration is g=9.81ms-1

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Journey of Drag: Part 2

  • Velocity increases, and the drag force also increases (as d∝v2)

  • Fnet=W-D so net force decreases and acceleration decreases (mass remains the same)

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Journey of Drag: Part 3

  • Drag force equals the weight

  • Fnet=0 so acceleration=0

  • Object moving at terminal velocity

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Definition of terminal velocity

Terminal velocity is the constant velocity an object falls at when the drag force is equal to the gravitational pull on the body

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Determining Terminal Velocity in Fluids: Step 1

Fill a vertical tube with a viscous liquid like hand sanitizer

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Determining Terminal Velocity in Fluids: Step 2

Use a metre ruler, and mark regular consecutive intervals (like very 10cm) using rubber bands

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Determining Terminal Velocity: Step 3

Drop a ball bearing into the tube and (with a timer) record the time for the ball bearing to reach each individual interval

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Determining Terminal Velocity: Step 4

Repeat several times (at least 3) and calculate the average

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Determining Terminal Velocity: Step 5

Use average values, calculate the velocity (distance of each interval / time taken)

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Determining Terminal Velocity: Step 6

Plot a velocity-time graph and draw the line of best git (representative of acceleration)

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Determining Terminal Velocity: Step 7

You should observe an initial linear section which eases off into a horizontal line. The velocity at the horizontal line is the terminal velocity

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Exam Definition of a Moment

The turning effect of a force about a point and is the product of the force and the perpendicular distance of the line of action of force from the point of rotation (units are Nm)

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Grace-Friendly Definition of a Moment

  • Turning effect of a force

  • They occur when forces cause objects to rotate about same pivot

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Equation for Moment

moment= Fx


F= Force (N)

x= Perpendicular distance (m)

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Perpendicular force/distance with moments

Force and distance NEED to be perpendicular to each other in calculations with moments


May have to resolve vectors with equations:

Fx=Fcosθ

Fy=Fsinθ

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The Principle of Moments

For a body in rotational equilibrium, the sum of anti-clockwise moments about any point is equal to the sum of the clockwise moments about that same point

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What is a couple

A pair of equal and opposite, parallel, coplanar forces that act to produce rotation only

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What do coplanar forces mean

Forces that act in the same plane

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What do couples consist of

A pair of forces that are…

  • Equal in magnitude

  • Opposite in direction

  • Perpendicular to the distance between them

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Resultant force of a couple

  • Net force is zero (due to Newton’s Second Law)

  • Object doesn’t accelerate

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Moment of a couple

  • Doesn’t depend on a pivot

Moment of a couple= force x perpendicular distance between the lines of action of the forces

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Torque

Is the rotational effect of a couple

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Equation of a torque

T=Fd


T= Torque (Nm)

F= one of the applied forces (N)

d= perpendicular distance between the forces (m)

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Centre of Mass

A point through which any externally applied force produces straight line motion but no rotation

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Centre of Mass for Objects with a Uniform Density

Is located at point of symmetry

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Centre of Mass and stability

Most stable objects have wide bases and low centres of masses

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Centre of Gravity

An imaginary point where the entire weight of an object appears to act

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Centre of Gravity in a Uniform Gravitational Field

The centre of gravity is identical to the centre of mass

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Determining Centre of Mass and Gravity: Step 1

Drill a small hole in the object and hang it so it is free to swing without obstruction

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Determining Centre of Mass and Gravity: Step 2

Hang a plumb line from the same suspension point, and mark the vertical line directly below the suspension point

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Determining Centre of Mass and Gravity: Step 3

Drill a hole at a different location within the object

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Determining Centre of Mass and Gravity: Step 4

Hang a plumb line to determine the vertical, and mark it on

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Determining Centre of Mass and Gravity: Step 5

The point at which the two marked lines cross is the centre of mass and gravity

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Systems in Equilibrium

All the forces are balanced in the system

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Conditions for Equilibrium

  • No net resultant force acting on an object

    • Net moment of zero acting on an object

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Objects in Equilibrium

  • Will remain at rest OR at a constant velocity

  • Won’t rotate

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Density

Mass per unit volume of a substance

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Equation for density

ρ=m/v


ρ= density (kgm-3)

m= mass (kg)

v= volume (m3)

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Determining Density

For liquids you can use a measuring cylinder to determine the volume. The volume of a regular-shaped solid can be calculated from measurements taken with a ruler, digital callipers or a micrometre. The volume of irregular solids cam be determined by displacement

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Pressure

How a force presses on a surface

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Pressure and area

Inversely proportional

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Equation for pressure

p= F/A


P= pressure (Pa)

F= force (N)

A= area (m2)

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Type of quantity: Pressure

Scalar

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Pressure in Fluids

Fluids exert a pressure on surfaces because of constant bombardment by their molecules

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Equation for pressure in fluids

p=hρg


p= pressure (Pa)

h= height of fluid column (m)

ρ= density of fluid (kgm-3)

g- acceleration due to gravity (ms-2)

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Upthrust

The upward force that a liquid or gas exerts on a body floating in it

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Archimede’s Principal

An object submerged in a fluid at rest has an upward buoyancy force (upthrust) equal to the weight of the fluid that the body displaces

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It will sink!

The upthrust is less than the weight of the object

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It will float!

The upthrust is equal to the weight of the object

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How to find the magnitude of upthrust

F=ρgV


F= upthrust (N)

ρ= density of fluid (kgm-3)

g= acceleration due to gravity (ms-2)

V= volume displaced (m3)