STUDENTS - FORCES & MOTION by Lizelle Swanepoel

NOTES

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DEFINITIONS

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OUR STORY STARTS HERE...

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Sir Isaac Newton under the apple tree in his garden - contemplating what causes an apple to fall straight down to earth.

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FORCES - Definitions (memorize)

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Mass = The amount of matter in an object.
Weight = The downward force of attraction towards the center of the Earth on an object within its gravitational field.
Force = A push or a pull.
CONTACT FORCES
- NORMAL FORCE = The upward force of a solid surface on an object.
- TENSION = The pulling force transmitted in a string, cable, chain, rod.
- FRICTION = A force that opposes motion.
- BUOYANT FORCE = Upward force of a liquid surface on an object.
- AIR RESISTANCE = Force of air particles on an object falling in air.
- DRAG = Force of air particles on a HORIZONTALLY moving object in air.
- LIFT = The aerodynamic force acting on aircraft that opposes gravity.
NON-CONTACT (FIELD) FORCES
- Gravitational force =The attractive force between masses.
- Electrostatic force = The force acting between charges, either attractive or repulsive. Like charges repel and unlike charges attract.
- Magnetic force = The force between magnets, either attractive or repulsive. Like magnetic poles repel; opposite poles attract.
FIELD = The region around an object in which another object with specific properties will experience a force.
- Gravitational field = The region around a mass in which another mass experiences a force of attraction.
- Electrostatic field = The region around a charge in which another charge experiences a force.
- Magnetic field = The region around a magnet in which another magnet will experience a force.

CONTACT FORCES

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Normal Force

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Tension

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Tension

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The ends of a string (or rod or cable or chain transmitting tension) will exert a pull force on the objects to which it is connected.
Tension acts in the direction of the string at the point of attachment.

NON-CONTACT FORCES

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GRAVITATIONAL FORCE

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This is:

The same as Weight
The force between masses
It is only ATTRACTIVE
It is a non-contact (distance) force

We calculate weight using this formula:
Weight = m.g (where g is acceleration due to gravity, g=9,8 N/kg)

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Exercise 3 - Worked Example

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ELECTROSTATIC FORCE

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This is:

a non-contact (distance) force
the force between CHARGES.
an attractive or repulsive.
following the principle: "Unlike charges attract, like charges repel".

Balloon rubbing

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BUILDING UP CHARGE

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IN A VAN DE GRAAFF GENERATOR

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LIGHTNING IS TRANSFER OF EXCESS CHARGE

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Separation of charge:

Clouds are suspended water droplets.
When water from the ground evaporates, it rises up towards the cloud and forms droplets.
This rising moisture collides with water droplets in the clouds, causing friction, and some of the molecules' electrons are ripped off.
The rising moisture is now positively charged, due to a loss of electrons from their molecules. This makes the top of the cloud POSITIVELY CHARGED.
The negatively charged electrons are left behind in the base, making the bottom of the cloud NEGATIVELY CHARGED.

Lightning formation:

When a large negative charge builds up in the base of the cloud, the excess electrons will discharge down to the ground.
We see this discharging stream of electrons a lightning bolt.
As air expands rapidly in the path of the hot electron bolt, sound waves in the form of shock waves are produced. This a a thunder clap.

MAGNETISM

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MAGNETIC FORCE = The attractiveness force between MAGNETS. It is a non-contact force.
MAGNETIC FIELD = A region around a magnet where another magnetic material will experience a force.
Magnetic field direction is N to S outside the magnet.
Magnetic field direction is S to N inside the magnet.

Unit for Magnetic field strength = Tesla
ONLY THREE METALS ON EARTH ARE NATURALLY MAGNETIC:

Fe, Co, Ni. (they are called ferromagnetic)

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FRICTION

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Friction = A force opposing motion.

Solid surfaces are subjected to two types of friction:

static friction and
kinetic friction

Static friction:

acts when the surfaces are stationary — think of a box on the floor.
Static friction is what keeps the box from moving without being pushed,
and it must be overcome with a sufficient opposing force before the box will move.
Static friction has to do with the microscopic roughness of surfaces.
The “rough” surfaces will interlock.
In order to move, these interlocked areas must be broken or plastically deformed before the surfaces can move.

Kinetic friction:

is the force that resists the relative movement of the surfaces once they are in motion.

Coefficient of Friction

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Coefficient of friction (µ) = A measure of the “stickiness” of two surfaces in contact.

Wood-steel boundary has a low µ.
Wood-sandpaper boundary has a high µ.
Just as there are two types of friction, there are two types of coefficients of friction:
- static µ (µ_s)
- kinetic µ (µ_k)

Coefficients of friction for different materials in contact

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Static friction > Kinetic friction

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f_s> f_k
Because:

The interlocked areas of two static surfaces must be “broken off” or plastically deformed before the surfaces can move.
Abrasion must occur, which requires a large amount of energy.
Once the surfaces are moving, the interlocked areas have already broken off and plastically deformed, requiring less energy to keep moving.
To overcome static friction requires more energy than to overcome kinetic friction, due to more energy required for ABRASION OF THE SURFACES WHEN APPLYING FORCE ON STATIONARY SURFACES.
Ultimately, it is easier to keep an object moving than to make it move.

FRICTION LAB:

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AIM: Investigating the relationship between normal force and friction.

INVESTIGATIVE QUESTION: What is the relationship between the normal force and friction?

Experiment at a glance:

A container with mass piece inside is pulled along on a flat, level surface.
By adding more mass to the container, we will measure the resulting frictional force.
The frictional force acts between the container and supporting surface.
We must determine how the frictional force relates to the normal force.

REMEMBER:

Where the object is on a flat level surface, the normal force is equal to the weight.

F_N = Weight = m × g

MATERIALS AND APPARATUS:

container (with string attached) holding mass pieces
extra mass pieces
spring balance
weighing scale

METHOD:

Measure the mass of the container with one mass piece in it on mass scale. Record in a data table.
Calculate and record the weight and then the normal force on the mass.
Put the container with mass piece on the table or solid surface.
Attach the spring balance to the hook.
Make a small mark on the desk from which to start pulling the container with mass piece.
Pull sideways to the point that the block just starts moving.
Record the force reading in a data table.
Put a second mass piece into the container. Record the total mass.
Calculate and record the normal force.
Pull it sideways to the point that it starts to move. Record the force on the spring balance.
Repeat this three times. In each case, start the block from the same position and pull gently.
Repeat the experiment for larger masses and complete the table.

RESULTS:
Table to record the force required to overcome the frictional force and move the block.

Picture below of prac setup:

We have gathered the following data.

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Plot a graph.

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1. Plot a graph of the Friction on the block at rest against the Normal force on the block. (6)

ANSWER THESE QUESTIONS - HAND IN ON GOOGLE CLASSROOM AS PART OF YOUR PRACTICAL WRITE UP IN 24 HOURS

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NOTE:
The force applied to the point at which the block starts to move is equal to the frictional force acting on the block.

2. Answer the following questions.

Hand it in, with your graph, on Google Classroom on 8 April before 8:00 (this means before 8 AM).

2.1. What is the dependent variable?

______________________________________________________ (2)

2.2. What is the independent variable?

______________________________________________________ (2)

2.3. Identify one control variable.

______________________________________________________ (2)

2.4. Draw a labelled free-body diagram of ALL the forces acting on the mass container just as it is about to start moving.

(4)

2.5. Why is the weight on the mass container being changed when the aim of the investigation is to find out how the normal force affects frictional force?

_______________________________________________________

_______________________________________________________ (2)

2.6. Why should 3 readings be taken for each setup and an average calculated?

_______________________________________________________ (2)

2.7. What is the shape of your graph? Be specific with your answer.

_______________________________________________________

_______________________________________________________ (2)

2.8. What does the shape of your graph tell us about the relationship between frictional force and normal force?

_______________________________________________________ (2)

2.9. What do you think would happen if the mass container was placed on a rougher surface than what was used in the video practical?

_______________________________________________________

_______________________________________________________ (2)

2.10. Write down a conclusion for this experiment.

_______________________________________________________

_______________________________________________________ (2)

2.11. During the investigation, you should have noticed the readings from the spring balance differ once the mass container is moving with a constant speed. How do the readings for the stationary mass container and a moving mass container compare? Give a reason for your answer.

_______________________________________________________

_______________________________________________________ (2)

TOTAL: 30

FREE BODY DIAGRAMS

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A free body diagram is a sketch of a body:

with all of the forces acting on the body shown.
- the body is free from the surrounding objects
- the body is drawn as a dot
- all the forces acting are drawn pointing outwards from the center of the body

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FREE BODY DIAGRAMS OF DIFFERENT SCENARIOS

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A man sitting on a chair:

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F _Net = 0

A plane flying forward at constant velocity:

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F _Net = 0

A car accelerating:

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F _Net > 0

FBD

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A car decelerating (applied force from engine < friction on road):

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F _Net < 0

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A car braking (decelerating under the influence of friction with no engine applied force):

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HOMEWORK

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EXERCISES 1-3

STATIC FRICTION VS. KINETIC FRICTION

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Which one is smaller?

FBD OF STATIC FRICTION ON AN INCLINED PLANE

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FREE BODY DIAGRAMS

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free body diagram is a sketch of a body:

with all of the forces acting on the body shown.
- the body is free from the surrounding objects
- the body is drawn as a dot
- all the forces acting are drawn pointing outwards from the center of the body

EVERYDAY SCENARIOS OF FORCES ON OBJECTS AND THEIR FBDs

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PAGES 39-50 (Notes)

You need to know:

7 different scenario's of everyday examples of forces acting on an object
and how to draw their FBD's
and how to calculate the net force in each case

1) SIMPLE SCENARIO: A BOOK (ANY OBJECT) ON A TABLE, BEING PUSHED TO MOVE AT CONSTANT VELOCITY

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FBD

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TWO OBJECTS IN DIRECT CONTACT THAT ARE BEING PUSHED IN ONE DIRECTION

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FBD

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ANSWERS - FORCES NOTES

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REVISION - PART 1

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WORKSHEET 1

REVISION - PART 2

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WORKSHEET 2

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NEWTON III

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Gravity is the weakest of all forces in the Universe

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F_g is 10 duodecillion (10⁴⁰) times weaker than the strong force.

MEMO WORKSHEET 1

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MEMO WORKSHEET 2

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