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Unit 2: Forces and Motion

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Justin Evanson

on 2 June 2014

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Transcript of Unit 2: Forces and Motion

Speed, Velocity and Acceleration (Justin)
Forces and Motion
Motion graphs are an important tool which is used to show graphs of speed, position and time.
Using graphs is a easier way to find out speed or position change over the time,
Motion Graphs

Recording Motion (Bares)
Free Fall ( Mona)
More motion graphs (Justin)
Forces in balance
Force, mass and Acceleration
- The force that tries to stop materials sliding across each other
e.g. Hands, Shoes
- Measure of how hard it is to slide one object over another
Friction and braking

By: George, Ron, Bares, Mona and Justin 10A

Force, Weight and Gravity
Action and Reaction
More about Vectors
Moving in Circles (Geo)

When two forces acting on an object are equal in size but act in opposite directions, we say that they are balanced forces.
Unbalanced forces
When two forces acting on an object are not equal in size, we say that they are unbalanced forces.
If the forces on an object are unbalanced this is what happens:
- an object that is not moving starts to move
-an object that is moving changes speed or direction
If the forces on an object are balanced (or if there are no forces acting on it) this is what happens:
-an object that is not moving stays still
-an object that is moving continues to move at the same speed and in the same direction
For every action, there is an equal and opposite reaction.
Action: Weight of book = gravitational attraction of earth on book (downwards)
Reaction: Gravitational attraction of book on earth (book pulls earth upwards towards it!)
-statement means that in every interaction, there is a pair of forces acting on the two interacting objects.
-The size of the forces on the first object equals the size of the force on the second object.
Using ticker tape is an effective way of recording motion. The faster the motion the bigger the gaps between dots are.
The average speed between two points can be calculated by (average speed = distance moved/time taken)
Unit: if distance is measured in meters (m), and time in seconds (s) the unit is m/s
To find the actual speed of car, how far the car moves in shortest time have to be measured (speed = distance/time)
Speed of something with direction of travel
Motion in straight line can use + or – to indicate direction
Might have vectors to show direction (will be discussed later)

An object is to be said accelerating if its velocity is changing
Calculation (= change in velocity (final-initial)/time taken
Measured in m/s^2 (- square)
Acceleration is a vector

All masses attract each other
Greater mass, greater force
Closer the mass, stronger force
Earth is massive so its gravitational pull is strong
Gravitational Force

Same as Earth’s gravitational force on an object
Newton (N)
Near Earth’s surface 1 kg = 9.8 N
In Qs 1 kg = 10 N
Gravitational Field Strength (g)
A region for a mass to experience fore
Near earth’s surface 10 N per 1 kg (N/kg)
Weight = mass x g
Weight is not mass. Mass is fixed, weight can change
Example: on Moon weight is more due to stronger gravitational weight but mass is the same
Acceleration (Force = mass x acceleration)

Uniform and Non-Acceleration:
Uniform acceleration is steady, gaining velocity at steady rate

Stone falling from a height with no air resistance (uniform)
Stone falling from a height with air resistance (non-uniform)

The meteors are falling in
to the Earth with gravitational (g)
force of 10N/kg and assume that
there isn't any air resistance in space
and in Earth’s sky. Meteor A is 5 kg and
Meteor B is 10 kg.

a. What is the weight of each Meteor?
b. What is the acceleration of each Meteor?
c. What is the gravitational field strength?
Explain why if a car is traveling
at 200 km/h braking distance is more
than the speed of the car
traveling only at 100 km/h
Solution to Question
Number 1
a. Meteor A 50N (5 kg * 10N),
Meteor B 100 N (10 kg * 10N)

b. 10 m/s^2, (5 kg * 10N) -> resultant force
(since air resistance is 0), 50/5), (10 kg * 10N) ->
resultant force (since air resistance is 0), 100/10)

c. 10 N/kg

Number 2
The kinetic energy needed to be lost to stop the car from speed is more. Apart from theory, the kinetic energy might be even more than doubled (due to real life factors)
> Downward movement under the force of gravity in a vacuum

> Works in vacuum, which makes every objects have the constant speed when it fall

> Free Fall happen because there's less or no air resistance

> Gravity makes free fall happen

> In free Fall the acceleration remain constant
> 2 types of quantities: Vector and Scalar

> Vectors he force that applied in a direction as well as a magnitude (size)

> Quantities is the
> Vector work in a direction But, scalar have no direction
> Example for vector: Velocity and force
> Example for scalar: Mass and Volume

Static and dynamic Friction
-two surfaces in contact are not sliding relative to each other
- higher friction
-two surfaces sliding relative to each other
-heats materials up
-less friction
Stopping Distance
thinking distance : before brakes are applied, driver still reacting
breaking distance : after brakes have been applied
Centripetal force
mass of an object is increased
speed of an object is increased
radius if a circle is reduced
Changing velocity
velocity is speed in a particular direction
change in speed or a change in direction
it has acceleration
Physics IGCSE Textbook by Stephen Pople
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