**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.

Example

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

Velocity

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)

Acceleration

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

Weight

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

Example:

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

Stone falling from a height with air resistance (non-uniform)

Questions

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

Static

-two surfaces in contact are not sliding relative to each other

- higher friction

Dynamic

-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

References

Physics IGCSE Textbook by Stephen Pople

http://www.mysciencesite.com/motion_graphs.pdf

http://www.physicsclassroom.com/class/newtlaws/u2l1d.cfm

http://www.physicsclassroom.com/class/newtlaws/Lesson-4/Identifying-Action-and-Reaction-Force-Pairs