force and acceleration simplified

Force and acceleration

IGCSE 1

Forces

• What is a force?

• A force is a push or a pull.

• An objects behaviour is dependant on all the forces acting on it.

• Force is measured in Newtons (N)

Effects of forces

• The size and direction of the forces acting on an object will determine how that object moves or behaves.

Forces can:1. Change the speed of an object2. Change the direction of movement of an object3. Change the shape of an object

Types of forcesWeight – the gravitational force between the earth and an object.

What is the difference between mass and weight?

Weight = mass x gravitational acceleration

The weight of an object changes according to its position in the universe because the value of g changes.

Other forces

• Tension:This is the force that acts through a rope.

• Friction: Friction is a force that acts to oppose motion.

• Normal:The normal force is the force exerted by a surface on an object with which it is in contact.

Adding Forces

Forces acting in a straight line:10N5N

10N 5NWhat will be the total Force?

10N

4N

10N 4N

Adding Forces

Forces acting in a straight line:8N

3N

8N 3NWhat will be the total Force?

10N

4N

10N 4N

A car of mass 900kg is travelling along a road at .

Forces that act on the car:Accelerating force of the engine (Weight of the car ()Friction of road on tyres ()Normal force of the road ()

Adding Forces: Parallelogram Law

Go to page 81 in your textbook and follow the instructions on the Parallelogram Law.

Worked Example• Find the resultant of two forces of and acting at an angle

of to each other.

• Using a scale of 1cm = 1N, draw parallelogram ABDC with AB = 5cm, AC = 4N and the angle .

• Use the parallelogram law to find the resultant of these two forces. (it must have magnitude and direction).

• Answer = 8.3N acting at an angle of to the 5.0N force.

Vectors and Scalars• Scalars are quantities with magnitude ONLY e.g. speed,

distance

• Vectors are quantities with BOTH magnitude AND direction e.g. velocity, displacement, acceleration

• A vector is represented by an arrow, where the line indicates the magnitude and the arrow points in the direction of motion.

•

Questions

• Do the questions on pg 70

Newton’s 1st Law

• What causes a car to come to rest when the engine is switched off?

• What would happen if these forces weren’t around?

Newton’s 1st Law

• Force is not needed to keep a body with uniform velocity in motion as long as no opposing forces act on it.

• This is summed up in Newton’s First Law

A body stays at rest, or continues to move at constant velocity, unless acted upon by an

external force.

Mass and Inertia

• Inertia is the property of a body that resists a change in it’s motion. i.e. if it is at rest, it wants to stay at rest.

The Egg Experiment

Mass and Inertia

• The larger the mass of a body, the larger its inertia (resistance to movement)

Newton’s Second Law

If there is an unbalanced force acting on an object, the object will accelerate in the

direction of the force. The acceleration is directly proportional to the force and inversely

proportional to the mass of the object.

Newton’s Second Law

• A CONSTANT force causes a CONSTANT acceleration

• CONSTANT VELOCITY means there is NO resultant force acting on the object.

• CONSTANT ACCELERATION = CONSTANT FORCE acting on the object

Worked Example

1. A block of mass 2kg is pushed along a table with a constant velocity by a force of 5N. When the push is increased to 9N, what is:

a) The resultant forceb) The acceleration

Worked Example

1. A car of mass 1200kg is travelling at is brought to rest in 4s. Find:

a) The average decelerationb) The average braking forcec) The distance moved during the deceleration.

Newton’s Third Law

If body A exerts a force on body B, then body B will simultaneously exert a force on body A. The

forces are equal in magnitude but opposite in direction.

These forces are called an ACTION-REACTION pair.

Note: the forces are applied to different objects so no resultant force can be found.


F1: Earth pulls book down F4: Book pulls

earth up

F3: Book pushes table down

F2: Table pushes book up

• The book does not move because F1 = F4 (balanced forces)• What are the action-reaction pairs? (F1+ F2 and F3+F4)


Example 3: Horse pulling a cart

Horse pulls cart forwardCart pulls horse backwards

Friction

The cart will only move when: pull on cart > friction on cart

The cart will not move if pull on cart = friction on cart

Newton’s Third LawExample 4: Motion of a rocket

Exhaust gases push rocket up = THRUST

Engine pushes exhaust gases down

Earth pulls rocket down

Rocket pulls earth up

Questions

• Do the questions on page 108

Hookes LawWhen an elastic object - such as a spring - is stretched, the increased length is called its extension. The extension of an elastic object is directly proportional to the force applied to it:

• F is the force in newtons, N• k is the 'spring constant' in newtons per metre, N/m• is the extension in metres, m

• This equation works as long as the elastic limit (the limit of proportionality) is not exceeded. If a spring is stretched too much, for example, it will not return to its original length when the load is removed.

The spring constant

The spring constant k is different for different objects and materials.

It is found by carrying out an experiment. For example, the unloaded length of a spring is measured.

Assuming the limit of proportionality (elastic limit) is not exceeded, a graph of force against extension produces a straight line that passes through the origin. The gradient of the line is the spring constant, k. The greater the value of k, the stiffer the spring.

How are materials affected by stretching?

• A spring (or length of wire) will stretch if weight is added .

• The wire will stretch in proportion to the load up to a certain point (the limit of proportionality)

• What happens after this point?

Moments

• Moments make things turn or rotate.

• A moment is the turning effect of a force around a fixed point called a pivot.

MomentsThe size of a moment depends on two factors:

1. the size of the force applied 2. the perpendicular distance from the pivot to the line of

action of the force

Moments

SMALL MOMENTThe distance from the fulcrum to the

line of action of force is very small

LARGE MOMENT The distance from the fulcrum to the

line of action of force is large

Moments

• M = the moment of the force in newton-metres, Nm

• F = the force in newtons, N• d = the perpendicular distance from the line of

action of the force to the pivot in metres, m

Worked example

A spanner is used to undo a nut. A force of 25 N is applied to the end of the spanner, which is 10 cm away from the centre of the nut. Calculate the moment when the spanner is horizontal.

Balancing moments

• Where an object is not turning around a pivot, the total clockwise moment must be exactly balanced by the total anti-clockwise moment.

sum of the clockwise moments = sum of the anti-clockwise moments

Are they balanced??

Levers• A lever is a simple machine that makes work easier to do.

Examples of simple levers include cutting with scissors, or lifting the lid on a tin of paint with a screwdriver.

• Levers reduce the force needed to perform these tasks.

• When someone uses a lever, they exert a force (the effort) around a pivot to move an object (the load).

Levers

Practical: Verifying the Principle of Moments

• Follow the instructions on pg 60 of your textbook to verify the principle of moments

When a body is in equilibrium the sum of the clockwise moments about any point equals the

sum of the anticlockwise moments about the same point.

Questions

• Do the questions on pg 62

Centre of Mass

• Mass is the amount of matter an object has.

• Centre of Mass: This is the point at which any object with mass will balance.

Finding the centre of mass of an irregular object: plumb line

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

2. Hang a plumb line (a piece of string with a weight hanging from it) from the same suspension point. This lets you mark the vertical line directly below the suspension point.

3. Drill another hole at a different location within the object.

4. Again hang a plumb line to determine the vertical and mark it on.

5. The point at which the two marked lines cross is the centre of mass.

Toppling

• A body topples when the vertical line through its centre of mass falls outside its base.

Which is more stable? Why?

What makes objects stable?

Toppling

The stability of a body is increased by:

1. Lowering its centre of mass.2. Increasing the area of its base.

Motion in a curved path due to a perpendicular force.

• Newton’s 1st law – an object with no resultant forces acting on it will continue to move in a straight line at constant velocity.


• If an object is moving in a circular path the direction is always changing – but a force must be present to create this change!

• If the direction is constantly changing then the force must also be constantly changing.


• Centripetal force - the force that always acts towards the centre of the circle.

• This force acts perpendicularly to the motion of the object


Examples of circular motion:1. The moon orbiting the earth2. A car turning a corner3. A train going around a bend


What would happen if the string that was spinning an object around broke?

Questions

• Do questions 1, 3, 4 and 5 on page 67

force and acceleration simplified

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