1.5 Forces - IGCSE Physics

Force

A force is a push / a pull

It can cause an object at rest to move/ if the body is already moving it can change the speed/direction of motion

A force can change a body's shape/size

Extension in springs

Robert Hooke found that the extension was proportional to the stretching force provided the spring was not permanently stretched

Extension is ∝ to the stretching force

It is true if the limit of proportionality of the spring was not exceeded

Spring constant

Spring constant is the force per unit extension

Forces & Resultants

Force has both magnitude (size) & direction

Usually more than 1 force acts on an object

If the forces act in the same straight line, the resultant is found by simple addition/subtraction

If they don't, they are added by parallelogram law

Parallelogram Law

The parallelogram for adding 2 forces is :

If two forces acting at a point are represented in size and direction by the sides of a parallelogram drawn from the point, their resultant is represented in size and direction by the diagonal of the parallelogram drawn from the point

Newton's First Law

"An object stays at rest or continues to move in a straight line at constant speed unless acted on by a resultant force."

The smaller the external forces opposing a moving body, the smaller is the force needed to keep it moving with constant velocity

Resultant force may change the velocity of an object by changing its direction of motion/its speed

Inertia

The longer the mass of a bod, the greater its inertia i.e. the more difficult it is to move it when at rest & to stop it when in motion. The mass of a body measures its inertia.

Inertia is the resistance of any physical object to any change in its velocity. A property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force.

Newton's Second Law

i) Acceleration, a is directly proportional to the applied force F for a fixed mass, i,e, a is directly proportional F and

ii) inversely proportional to the mass, m for a fixed force, i.e. a ∝ 1.m

Therefore :-

a∝ f/m F = kma

k (is the constant of proportionality)

F-> Resultant/ unbalanced force causing acceleration a in the same direction as F. F must be in newtons, m in kg and a in m per second

The law proves that a will be the largest when F is large & m small

Friction

Force that opposes one surface moving, trying to move, over another.

Newton's 3rd Law

"If a body A exerts a force on body B, then body B exerts an equal but opposite force on body A."

Forces never occur singly but always in pairs as a resultant of action between 2 bodies.

Circular Motion

A movement of an object along the circumference of a circle or rotation along a circular path

It can be uniform, with constant angular rate of rotation & constant speed, or non-uniform with a changing of rotation

Centripetal Force

A centripetal force is a net force that acts on an object to keep it moving along a circular path. The tension force in the string of a swinging tethered ball and the gravitational force keeping a satellite in orbit are both examples of centripetal forces.

Balancing a beam

To balance a beam about a pivot, the weights must be moved so that the clockwise turning effect equals the anticlockwise turning effect & net moment on the beam becomes zero.

Principal of moments

Law of moments stated that :-

"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. There is no resultant moment on an object in equilibrium."

Levers

A device which can turn about a point

In a working lever a force called the effort is used to overcome a resisting force called the load.

Conditions for equilibrium

i) The sum of the forces in one direction equals the sum of the forces in the opp. direction

ii) The law of moments must apply

When there is no resulttant force & no resultant moment, an object is in equilibrium

Equilibrium Experiment

In this experiment, diff. weights (F) are suspended either side of the central pivot and the distance (d) of each from the pivot is measured when the beam is balanced (in equilibrium)

The clockwise & anticlockwise moments (Fxd) are then calculated for each weight. It's found that when the beam is in equilibrium the clockwise anticlockwise moments are equal in magnitude & there is no resultant moment (i.e. no net turning effect) on the beam.

Centre of gravity

The point through which all of an object's weight can be considered to act.

Toppling

The position of the centre of gravity of an object affects whether or not it topples easily

An object topples when the vertical line through its centre of gravity falls outside its base. Otherwise it will remain sable when the object will not topple.

The stability of a body is therefore increased by

i) lowering its centre of gravity

ii) increasing the area of its base

An object is in stable equilibrium if ehen slightly displaced & then released it to its previous position.