Physics - EDEXCEL IGCSE - Forces and Motion

Section 1 - Forces and Motion

average speed = distance moved / time taken

acceleration = change in velocity / time taken

Force - a push or pull of one body on another.

Examples of forces are:

• Applied force - transferred from a person or object to another. e.g. a man pushing a door.
• Gravitational force - a force caused by the attraction between objects by gravity.
• Normal force - when an object is in contact with another. e.g. a cup on a table.
• Friction force - when an object moves across a surface. e.g. a car driving.
• Air resistance - when an object travels through the air. e.g. a plane flying.
• Tension force - when an object is pulled in opposite directions.
• Spring force - when a compressed or stretched spring returns to its inert state.
• Electrical force - an attraction between oppositely charged objects.
• Magnetic force - an attraction caused by magnets.
• Upthrust force - buoyancy. e.g. the lift on a plane.

Vectors - something with both magnitude and direction. e.g. velocity, displacement.

F = MA

force = mass * acceleration 

W = MG

weight = mass * gravitational strength

p = Mv

momentum = mass * velocity

force felt = change in momentum / time taken

Airbags and such increase the time it takes for momentum to reach 0, thereby reducing the force felt.

Centre of mass - the point through which the weight of an object always acts through. 

Stability:

• Stable - the centre of gravity rises and returns to its equilibrium position. e.g. A ball in a bowl.
• Unstable - the centre of gravity falls and moves away from its equilibrium position. e.g. A ball in an inverted bowl.
• Neutral - the centre of gravity remains on the same level. e.g. a ball on a flat plane.

Moments:

Moment = force * perpendicular distance from the pivot

Taking the moment around ‘a’:

The anticlockwise moment = B * 10

The clockwise moment = 200,000 * 5 + 500 * 3

Therefore 10B = 1,001,500Nm

B = 100,150N

A + B = 200,500N

A = 100,350N

Taking the moment around ‘b’:

The clockwise moment =  A * 10

The anticlockwise moment = 200,000 * 5 + 500 * 7

Therefore 10A = 1,003,500Nm

A = 100,350N

A + B = 200,000 + 500

B = 100,150N

You can also take the moment around ‘c’:

Anticlockwise moment = B * (x + 10) + A * x

Clockwise moment = 200,000 * (x + 5) + 500 * (x + 3)

Therefore Ax + 10B + Bx = 1,001,500 + 200,500x

(A + B)x + 10B = 1,001,500 + 200,500x

(A + B - 200,500)x + 10B = 1,001,500

A + B = 200,500N

(0)x + 10B = 1,001,500

B = 100,150N

A = 100,350N

This proves that no matter where you take the moment from the result is always the same.

Stretching materials:

Hooke’s Law - the deformation of a body is directly proportional to the force applied to it, provided that the limit of proportionality is not exceeded.

This is translated into the formula F = kx where F is the force applied, x the extension of the spring and k the spring constant.

The limit of proportionality is the point at which the force and the extension of the spring are no longer proportional.

The elastic limit is the point at which the spring will not return to its original length when the force is removed. 

Elastic behaviour - materials are said to be elastic if they can regain their original size and shape.

Plastic behaviour - materials are said to be plastic if they do not return to their original size and  shape.