Physics P3 - Revision Cards

How to find the Centre of Mass on a thin sheet of material

1.) A small hole is made in the material

2.) A plumb line is suspended from the hole

3.) Wait til the sheet stops moving

4.) Draw the plumb line

5.) Repeat 2,3,4

6.) The Centre of Mass is where the two lines cross

The Centre of Mass is the point at which the mass of a body is thought to be concentrated

The centre of mass of a symmetrical shape (body) is along the axis of symmetry.

Factors that affect Stability

• A wide base (stable)
• A narrow base (unstable)
• A low centre of mass (stable)
• High centre of mass (unstable)

If the line of action falls outside thebase of the body, the object is NOT stable as there is a resultant moment and will topple over.

A moment is the turning effect of a force

Force x Perpendicular Distance = Moment

(Newton) N x (metre) m = (Newton metre) Nm

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If the object is NOT TURNING, the anticlockwise moment must be balanced by the clockwise moment

Anticlockwise Moment = Clockwise Moment

Example - 3N x 4m = 12Nm

1.) The Penpendicular Distance from the pivot - the greater the perpendicular distance, the greater the size of the moment

3N x 10m = 30Nm

2.) The size of the Force applied - the greater the force applied, the greater the size of the moment

10N x 4m = 40Nm

3.) To get the maximum moment from a given situation, the force needs to be at right angles to the pivot (perpendicular distance). Pushing at any other angle means a smaller moment as the perpendicular distance between the line of action and the pivot is smaller

Velocity is speed, with a given direction

Newtons First Law - "If an object continues in a straight line at constant velocity, all forces acting on the object are balanced."

If a body moves in a circle, the forces acting on it are not balanced, it is constantly changing direction

When a body moves in a circle, it continuously accelerates towards the centre of the circle

This acceleration changes the direction of the motion of the body, not the speed.

The resultant force is called the centripetal force

The centripetal force increases as -

1.) The mass of the body increases

2.) The speed of the body increases

3.) Theradius of the circle decreases

The Earth, Moon and Sun and all other bodies are gravitationally attracted to each other

The bigger the masses of the body, the bigger the gravatational attraction

As the distance between two bodies increases, the gravitational attraction between them decreases

The further the object, the greater the time taken for the object to complete the orbit

Gravitational force provides the centripetal force that allows planets and satellites to maintain their circular orbits

To stay in orbit, the satallite must move at a particular speed.

There are 2 types of satallite orbits -

Geostationary

• Above the equator
• High
• Takes 24 hours to complete
• Uses - Communications (Telephone,TV etc)

Low Polar Orbit

• Over Poles
• Low
• < 6 hours
• Uses - weather, spying, observations of the earth

Light is an electromagnetic wave that travels in straight lines until it is either reflected or refracted

When light strikes a plane (flat) mirror, it is reflected -

There is an incident and reflected ray;

The angle of incidence and the angle of refraction are measured from the normal line;

The normal line is a construction line that is at 90 degrees to the surface of the mirror.

The angle of incidence = angle of reflection.

There are two types of images that you need to know about -

REAL IMAGE

• Formed when real light rays intersect to from an image
• Represented by a solid line

VIRTUAL IMAGE

• Formed when virtual rays intersect to form an image
• Represented by a dotted line

To describe an image, you need to -

1.) Say how BIG it is compared to the object

• Magnified (Bigger than the object)
• Diminished (Smaller than the object)

2.) Say whether it is the RIGHT WAY UP

• Upright (the right way up)
• Inverted (Upside down)

3.) Say whether it is REAL or VIRTUAL

There are 2 types of Curved Mirrors -

Convex

• Curve OUTWARDS
• Shiny on the outside
• DIVERGES light

Concave

• Curve INWARDS
• Shiny on the inside
• CONVERGES light

Centre of Curvature (C)

Uniformly curved mirrors are like a round portion of a sphere. The centre of the sphere is the centre of curvature.

Vertex

The centre of the mirrors surface

Focal Point (F)

Halfway between C and the vertex

Axis

The line down the centre of the mirror - C, the vertex and F lie on this

1.) An incident ray parallel to the axis will pass through the focal point when reflected

2.) An incident ray that passes through focal point will be parallel to the axis when reflected

3.) An incident ray that passes through the C is reflected back

Lines you need to draw for the exam -

• Parallel to axis --> reflected through focal point
• Passes through C --> reflected back

Between F and Mirror -

• Upright
• Virtual
• Larger than object

At F -

No image formed as reflected rays are parallel

Between C and F -

• Inverted
• Real
• Larger than object

At C -

• Inverted
• Real
• Same size as object

Beyond C -

• Inverted
• Real
• Smaller than object

USES -

Make Up Mirror (between F and Mirror)

1.) An incident ray parallel to the axis will pass through the focal point when reflected

2.) An incident ray that passes through focal point will be parallel to the axis when reflected

3.) An incident ray that passes through the C is reflected back

Lines you need to draw for the exam -

• Parallel to axis --> reflected through focal point
• Passes through C --> reflected back

At any point

• Virtual
• Upright
• Smaller than object

USES -

Shop security mirrors

Road Corners

All lenses change the direction of light rays by refraction

Light is refracted when is enters and leaves glass prisms

The ray bends towards the normal as it enters the denser medium

It bends away from the normal as it emerges into the less dense medium

Light can be split up into the different wavelengths of the spectrum in a prism

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