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Slide 1

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Unit 1:Turning forces and centre
of mass
Moment = Force x Distance (Between line of action and Pivot)
The force of the object would causes a turning effect:
Large the force would cause a large moment
Longer the object cause a larger moment
Turning at a right angle causes a larger turning force
Centre of mass
Centre of mass is where the whole mass is concentrated
An object will swing until the centre of mass is vertically below the
point of suspension
The object weight acts at a distance from the pivot which creates a
moment about the pivot . This make is swing
To find the centre of mass on simple shapes draw the line of
symmetry and were the lines cross is were the centre of mass is
Physic 3
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Slide 2

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Balance moments and stability
Stable object have a wide base
2m Y
An object will begin to tip over if the
centre of the mass moves beyond the
edge of the base
300 N 700 N
Moment- if the weight doesn't act in
2 x 300 700 x y
line with pivot there will be a resultant = 600 = 600
moment. This will either right the
object or tip right over
If the anticlockwise is equal to clockwise
moment the object wont turn. Vice versa
Anticlockwise moment = clockwise moment
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Slide 3

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Circular motion
Velocity is constantly changing
Velocity is both the speed and direction of a object
If an object travelling in a circle and continently changing direction this means it is accelerating
The force act towards the centre of the circle. This force making the object travel in a circle is called
centripetal force
As force which is actual providing a centripetal force in a given
Friction ­ the centripetal force on the race track is towards the centre of the circle .
The force is friction between the car tyres and road
Tension- the centripetal force comes from tension in the rope. The rope breaks the
bucket would fly of the tangent
Gravity- the centripetal force keep the earth in orbit with the help of gravity
keeping it in a circle
Centripetal force depends on mass, speed and radius
·The faster the object the bigger the centripetal force keeps it moving in a circle
·The heavier the object the bigger centripetalPhysic
force 3 keeping it in the circle
·Larger force keep an object in a small circle SR…read more

Slide 4

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Gravity and planetary orbits
Gravity is the centripetal force that keep planets in orbit
Gravity is a force of attraction between mass. Larger the mass the greater the force
The further the planet is from the sun the longer it takes to orbit because gravity decreases
The orbits have slightly elliptical elongated circle with sun at one focus of the ellipse
Orbit is possible when their is a balance between a forwards motion of the object
and gravitational force pulling inwards. If this doesn't happen it would get pulled in
or fly of the tangent
Gravity decrease quickly as you get further
With larger mass like stars and planets, gravity is big and felt long way out
Closer you are to the sun, the stronger the attraction
The more closer you are to the sun the less time it take to orbit the sun
Satellites, spaces stations and comets are kept in orbit as well
Long the distance the less gravity you have Physic 3
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Slide 5

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Gravity and planetary
Geostationary satellites used for communication
These sorts of satellites ideal for telephones and TV
because they stay in the same place
The satellites transfers signals from one side to
24 hours to complete orbit
Low polar orbit satellite
These satellites scan the planet each day
Good for weather and spying
For each orbit take few hours, sweeps past both post
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Slide 6

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Plane mirror reflections
When light hits an uneven surface, light reflects at different angles
this is called diffuse reflections .When light hit a even surface the light meets
for form a picture, this is how plane mirrors work.
· Angle light ­ angle of reflection
· Incidence light ray- angle of incidence
· The dotted line is called normal
· Refraction means a bend of light as it enters an object
· Refraction is caused by the change of speed
· Refraction is the reason why many objects in the water
· appear to be closer due to the fact when light hit the bottom it
speed up
· Real image is where the rays of light meet and can be put on a screen
· Virtual image is where the rays diverge e.g. Mirror
· Another virtual image could be a magnifying glass
To describe an image you need to say
· How big it is compared to the object ?
· Whether it is upright or inverted?
Whether it is real or virtual?
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