Physics

A moment is the "turning effect of a force" and is measured in newton metres, or Nm

moment (Nm) = force (N) X perpendicular distance from line of action to axis of rotation (m)

If something isn't turning the anti clockwise and clockwise moments must be balanced

The centre of mass is the point at which the mass of an object acts, but can often be where there is no physical material

For a symmetrical shape, it lies along the axis of symmetry

You can find the centre of mass by suspended an object, eg a piece of paper, and drawing a line directly down from the point from which is hanging. Then change the point, and repeat. Where the lines cross is the centre of mass.

Stability is how likely an object is to stay in a certain position

The line of action is the line of force that can be drawn from the centre of mass straight to the ground. When this line of action falls outside of the base, the object topples over.

TO INCREASE STABILITY:

• Lower centre of mass (object needs to be tilted more before the line of action falls outside of the base)
  
• Increase width of base (see above)

(For better explanations, see my PowerPoint)

ACCELERATION= a change in velocity, and velocity depends on direction

Therefore an object moving in a circle is accelerating as it's velocity/direction is changing.

CENTRIPETAL FORCE= force which keeps an object moving in a circle, acting towards the centre of the circle

FORCE = MASS X ACCELERATION

CENTRIPETAL FORCE DEPENDS ON:

• Mass of object (larger mass=greater force)
• Speed of object travelling (greater speed=greater force)
• Radius of circle (smaller radius=greater force)

Because planets move in elliptical, almost circular orbits around the sun, we can apply theories about circular motion to them.

(an ellipse has 2 focus points, whereas a circle only has 1 in the centre)

GRAVITY= the centripetal force which keeps planets orbiting. The rules that apply to centripetal forces apply to gravity also:

• Planet further away = smaller gravity/force
• Planet with more mass= greater gravity/force

The further away a planet is the longer it's orbit time because...

• It has further to go
• It moves more slowly due to the fact that it needs to move at the correct speed to keep in orbit

When the speed of movement is constant:

• Twice the radius of an orbit=half the force needed to keep orbiting

When the distance from the sun increases, the force of gravity acting on it decreases. The planet needs to slow down because other wise, due to the fact that a greater speed requires a greater force, the gravitational force would not be great enough to keep it in orbit.

• The speed of a planet in orbit does not depend on its massbecause the larger force need to keep in orbit and the larger force generated by its extra mass balance each other out

This stuff is confusing!!

PTOLEMY:

• Earth in centre
• Didn't predict planets movements accurately
• Didn't explain why they don't always move smoothly west ---> east

NICOLAS COPERNICUS:

• Sun at centre, circular orbits
• Observations matched theory better, but not perfectly
• Conflicted with Christian ideas

GALILEO GALILEI:

• Found out that objects could orbit around things other than earth, using a telescope to observe moons around jupiter