Turning Forces

A moment is the turning effect of the force

Moment = force x perpendicular distance from the pivot

(M = F x D)

To increase the moment:

• Increase the size of the force
• Increase the length of the object

The centre of mass of an object is the point at which most of its mass is thought to be concentrated.

For a symmetrical object, the centre of mass is where the lines of symmetry meet

When a suspended object comes to rest (in equilibrium) its centre of mass is directly below the point of suspension

The stability of an object is increased by:

• Making its centre of mass as low as possible
• Making its base as wide as possible

The object will topple over if the line of action of its weight is outside its base.

For an object moving round a circle at a constant speed:

• The objects velocity is along the tangent
• The change in velocity is towards the centre of the circle

The object therefore accelerates towards the centre of the circle.

So the force on the object acts towards the centre of the circle

Any object moving in a circle must be acted on by a resultant force towards the centre of the circle. We call this resultant force the centripetal force.

• The centripetal force needed increases as speed increases
• The centripetal force needed increases as the radius of the circle decreases

• Is an attractive force
• Is bigger, the greater the mass of each object
• Is smaller the bigger the distance between the two objects

Gravitational field strength on earth = 10N per Kg

To stay in orbit, a planet must stay at a particular speed around the sun:

• If the speed is too low, the planet will spiral into the sun
• If the speed is too high, it will fly off its orbit

The further the planet from the sun, the less its speed as it orbits the sun because gravity is weaker further from the sun.

The further a planet from the sun, the longer it takes to complete an orbit because it has further to travel and travels at a lower speed.

Time for orbit = Circumference/Speed

If a satellites speed is too low it will fall to the ground. If its initial speed is too great, it will fly off into space.

A period of satellite is the time it takes to make one orbit. A satellite with a higher orbit will have a longer period than a lower one because it is travelling at a slower speed and it has further to travel.

Communications Satellites are at a particular height above the equator so their period of orbit is 24 hours and they orbit in the same direction as the earths spin. We call this geostationary.

Monitoring Satellites can be used for weather forecasting, military and police surveillance and environmental monitoring. They are in low orbits and their period of orbit is two/three hours