- The moment of a force F about a pivot is F x d, where d is the perpendicular distance from the pivot to the line of action of the force.
Moment = Force x perpendicular distance from the pivot
A force of 50N is exerted on a claw hammer of length 0.30m. Calculate the moment of the force.
Moment: 50N x 0.30m = 15Nm.
Therefore the moment is 15Nm.
CENTRE OF MASS
- The centre of mass of an object is the point where its mass may be thought to be concentrated.
- When a suspended object is in equilibrium, its centre of mass is directly beneath the point of suspension.
- The centre of mass of a symmetrical object is along the axis of symmetry.
MOMENTS IN BALANCE
- For an object in equilibrium, the sum of the anticlockwise moments about any point = the sum of the clockwise moments about that point.
- The stability of an object is increased by making its base as wide as possible and its centre of mass as low as possible.
- An object will tend to topple over if the line of action of its weight is outside its base.
- For an object moving in a circle at constant speed:
- the object accelerates continuously towards the centre of the circle.
- the centripetal force needed increases, either as the mass or the speed of the object increases, OR as the radius of the circle decreases.
- The force of gravity between two objects is:
- an attractive force
- bigger the greater the mass of each object is
- smaller, the greater the distance between the two objects is.
- A satellite in a geostationary rbit has a period of 24 hours and stays at the same position directly above the earths equator.
- Geostationary orbits are usually used of communication satellites.
- Monitoring satellites are usually in low polar orbits.
- The larger an orbit is, the longer the orbiting body takes to go round the orbit.
To stay in orbit at a particular distance, a planet must move at a particular speed around it's star.
- If it's speed is too low, it will spiral into it's star.
- It it's speed is too high, it will fly off its orbit and move away from it's star.