Turning Forces AQA Physics Unit 3

Covers the topic 'Turning forces' in AQA's Physics Unit 3.

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- 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.

Calculating Moments:

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.

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- 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.

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- For an object in equilibrium, the sum of the anticlockwise moments about any point = the sum of the clockwise moments about that point.

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- 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.

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- 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.

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- 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.

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- 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.

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- 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.

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