# Physic Unit 3 Chapter 2

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• Created by: A.B.
• Created on: 16-04-13 18:02
• Physics Unit 3 Chapter 2
• Moments
• The moment of a force is the measure of the turning effect of the force on an object.
• The moment of a force F about a pivot = F x d, where d is the perpendicular distance from the line of action of the force to the pivot.
• To increase the moment of  a force F, increase F of increase d.
• The weight of an object is called the load. The force the person applies is called the effort.The point at which the item moves is called the pivot or fulcrum.
• A moment is measured in Nm.
• Centre of Mass
• The centre of mass of an object is that 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.
• Same as distance x load on one side = distance x load on the other side.
• Can be applied when no obvious load, just the weight of the beam.
• If an object at rest doesn't turn, the other statement must apply.
• For an object in equilibrium, the sum of the anticlockwise moments about any point = the sum of the clockwise moments about that point.
• Same as distance x load on one side = distance x load on the other side.
• Can be applied when no obvious load, just the weight of the beam.
• To calculate the force needed to stop an object turning we use the equation to the right. We need to know all the forces that don't act through the pivot and their perpendicular distances from the line of action to the pivot.
• Stability
• 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.
• An object topples over if the resultant moment about its point of turning is not zero.
• If the clockwise moment is not balanced by the anticlockwise moment, which happens if inside the base area.
• Hydraulics
• Pressure is force divided by the area which the force acts on. The unit of pressure is the pascal (Pa) which is equal to 1 N/m3.
• The pressure in a fluid acts equally in all directions.
• A hydraulic system uses the pressure in a fluid to exert a force.
• The liquid used is virtually incompressible.
• The force exerted by a hydraulic system depends on the force exerted on the system, the area of the cylinder which this force acts on and the area of the cylinder that exerts the force.
• When there are 2 different areas P(F x A1)  x A2.
• The hydraulic system is a force multiplier.
• Circular Motion
• The velocity of an object moving in a circle at constant speed is continually changing as the object's direction is continually changing.
• The acceleration constantly increasing towards the centre of the circle.
• Centripetal acceleration is the acceleration towards  the centre of the circle of an object that is moving round the circle.
• Centripetal acceleration is the acceleration towards  the centre of the circle of an object that is moving round the circle.
• The centripetal force on an object depends on its mass, its speed and the radius of the circle.
• The Pendulum
• The time period of a simple pendulum depends only on its length.
• This is the time it takes to complete one full cycle.
• To measure the time period  of a pendulum, we can measure the average time for 20 oscillations and divide the timing by 20.
• Friction at the top of a playground swing and air resistance will stop it oscillating if it is not pushed repeatedly.
• An oscillating motion is one that moves to and fro along the same line.
• The amplitude is the distance from the equilibrium position (centre) to the highest point.
• Time period (s) = 1/ frequency (Hz)