P3.2 Using Physics to Make Things Work Flashcards
What is the centre of mass of an object?
The point at which all the mass of the object is concentrated
What acts through the centre of mass of an object?
Its weight
What is a freely suspended object?
An object that rotate freely around one point of suspension
When a freely suspended object is at rest, where will the centre of mass be in comparison to the point of suspension?
When at rest, the centre of mass of the object will be vertically below the point of suspension
How do you find the centre of mass of an object?
Drawing lines according to the plumb line - the point where the two lines meet is the centre of mass
When does an object which is freely suspended swing?
When the object’s centre of mass isn’t vertically in line with its suspension point
What is the time period of a pendulum?
The time taken for the pendulum to swing from one side to the other and back
Why are pendulums used in clocks?
Because their time periods are always the same
What is the formula for working out a time period?
T = 1 / f ==> Time period in s = 1 / Frequency in Hz
How does the length of a pendulum affect its time period?
The longer the pendulum, the greater the time period
What is the length of a pendulum?
The distance from its suspension point to its centre of mass
What are three uses of pendulums?
Clocks, playground swings and fairground rides
What is a moment?
The turning effect of a force
What is the line of action of a force?
The straight line passing through the point at which F acts and in the same direction as F
What is the equation for finding a moment?
M = F x d ==> Moment of force in Nm = Force in N x Perpendicular distance in m
What is the perpendicular distance?
A line from the line of action of the force to the pivot in m
How do levers ‘multiply the force’ put into them?
By increasing the distance from the pivot at which the force is applied
What are two examples of levers?
Wheelbarrows and scissors
When doesn’t an object turn/when is an object balanced?
When the total anticlockwise moments on an object about any pivot are equal to the total clockwise moments, the object won’t turn
For a balanced object, how do the anticlockwise/clockwise moments relate?
Total anticlockwise moment(s) = Total clockwise moment(s)
What do the most stable objects have? (2)
A wide base and a low centre of mass
When will an object tip over?
When the line of action of its centre of mass moves beyond the edge of its base
How can you use moments to describe how an object topples over?
Once the line of action is beyond the edge of an objects base, the base acts as a pivot and a resultant moment causes the object to rotate around it (topple over)
How can you prevent objects from toppling over? (2)
By giving them a heavy round base
How does a heavy base help to stop an object from toppling over?
The centre of mass is closer to the ground
How does a round base help to stop an object from toppling over?
The outer edges of the base (the pivot) moves away from the centre of mass when you push it (as the object rolls) basically giving it a wider base
What does a liquid being incompressible mean?
You can’t squash it, the volume and density stay the same
What happens when a force is applied to one point in a liquid? Why?
Because they are incompressible, a force applied to one point in the liquid will be transmitted to other points in the liquid
How is pressure transmitted throughout a liquid?
Pressure in a liquid is transmitted equally in all directions
How do you calculate the pressure caused by a force?
P = F/A ==> Pressure in Pa = Force in N / Area in m²
Why is the force in a hydraulic system multiplied?
The pressure is equal at both pistons (the slave and master piston) as it contains a liquid, and due to the second piston having a larger cross-sectional area, it produces a greater force
What are hydraulic systems used in? (3)
Car breaking systems, hydraulic car jacks, and the manufacturing/deployment of landing gear on some aircraft
What is velocity?
Both the speed and direction of an object
What happens when an object is travelling in a circle?
It is constantly changing direction
Why does an object when travelling in a circle?
Because, even if its speed isn’t changing, its velocity is constantly changing
In what direction does an object accelerate towards when going round a circle?
It accelerates towards the centre of the circle
What is the centripetal force?
When an object travels around a circle, the centripetal force is a resultant force that is acting on an object which causes it to accelerate towards the centre of the circle
Which 3 forces provides the centripetal force in a given situation?
It can be tension, friction or gravity
When will friction be the force causing the centripetal force acting on an object?
E.g. a car around a bend - the centripetal force comes from the friction between the car’s tyres and the road
When will tension be the force causing the centripetal force acting on an object?
E.g. a bucket whirling around on a rope - the centripetal force comes from the tension in the rope
How does speed affect centripetal force?
The faster an object’s moving, the bigger the centripetal force has to be to keep it moving in a circle
How does mass affect centripetal force
The larger the mass, the bigger the centripetal force has to be to keep it moving in a circle
How does the radius of the circle affect centripetal force
The smaller the circle it’s moving in, the bigger the centripetal force has to be to keep it moving in a circle (as it has more ‘turning’ to do)
