The Turning Effect of Forces Flashcards
What is the equation for the moment of a force?
Moment = force x perpendicular distance from pivot
What is the moment of a force?
It’s ‘turning effect’
How do levers use the moment of a force?
Levers are used to increase the turning effect of a force by increasing the distance from the pivot (think about the equation, if larger perpendicular distance from the pivot, smaller force is needed for the same moment or ‘turning effect’)
What is the equation for the moments in a balanced system?
The sum of clockwise moments = the sum of anticlockwise moments
(just another way of saying that: force x perpendicular distance on one side = force x perpendicular distance on the other)
What is the centre of gravity of an object?
Where the whole weight of an object acts through one point of the object.
How do you find an objects centre of gravity?
If a shape is suspended from any point, it will come to rest with the centre of gravity immediately below the point at which it is suspended. If asked to find the centre of mass of an object in an experiment, just hang it from two different points (three if you want to be accurate). Draw the lines of the centre from where it hangs on and find the point where said lines meet.
What two conditions must be fulfilled if a beam is stationary?
- The sum of upward forces acting on the beam is equal to the sum of downward acting forces.
- The sum of clockwise moments acting on the beam about any point is equal to the sum of anti-clockwise moments acting on the beam.
How is the principle of moments used in a simple system of parallel forces acting in one plane?
There are many practical situations in which an object can rotate about a pivot in both directions (e.g. seesaw, steering wheel)
The object in question will be in equilibrium (i.e. it will either be stationary or turning at a constant speed) if the total moment it turn it one way is equal to the total moment trying to turn it the other
What does it mean for an object to be in equilibrium?
It will either be stationary or turning at a constant speed
How do upwards forces on a light beam, supported at its ends, vary with the position of a heavy object placed on the beam?
Consider a car weighing 8000N crossing a beam bridge. We will ignore the weight of the beam.
When the car is in the middle, the forces exerted by the supports at each end will be equal and must add up to 8000N - i.e. they are both 4000N
But when the car is closer to one end, the support force will be larger at that end.
How can you calculate the support forces on a car on a beam that weighs 8000N and is 12.5m from one end and 7.5m from one end (20m in total)
Anticlockwise moment = clockwise moment, so taking moments around the left hand support (F1):
F2 x 20m = 8000N x 12.5m = 100,000 Nm
F2 = 100,000nm / 20N
F1 = 8000N - 5000N = 3000N
What is the unit of moment of force?
Newton-metres (Nm)
