Physics- Unit 2- Forces in Equilibrium

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  • Created by: FireDwarf
  • Created on: 29-01-14 20:46
What is a vector?
A physical quantity with a direction as well as magnitude.
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What is a scalar?
Physical quantity that is not directional.
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How to find the resultant force using a scale diagram? When would I use it?
Put the two lines into a triangle shape. The angle diffrence between the two should come out of the corner they meet. Resultant is the connection between both. Useful for when we have 2 forces not at right angles.
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Using phythagarous?
Useful for right angles. a2 + b2= c2. Get values into triangle and use equation to find the resultant force.
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To find angle?
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How to write down the direction?
Can give it as a compass point eg: North of west or west of north. Can give to the horizontal and to the vertical if appropiate.
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Two forces in the same direction to find resultant force?
Add them
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Two forces in opposite directions to fnd resultant force?
Take them away.
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If I have the resultant force and the angle, what can I work out?
2 perpendiular componants. The y axis = the line x sin of angle, the x axis- the line x cos of angle.
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When is an object at equlibrium when two forces are acting on it?
When two forces acting on an object are equal and opposite to each other.
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When is an object at equlibrium when three forces are acting on it?
When two of the forces are opposite and equal to the other force.
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What is a moment?
Force x the perpendicular distance from the line of action to the point.
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Moment (nm) = (Force n) x (Distance m)
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When does a body turn?
If a force is applied onto it at any point but its center of mass.
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What happens if a body is effected by 2 or more forces but is in equlibrium?
The turning forces must be equal.
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Equation is therefore?
For moments of a force at equlibrium , sum of clockwise moments = sum of anti-clockwise moments.
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What is the center of mass?
It is a point where a single force on the body has no turning effect (where gravity and weight act)
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How do we find the center of mass? Uniformed shapes and un-uniformed?
Uniformed- Balance. Un-uniformed- Puncture holes in the shape. Hold the shape from that hole and draw a plum line directly down. Put other holes and draw plum lines. Where they all meet = CoM.
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How can we calcuate the weight of a ruler?
Balance it and work out CoM therefore. Use a known weight. Posiston the weight onto the ruler so it is agian at equlibrium. We should see the weight and the CoM now acting as moments to the pivot. Calcuate moment of W, then work out moment of CoM.
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How do work out the support force if there are two support forces in action?
If the two forces are x and y, then support force y is calculated by ; total weight multiplied by the distance from the weight to the x force/ total distance between x and y.
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What is a couple?
Two forces acting on a body that are equal and opposite but are not on the same line.
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How to caculate the moment of a couple?
Multiple the distance between the couples by 1 of the forces (there equal).
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What are the forces like if a body is in a stable equlbirium?
Its weight is in the same line as its support force, therefore making the forces opposite and equal, establishing an equlibrium.
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So if an object at equlibrium is displaced?
It will move back towards equlibrium by making its weight's line of action pass through the support force.
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What is an unstable equlibrium?
When a body is in equlibrium by having its forces balanced (eg: weight and the support force) but if displaced, its weight will not be in the same line of action as support, so not equlibrium. as weight moves further, will move away from equlibrium.
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What must the clockwise and anticlockwise be for tilting to occur.
The clockwise moment (or anti) must be larger then the clockwise moment.
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How do we calculate tilting?
assume clockwise = tilting force and anti= body. Clockwise is calculated by applied force multiplied by distance from pivot.
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anticlockwise calculated by the weight times the base/2.
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When does tilting turn to toppling?
An object topples if its line of action of weight suppases the pivot point.
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Talk about center of mass and toppling?
Lower the center of mass, more stable the object.
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When we talk about wind, what is this an example of? What should we discuss?
Wind is a tilting force. Can mention the heaviner it is, higher anticlockwise moment, so therefore higher clockwise needed (Force). But heavier, higher CoM.
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Talking about slopes, what is this an example of? Disuss?
Example of toppling. Heavier, higher CoM therefore less stable. Wider, lower CoM, more stable.
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Speed and Velocity
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What is velocity?
Change in displacement per unit time.
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distance per unit of time.
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On a distance time graph, what goes on the x axis and what goes on the y?
Time on the x and distance on the y.
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How do we calculate the speed?
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How can we relate speed to velocity?
Speed and velocity are the same if the object travels at the same speed in the same direction, but velocity changes if it changes direction.
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What is a displacement time graph? x and y?
Time on the x and displacement on the y. Take + as up for instance and - as down. We can get a curve if its thrown up, then comes back down.
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What is acceleration?
Change of velocity per unit time.
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What is uniform acceleration?
When the velocity of an object moving along a straight line increases at a constant rate .
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What is therefore constant?
The acceleration
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x and y axis on a speed time graph? How to calculate acceleration?
Time on x and speed on y axis. Acceleration is the gradient.
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What is non-uniform acceleration?
When the direction of motion or its speed changes, so its acceleration is not constant.
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How to find the acceleration therefore?
Depending on the value were looking at, we want to draw a line off this point. Then work out the height of the gradient traingle, then the base of the triangle. Divide the height by the base.
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What are the three types of speed time graphs?
Moving at a constant speed, moving at a constant acceleration, moving at a non-uniform acceleration.
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Constant speed?
Area under the graph, which is a rectangle, so work out area from this.
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Uniform acceleration?
Area underneath is a trapezium which can be calculated via half of height x base.
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Why is it a trapezium?
Because at 0, the speed is not 0 and is higher, not forming a triangle.
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If its non-uniform we must work out the area for each square, then count the squares present.
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Do heavier objects fall faster then lighter objects? Why?
Because there are not other external forces acting on the object apart from force of gravity, the value of acceleration is therefore 9.81..
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What is the inclined plane test?
Dripping water clock timed the procedure- more volume of water, longer time. Measured how long a ball took to travel equal distances down slope to rest.
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Ball gained speed as it went down the slope. The ball was therefore accelerating.
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Another test? (photos)
Release a ball from a height and use a ruler to measure distance. Camera flashes at a constant rate. As the images increase, we should see the ball travel further distances between images as its accelerating.
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What is a projectile?
any object where the only force acting upon it is gravity.
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3 Principles?
Acceleration of the object is always equal to g and downwards because of the force of gravity. Only effects verticle. Horozontal is constant. Independant of each other.
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What is the formula for calculating horozontal componant of displacement?
x= Ut because its in constant acceleration. (distance)
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Verticle compontant? (distance)
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What is a scalar?


Physical quantity that is not directional.

Card 3


How to find the resultant force using a scale diagram? When would I use it?


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Card 4


Using phythagarous?


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Card 5


To find angle?


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