Mechanics Flashcards
Moment definition
The force multiplied by the perpendicular distance from the pivot to the line of action of the force
Unit of moment
Nm
Principle of moments
If a system is in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about the same point
Moment equation
M=Fd
What’s a couple
A pair of forced of equal size which act parallel to eachother but in different directions. Coplaner forces
How to calculate moment of a couple
Size of on of the forces x perpendicular distance between lines of action
Centre of mass definition
The single point an objects whole weight acts through
Find centre of mass
-Cut piece of card into irregular shape
-Hang card with string freely from a point
-Use ruler to mark a line where the string is
-Hang from a different point repeat 3x
-Centre is where lines intersect
Centre of mass: When will an object be stable?
Low centre of mass + wide base area
Centre of mass: When will an object topple over?
If a vertical line drawn downwards from centre of mass falls outside base area
What does the gradient of a displacement time graph represent
Velocity
What does a curved line show on a displacement time graph
Change in velocity
Acceleration
What does the gradient of a velocity time graph represent?
Acceleration
How to find displacement on a velocity time graph
Area under graph.
Area above x= positive change
Area below x= Negative change(returning to starting point)
What’s does a curved line show on a velocity time graph
Increasing /decreasing acceleration
Area under line on acceleration time graph
The change in velocity
Newton’s 1st law
If there are no resultant forces acting on an object it will either stay at rest or travel at constant speed in a straight line
Newton’s 2nd law
The acceleration of an object is proportional to the resultant force acting on it
Newton’s 3rd law
If object A exerts a force of object B, then object B exerts an equal but opposite force on object A
When is an object in freefall
When the only force acting on it is its weight
Galileo experiment
-Rolled balls with similar masses down a slope
-Acceleration still caused by gravity but rolled slower: easy for timing /air resistance negligible.
-He timed how long it took for the balls to roll down different lengths - Found distance travelled was always the same
-Proved acceleration was uniform regardless of mass
Projectile definition
An object where the only force acting on it is its weight.
What to use to calculate vertical components
Suvat equations
Effects of air resistance on projectiles
-Horizontal velocity will slowly decrease to 0 as horizontal force is now acting
-It will therefore not travel as horizontally
-Projectile will gain vertical speed more slowly
How to calculate horizontal component
D=ST
Friction definition
A force that opposes motion, turns kinetic into thermal energy, can never speed things up or start them moving
Drag definition
Friction caused by a gas or liquid
How does a wing generate lift
1) It diverts air down changing direction of the air by forcing it down
2)Newtons 3rd law air exerts equal and opposite force up on wing
3) Bernoulli’s principle
What does drag increase with
-Surface area
-Speed
-Density
How does an object reach terminal velocity moving down through fluid
-Initially accelerates
-As velocity increases drag increases
-Weight and drag forced will balance out
What is terminal velocity
When frictional forces are equal to driving forces acting on an object making resultant force 0. Object moves in constant velocity
principle of conservation of momentum
The total momentum before a collision is equal to the total momentum after the collision provided no external forces act
m1u1+m2u2=m1v1+m2v2
