Forces in Action Flashcards
What is the equation linking mass, acceleration and force.
Force = mass x acceleration
Resultant force = mass x acceleration
F (N) = m (kg) x a (ms^-2)
What is weight?
Weight is the gravitational force on an object due to gravitational field strength and mass.
(the force exerted on a body due to gravity)
Weight = mass x acclerartion due to gravity
W (N) = Mass (kg) x g (ms^-2)
What is the value of g at the earths surface?
9.81 N/kg
9.81 ms^-2
- They are both the same
- They both decrease with altitude
What is the centre of mass of an object?
COM - the point at which all of an objects mass appears to act
(the point of average mass)
also the point at which the resultant weight acts from.
How do you calculate the COM of a ‘2D’ flat object?
1) Cut 3 different holes in it at different positions along its edge.
2) hang it from a point and let it come to rest
3) Use a plumb line to identify a vertical line from below the point and draw a vertical line down.
4) repeat steps 2 and 3 twice more
5) the intersection of the lines is the COM/COG
What is a free body diagram?and
What are the rules for drawing them?
A free body diagram is a diagram isolating all the forces acting on a particular object.
RULES:
1) each force vector is represented by an arrow labeled as the force it represents
2) each arrow is drawn to the same scale
3) the forces act from the objects COM
What are the 6 main “types” of force?
1) Weight - gravitational force acting on an object through its COM
2) Friction - The force that acts when two surfaces move against each other
3) Drag - the resistive force on an object as it travels along through a fluid
4) Upthrust - a buoyancy force acting on an object when it is in a fluid, relates to g
5) Normal Contact force - A force arising when an object rests against another. Always perpendicular to the surface.
6) Tension - the force within a streched cable or rope.
How do you check which is which?
Eg wcostheta and wsintheta
you take it to the extreme where the slope rests on the horizontal.
What is drag force?
What does it depend on?
(all in terms of collisions)
Drag force is a frictional force that opposes motion due to the collisions between the object and particles in the fluid.
The magnitude of the force depends on variety of factors with the main ones being:
1) Surface area & shape - increased area for collisions
2) Density of the fluid - increased particles for collisions
3) Speed of the object - increased frequency of collisions
What is the relationship between speed and drag?
Cross sectional area and drag?
Drag force proportional to velocity^2
(D = KV^2 —> 2 x v = 4 x D) (because of a complex equation we don’t need to know)
Drag force proportional to Cross sectional area
(D = kA —> 2 x D = 2 x A)
What is terminal velocity?
How is it reached?
- An object begins to fall, there is no air resistance so acceleration is due to the objects weight
- As the objects velocity increases, so does the opposing force of air resistance, and so the resultant force, hence acceleration begins to decrease.
- When the force of air resistance is equal to its weight, there is no resultant force, hence the object doesn’t accelerate so it reaches terminal velocity.
a = Mg - D / m as F = ma —> a = F/m
What is the moment?
What is the equation?
The moment about a force is its turning effect about a point or Axis.
Moment = Force x perpendicular distance of the line of action of
force from axis or point of rotation
Moment (Nm) = Fx
How do you calculate perpendicular distance?
Perpendicular distance is the distance between the line of action of the force and the point where they meet at a right angle.
USE TRIGONOMETRY
What is the principle of moments?
The principle of moments states that:
For a body in rotational equlibrium (to be in equlibrium)
the sum of the anticlockwise moments about any point is equal to the sum of the clockwise moments about the same point.
What is a couple?
What do they result in?
A couple is a pair of equal but opposite forces which act parallel to one another in different lines
they result in rotational motion but not translational
