4 - Forces in action Flashcards
What is Newton’s 2nd Law?
F = ma.
Define geometric centre.
Middle of an object.
Where is the centre of mass of an object of uniform density?
Geometric centre of the object.
Why is the centre of mass higher than the geometric centre of a human?
Head more massive than feet.
What happens when a force is applied to an object’s centre of mass?
Straight line motion produced without rotation.
What is the centre of mass equal to?
The centre of gravity.
How do you find an object’s centre of gravity?
.Suspend another object that comes to rest in line with the centre of gravity.
.Use plumb-bob and trace line on the object.
.Rotate object and trace plumb-line again.
.Centre of gravity is where the lines intercept.
Define weight.
Gravitational force through centre of mass.
Define friction.
2 surfaces rub against one another.
Define drag.
Resistive force when travelling through a fluid.
Define tension.
Stretched cable or rope.
Define Upthrust.
Upward buoyancy in a fluid.
Define normal contact force.
One object rests against another.
What is responsible for acceleration down a slope?
Weight down a slope.
Why do objects experience drag force?
Object colliding with particles in the fluid (liquid or gas).
What factors increase drag on an object?
Cross-sectional area and velocity.
What is the relationship between drag and speed?
Drag proportional to speed^2.
What happens to drag as an object falls?
.g stays the same.
.No drag at t=0.
.Speed and drag increase until it balances with mg, then the object is at terminal velocity.
Define moment.
Turning effect about some axis or point - clockwise or anticlockwise. Need a pivot, force and distance.
What does it mean when a body is in equilibrium?
.Net forces/moments are 0.
.Not accelerating.
.Not rotating - clockwise = anti-clockwise.
What is the equation for a moment?
Force x perpendicular distance of the line of action of force from the axis or point of rotation.
M = Fx.
What if force and distance aren’t perpendicular?
Look in folder.
What happens when something is in static equilibrium?
.Moments at any point are equal.
.If B is unknown, pick A so it’s contribution is 0.
.
What happens if a force is applied anywhere but the centre of gravity?
It will rotate with translational motion (through space).
How do you stop an object from rotating with translational motion?
Apply equal and opposite forces to it.
Define couple.
Forces that produce rotational motion and no translational motion.
Define torque.
The moment of a couple.
How do you calculate torque?
One force x perpendicular separation between forces.
How do you get a triangle of forces?
.Split force horizontally and vertically.
.Make a triangle.
.Tsin0 = W.
.Tcos0 = F.
What is the equation for density?
Density = mass/volume.
What is the equation for pressure?
Pressure = Force/area.
Define density.
How much mass per unit of volume.
What is the relationship between density, mass and volume?
Mass and density directly proportional.
Volume and density inversely proportional.
What is 1cm^3 in m^3?
1 x 10^-6 m^3.
What is the average weight of a human?
70kg.
How do you work out the density of regular and irregular shapes?
.Measure mass on balance.
.Regular - Measure dimensions with a Calliper/Micrometer/ruler
.Irregular - displacement of liquid.
What is the pressure exerted by the Earth’s atmosphere?
1 x 10^5 Pa.
What is the relationship between pressure, force and area?
Force and pressure directly proportional.
Area and pressure inversely proportional.
How is pressure exerted by a fluid?
Molecules are in constant, random motion and collide with each other and the walls of the container, producing a pressure.
What is the equation for pressure of a liquid?
.P = m/v so m = pv so W = pVg
.v = hA so W = PhAg
p= F/A so p = W/a so phAg/a
.p = hpg.
What is the equation for the difference in force experienced at the top and at the bottom of an object?
xpgA.
Upthrust = Axpg.
Define Archimedes’ Principle.
The upthrust exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces.
For a floating object, what is it’s upthrust equal to?
It’s weight.
How do you measure terminal velocity?
Look in folder.
