4 - Forces in action Flashcards
What is mass?
A physical property of an object which describes how much matter it contains.
Define net force.
The sum of all of the forces acting on an object.
What does a net force cause?
Acceleration in the direction of the force.
What is weight?
The gravitational force acting on an object.
What is an object’s centre of mass?
A point in an object through which its entire weight can be thought to act.
How would you carry out an experiment to determine an object’s centre of mass?
- Hang object - free to spin
- Hang plumb line and mark line
- Repeat in two other positions
- Point where lines cross is centre of mass.
What is each force represented by in a free body diagram?
A labelled arrow, proportional to the size of the force.
What is friction?
The force that arises when two surfaces rub against each other.
What is drag?
The resistive force acting on an object that is travelling through a fluid.
What is tension?
The force within a stretched cable or rope.
What is upthrust?
An upwards buoyancy force acting on a liquid.
What is normal contact force?
A force arising when one object rests against another object.
What is drag proportional to?
velocity squared x surface area
What is the drag force experienced by objects travelling through air commonly called?
Air resistance
Define terminal velocity.
When an object stops accelerating and has a constant velocity because the drag is equal and opposite to its weight.
Describe how a falling object reaches terminal velocity.
- At release, acceleration is equal to g, downward force = mg, net force = mg
- As object accelerates, velocity increases, downward force = mg, upwards force = D, net force = mg - D
- Eventually, drag becomes equal to weight and so net force = 0, drag cannot increase because speed is no longer increasing
What is the only factor that can affect an objects terminal velocity when it is already travelling at terminal velocity?
Surface area
e.g. parachute
What is a moment?
The turning effect of a force around a point.
How do you calculate a moment?
Force x distance from pivot
What must you always ensure (and sometimes have to calculate) before calculating a moment?
That the force and the distance are perpendicular.
What is the principle of moments?
When a body is at rotational equilibrium, the sum of the clockwise moments must be equal to the sum of the anticlockwise moments.
What is a couple?
A pair of equal, parallel forces acting in opposite directions.
What is torque?
The moment of a couple.
How do you calculate torque?
One of the forces x distance between forces.
What does coplanar mean?
All the forces lie in the same plane e.g. 2d
Define density.
Mass per unit volume.
Define pressure.
Normal force exerted per unit area.
How could you determine the volume of a irregularly shaped object?
- Fill measuring cylinder with known volume of liquid
- Push object under water level and see how much the volume changed by
What is a fluid?
A substance that flows. Gases or liquids
What is the equation for pressure under a column of liquid?
P = hpg
Does the cross-sectional area of the column of water affect the pressure?
NO
What is pressure under water proportional to?
The depth below the water, h.`
In what direction is pressure the same at a certain depth under water?
All directions
What is pressure caused by?
Particles moving and colliding with objects.
What must you always remember to include when calculating pressure underwater?
The pressure exerted by the atmosphere above the water.
What is upthrust caused by?
A difference in pressure above and below an object.
When will an object float?
If the upthrust force is greater than the weight of the object.
How do you calculate upthrust?
F = pxgA
pressure below - pressure above
Pb = pgh + pgx Pa = pgh
P = pgh + pgx - pgh = pgx
p = F/A F = pA
F = pgxA
What does Archimedes’ Principle state?
The upthrust force exerted on a body is equal to the weight of fluid that the object displaces.
What conditions (density) must be met in order for an object to float?
The average density of the object must be less than the density of the fluid.
