Chapter 3.2 - Forces in Action Flashcards
Equation for weight
W=mg
Tension
A force experienced by a string or wire that is being pulled
Normal contact force
A force that acts perpendicular to the point of contact between two surfaces
Upthrust
The upward force that a liquid or gas exerts on a body due to the liquid or gas being displaced
Friction
A force that occurs between two surfaces that opposes motion. It is a result of intermolecular forces.
What you show on a free body diagram (3)
- The bodies
- The directions of each force
- The value of each force
Fluid
A liquid or a gas
Drag
The frictional force experienced by an object travelling through a fluid
Does friction depend on velocity
no sir
Does drag depend on velocity
o yes
How is drag related to velocity
D = kv^2
Factors affecting drag (4)
- Density of fluid
- Coefficient of drag
- Cross sectional area of object
- Velocity
The motion of an object falling in the presence of drag
- Initially accelerates because force of gravity acts downwards and there is minimal drag
- As speed increases the upwards force of drag increases
- Eventually the force of weight is equal to the force of drag and the object no longer accelerates and is moving at its terminal velocity
What happens when a parachute is opened while falling at terminal velocity
The drag force increases due to the increased cross sectional area. This results in a net upwards force and therefore an upwards acceleration, which causes the skydiver to slow down. As they slow down the drag force decreases until a new terminal velocity is reached
Terminal velocity
The speed of an object at which the weight is equal to the drag
Experiment to determine terminal velocity of a body in a fluid
Set up a cylinder filled with fluid and wrap elastic bands around it at regular intervals. Drop a ball of known mass into the tube and time how long it takes to reach each elastic band. Towards the bottom the times should remain constant and this value is used to calculate terminal velocity
Equilibrium
The resultant force acting on an object is zero
How to find the resultant of two forces
Add them
Moment
The turning effect of a force
Equation for a moment
moment = force * perpendicular distance
Principle of moments
For a body to be in equilibrium, the sum of the clockwise components must equal the sum of the anticlockwise components
Couple
Pair of forces of equal size that act in parallel but opposite directions, resulting in a turning force (torque)
Equation for torque of a couple
T = (magnitude of one force) * (perpendicular distance between the forces)
Weight
Force experienced by a mass due to a gravitational field
Centre of mass
A single point that you can consider the entire mass to exist at, regardless of orientation
Centre of gravity
A single point through which the entire weight of an object can be thought to act
When are the centre of mass and the centre of gravity the same
For a small body in a uniform gravitational field
Experiment to find centre of gravity
Hang the object freely from a point and draw a vertical line down it. Repeat from another point. Where these two lines cross will be the centre of gravity
Three forces act on an object in equilibrium. What must these forces be?
coplanar
Density
Mass per unit volume
Pressure
Force per unit area
Unit of pressure
Pascal (Newtons per metre squared)
Pressure in a fluid
p=hρg p - pressure h - depth ρ - density g - acceleration due to gravity
Upthrust
An upward force exerted by a fluid on an object that is fully or partially submerged
Archimedes principle
The upthrust is equal to the weight of fluid displaced
Equation for upthrust
F = (h2 - h1)ρgA
or
F = sρgA where s is the height of the object
Remember
Pressure increases as you go deeper into a fluid but upthrust is the same wherever you are (as long as you are fully submerged)
