4. Forces in action Flashcards
What is the equation for force?
F = ma
F = force / N
m = mass / kg
a = acceleration / ms^-2
Define 1N
A force of 1 newton will give a 1kg mass an acceleration of 1ms^-2 in the direction of the force
What is the equation for Weight?
W = mg
W = weight / N
m = mass / kg
g = acceleration of free fall, 9.81ms^-2
What is the centre of mass?
A point through which any externally applied force produces straight-line motion but no rotation
What are the steps to finding the centre of gravity with a plum-line?
- Make small holes along the edges of the object made from card
- Insert a pin through one of the holes and hold the pin firmly in a clamp
- Allow the object to swing freely, it will come to rest with its centre of mass vertically below the pin
- Hang a plumb-line from the pin and draw a line along the verticals string of the plumb-line
- Repeat the process for other holes
- The centre of gravity will be the point of intersection of the lines
What is weight?
The gravitation force acting on an object through its centre of mass
What is friction?
The force that arises when two surfaces rub against each other
What is drag?
The resistive force on an object travelling through a fluid (e.g. air or water)
What is tension?
The force within a stretched cable or rope
What is upthrust?
An upward buoyancy force actions on an object when it is in a fluid
What is normal contact force?
A force arising when one object rests against another object
What are the factors that affect the magnitude of drag?
- speed of the object
- cross-sectional area of the object
- density of the fluid the object is travelling through
- shape of the object
What is the relationship between drag force and speed (for most objects)?
drag force is directly proportional to speed^2
Describe how the forces acting on an object change as it falls through the air
At the instant the object starts to fall, there is no drag force on the object.
The force is equal to the weight.
The acceleration of the object is g.
As the object falls, its speed increases.
This increase the magnitude of the opposing drag force.
The resultant force on the object decreases.
The instantaneous acceleration of the object becomes less than g.
Eventually the object reaches terminal velocity, when the drag force on the object is equal and opposite to its weight.
At terminal velocity, the object has zero acceleration and its speed is constant.
What is the principle of moments?
For a body in rotational equilibrium, the sum of the anticlockwise moments about any point is equal to the sum of the clockwise moments about that same point
What is the equation for moments?
moment = Fx
moment / Nm
F = force / N
x = perpendicular distance
What is a couple?
A pair of equal and opposite forces acting on a body but not in the same straight line
What is a torque?
The moment of a couple
What is the equation for torque of a couple?
Torque of a couple = Fd
Torque of a couple / Nm
F = on of the forces / N
d = perpendicular separation between the forces / m
When an object is in equilibrium…
the net force is zero
What is the density of an object?
It’s mass per unit volume
What is the equation for density?
p = m/V
p = density / kgm^-3
m = mass / kg
V = volume / m^3
How do you determine density?
Obtain mass of object using a digital balance
Obtain volume of a:
liquid using a measuring cylinder
regular solid using ruler, digital callipers, or micrometer
irregular solid using displacement
Define pressure
The normal contact force exerted per unit cross-sectional area
