forces
DESCRIPTION
Forces. Force causes acceleration Force is measured in Newtons (N) There are several different types of forces that can be applied to bodies and structures. Static Forces. Static forces do not usually cause motion Consider a tall building The weight of the material - PowerPoint PPT PresentationTRANSCRIPT
Forces
Force causes acceleration
Force is measured in Newtons (N)
There are several different types of forces that can be applied to bodies and structures
Static Forces
Static forces do not usually cause motion
Consider a tall building
The weight of the material it is built from, and thepeople and furniture inside it are static loads
Static Forces
Examples:
Water sitting in a jug with uniform temperature.
A motionless weight hanging from a cable.
A block resting on a desk.
A poster hanging on a wall.
Dynamic Loads
Usually causes a movement The value of the force can be variable
Again consider a tall building
Variable winds add an extra force or load to the structure
The engineer must allow for this
Dynamic Loads
Examples:
o Tree swaying in the wind
o A bridge moving under the weight of traffic
o Someone pushing a swing
o Someone kicking a football
Bending Forces
Structures that carry loads across their length are subject to bending forces
Consider a car driving across a bridge
Shear Forces
These are tearing or cutting forces
Scissors are an example of these
Torsion Forces
Torque is a turning force which tries to twist a structure
Compression Forces
Compression forces try to squash a structure
Consider a column
The weight down is balanced by the reaction from the ground
The forces act to try and shorten the column
C O LU M N
W EIGHT FO R C E W(EXTER NAL FO R C EO N C O LUM N )
W
R
GROUND R EAC TIO N R(EXTER NAL FO R C E O N C O LU M N)
Forces in Tension
Tensile forces try to stretch a structure
Consider a crane’s lifting cable
The weight tries to stretch or pull the cable apart
Cables in tension can have small diameters compared to members in compression
LEVERS In its simplest form, a lever is a stick that is free to pivot or
move back and forth at a certain point.
Levers are probably the most common simple machine because just about anything that has a handle on it has a lever attached.
The point on which the lever moves is called the fulcrum.
By changing the position of the fulcrum, you can gain extra power with less effort.
LEVERS How do you move a heavy person?
If you put the fulcrum in the middle, you won't have a chance. But if you slide the fulcrum closer to the heavy person, it will be easier to lift.
Where's the trade-off?
Well, to get this helping hand, your side of the see-saw is much longer (and higher off the ground), so you have to move it a much greater distance to get the lift
LEVERS
Draw the universal system for a lever
Copy the line diagram of a lever
EFFO RT
LO AD
D ISTANC EM O VED
BY LO AD
D ISTANC EM O VED
BY EFFO RT
LEVER SYSTEM
INPUT FO RCE
INPUT M O TIO N
O UTPUT FO RCE
O UTPUT M O TIO N
Task 1
Draw a universal system diagram for a lever
Complete the following diagram, indicating clearly the LOAD, EFFORT and FULCRUM
INPUT O U TPU T
Lever Systems The lever shown is in
equilibrium (a steady state)
The input force exerts an anticlockwise moment
The output force exerts a clockwise moment
To be in equilibrium both moments must be equal
The Principle of Moments
The sum of the moments must equal zero
CWM = ACWM
Example: Prove that the following system is in equilibrium
Solution
• For equilibrium, the CWM = ACWM. • A moment is a force multiplied by a distance
CWM = ACWMF1¹ d1 = F2 d2
•The load exerts a clockwise moment (tends to make the lever turn clockwise)
Clockwise moment = 200 N 2 m = 400 Nm
•The effort exerts a anticlockwise moment.
Anticlockwise moment = 400 N 1 m = 400 Nm CWM = ACWM
• Therefore the lever is in a state of equilibrium.
Practice:
Questions:
For the system shown:
If the handle length is 250mm and the effort to turn it is 15N, what moment would close the tap valve?
What is the benefit of this type of tap?
Suggest a situation where this type of tap would be useful