forces

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