Forces

1. 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

2. Static Forces • Static forces do not usually cause motion • Consider a tall building • The weight of the material it is built from, and the people and furniture inside it are static loads

3. 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.

4. 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

5. Dynamic Loads Examples: • Tree swaying in the wind • A bridge moving under the weight of traffic • Someone pushing a swing • Someone kicking a football

6. Bending Forces • Structures that carry loads across their length are subject to bending forces • Consider a car driving across a bridge

7. Shear Forces • These are tearing or cutting forces • Scissors are an example of these

8. Torsion Forces • Torque is a turning force which tries to twist a structure

9. 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

10. 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

11. 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.

12. 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

13. LEVERS • Draw the universal system for a lever • Copy the line diagram of a lever

14. Task 1 • Draw a universal system diagram for a lever • Complete the following diagram, indicating clearly the LOAD, EFFORT and FULCRUM

15. 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

16. The Principle of Moments • The sum of the moments must equal zero • CWM = ACWM • Example: Prove that the following system is in equilibrium

17. Solution • For equilibrium, the CWM = ACWM. • A moment is a force multiplied by a distance • CWM = ACWM • F1¹ 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.

18. Practice:

19. 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