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FORCES AND STRESSES

FORCES AND STRESSES. Lesson objectives. To be able to name the different forces To be able to describe the different forces To be able to understand the impact of forces and stresses and the way materials can be reinforces and stiffened. Introduction to Forces. What are forces?.

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FORCES AND STRESSES

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  1. FORCES AND STRESSES

  2. Lesson objectives To be able to name the different forces To be able to describe the different forces To be able to understand the impact of forces and stresses and the way materials can be reinforces and stiffened

  3. Introduction to Forces

  4. What are forces? A force is an action that makes something or someone move. We are going to look at three different forces. push twist pull

  5. Push Click on the 2 pictures that show something being pushed. twist pull pull

  6. Pull Click on the 2 pictures that show something being pulled. twist twist push

  7. Twist Click on the 2 pictures that show something being twisted. pull push push

  8. Other Objects Some objects use more than 1 force to get them moving. When you ride a bicycle you push on the pedals and pull on the handlebars. When you open a door you twist the handle and pull or push the door.

  9. See if you can identify which force or forces are being used in these photos. The seed is being pushed into the ground.

  10. The boy is pushing the dough with his hand.

  11. The hand is twisting the knob to turn on the oven.

  12. The door is being pulled open.

  13. The little girl is pushing her friend in her wheelchair.

  14. The horses are pulling the old fashioned plough.

  15. The screwdriver pushes the screw into the wood.

  16. How did you do? Could you spot the different forces? Let’s have one more try. What forces can you see in these pictures twist pull push Did you get them right?

  17. Forces can make objects do FIVE things 3. Change Direction Like hitting a ball with a bat • Speed up • Like kicking a ball 2. Slow down Like drag or air resistance 5. Change shape Like stretching, twisting, compressing and Bending 4. Turn Like turning a spanner

  18. FORCES Forces can cause objects or materials to break or change shape. Materials are strong if they are good at withstanding a force without breaking. Force is measured in newtons (N) just like liquid is measured in litres or millilitres or kilolitres There are a few different types of forces ans stresses that you need to know about:

  19. FORCES • Compression • Tension • Bending • Torsion • Shear https://www.youtube.com/watch?v=8IN544ZKzmQ

  20. tension

  21. tension Tensile forces act to stretch an object by pulling it apart. • Eg tug-of-war, the rope in under tension caused by people pulling in opposite directions • Trampolines use springs with a high tensile strength when you land on a trampoline you stretch the springs. The springs resist being permanently stretched and try to return to their original length – this throws you back up into the air • Suspension bridges use cables with high tensile strength to support the weight of the bridge

  22. Take a piece of string and a strip of paper and use the tension forces on both.Which is the strongest? or

  23. compression

  24. compression • Compression is the opposite to tension. Compression forces are directed towards one another, and act to squash and shorten objects • Compression often occurs when a material is supporting a weight above it. • Chair legs support the person sitting on the chair. A bridge supports hold the weight of the bridge. • They are made from materials that have a very high compressive strength

  25. Place your hands together and push together.If your felling brave get a partner and have an arm wrestleWhich was harder and why? or

  26. shear

  27. shear • Shear forces often act in opposite directions but unlike compressive and tensile forces, they aren't aligned with one another. This can cause parts of an object to slide past one another • For example, scissors use shear force to cut (shear) materials. • Industrial cutting machines such as guillotines and die cutters use shear force to cut materials The scissor blades push the paper in the opposite directions causing it to be ripped or cut

  28. You can rip or shear a piece of paper using scissors or your hands easily.Could you do this with a piece of wood or metal? Why or How? or

  29. bending

  30. bending • A bending force is a type of shear force that causes materials to bend. Load • This is commonly occurs when a load is applied to a certain part of a material. • Eg, a bench had to resist bending forces when someone sits down in a certain place Bench legs push up on the bench

  31. Lets bend a plastic ruler !!!!Lets bend a steel ruler !!!!What's the difference of why? or

  32. torsion

  33. torsion • Torsion acts to twist objects and materials. The forces attempt to rotate the different parts of a material in opposite directions. • Propeller shafts connect a ships engine to the propeller which spins in the water and drives the ship forwards • These shafts have to be able to resist torsion – it allows them to transfer rotary motion without breaking or twisting Engine rotates the propeller shaft Water resists the rotation of the propeller and shaft • The water applies a rotary force in the opposite direction to the engine – so a twisting force is applied to the shaft.

  34. You have a piece of fabric and a piece of wood which is easier to twist and why? or

  35. Task one After our experiments you must complete the differentiated worksheets. The worksheets are to identify your knowledge and understanding of the different forces and stresses. Challenge yourself by choosing a differentiated worksheet.

  36. Effect of forces When a force is transmitted through a solid body, the body tends to undergo a change in shape. Can you match up the correct diagram with the correct force description? Tension Compression Shear Bending Torsion

  37. Effect of forces Bending The force combines the effects of push and pull or tension and compression. Material transforms from a straight form into a curved or angular one Compression The force acts to squash the object and thus has a push effect. Tension The force acts to stretch the material and hence has a pull effect Torsion The force acting on a body that tends to twist the body. Shear The force tends to make the surfaces of a material slide relative to each other. The material resists these forces internally and hence will be in a state of stress.

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