FORCES AND STRESSES 1 To be able to

FORCES AND STRESSES

1. To be able to name the different forces 2. To be able to describe the different forces 3. To be able to understand the impact of forces and stresses and the way materials can be reinforces and stiffened

INTRODUCTION TO FORCES

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

Click on the 2 pictures that show something being pushed. twis t l l u p pull

Click on the 2 pictures that show something being pulled. t s i w t push twist

Click on the 2 pictures that show something being twisted. push pull h s pu

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

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

The boy is pushing the dough with his hand.

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

The door is being pulled open.

The little girl is pushing her friend in her wheelchair.

The horses are pulling the old fashioned plough.

The screwdriver pushes the screw into the wood.

Could you spot the different forces? Let’s have one more try. What forces can you see in these pictures pull twist push

Forces can make objects do FIVE things 1. Speed up 2. Slow down 3. Change Direction Like kicking a ball Like drag or air resistance Like hitting a ball with a bat 4. Turn Like turning a spanner 5. Change shape Like stretching, twisting, compressing and Bending

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 a few different types of forces ans stresses that you need to know about:

FORCES § § § Compression Tension Bending Torsion Shear https: //www. youtube. com/watch? v=8 IN 544 ZKzm. Q

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

§ 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

§ 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

§ 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

§ 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.

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.

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

Effect of forces Bending Compression Tension The force acts to squash the object and thus has a push effect. The force acts to stretch the material and hence has a pull effect 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 Torsion Shear The force acting on a body that tends to twist the body. 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.