# Scope Scope of this presentation is : Calculating forces on static objects. This is an important skill to master, as understanding how forces must balance.

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Scope Scope of this presentation is : Calculating forces on static objects. This is an important skill to master, as understanding how forces must balance and how to work out the sizes of different forces is key in being able to work out if a structure will stand up.

On objects: Acceleration Deceleration

On materials: Tension Twisting Bending Compression

Effects of Forces 1)When forces push on an object, the material is compressed. This is what occurs to bricks in brick walls. Bricks are strong in compression, and structures such as arches are commonly made out of brick and stone as the arch structure ensures all bricks only experience compression forces. 2)When forces pull on an object, the material goes into tension. Wood and steel are strong in compression, so are often used for ties in roof structures. The tie is used to hold the two side of the roof together, to stop it collapsing. 3) Materials can also experience twisting, in objects such as axles or crankshafts. 4) Bending can also occur. This example shows bending occurring as an extreme form of compression in a column.

For something to be stationary, all the forces on it must be balanced 500 N

What force must you will apply to hold the bucket in mid air? Weight = 250 N Lifting Force = 250 N

What force must the chair exert to support the cat? Weight = 100 N Reaction Force = 100 N

What force must each of the chair legs exert to support the load? Weight = 300 N Reaction Force leg 1 = Reaction Force leg 2 = 150 N leg 1 leg 2

What force must the supports exert to hold the arch? Weight = 500 N Reaction Force 1 = 250 N Reaction Force 2 = 250 N

What about a 4 way tug of war? When does the centre point stay still?

Think of it like a compass: North-South forces must balance AND East-West forces must balance for it to be in equilibrium 500 N 300 N N E S W

What forces are required to hold the middle still? 100 N 900 N N E S W

What force is required to make the boat float? What drag force would result in a stationary boat? Weight = 9000N Upthrust = Force = 13,000N Drag = 9000 N 13,000N N E S W

What is the weight of the arch? What frictional forces can counter the wind force? Weight = Reaction Force 1 = 300 N 600 N Reaction Force 2 = 300 N 200 N Force = 400 N Friction 1 = Friction 2 = 200 N N E S W Answer : The total North force should equal the total South force. The total East force should balance the total West force.

But what about forces at angles? Easy – a force at an angle can simply be rewritten as two forces in easy directions N E S W

Eg. Imagine an angled force of 5N Draw the force 1 Draw a box around the force 2 Measure the length and width 3 5 cm 4 cm 3 cm 1cm = 1N

Look at the force applied by the man to the wheelbarrow What is the lifting force? What is the forward force? N E S W 50N 300N 340N 30N 1cm = 10N 5 cm 4 cm 3 cm 40N 30N

Look at the force applied by the weather What friction will resist sideways motion? What reaction force will resist downwards force? 1000N N E S W 1cm = 100N 10 cm 6 cm 8 cm 600N 800N 400N 500N

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