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Forces 1: 1.3 Examples of contact forces explained

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INDEX of my physics notes on FORCES Section 1. An introduction

Examples of contact forces between objects touching each other

If two objects have to be touching for the force to act, the force would be described as a contact force. The two objects will be pushing or pulling  on each other e.g.

(1) Friction is a contact force between two surfaces rubbing against each other, the force moving an object forwards is partially countered acted by a force of friction acting in the opposite direction e.g.

the tyre of a car in contact with the road,

pressing the brake pad onto the disc of a car's braking system,

rubbing your hands together.

(2) Air resistance is also a contact force as an object moves through the atmosphere, friction between the object and air, even though the downward motion is caused by the non-contact force of gravity.

An object resting on a surface involves weight and compression (see 'interactions' section further down the page).

When a spring is stretched you have 'in tension' force as the spring tries to recover to its original shape acting against the weight or force applied to stretch it e.g. when you use a spring force meter in the laboratory or a spring balance for weighing things.

You get the opposite force 'in compression' if you push a spring to compress it to a shorter length it tries to regain its original shape e.g. the weight of a car acting on its suspension springs. Forces - elasticity and energy stored in a spring

the tension in the wires attached to the hook of a crane, if stationary, the tension in the cable is balanced by the weight of the object the crane is lifting.


(3) Forces of tension and compression

When dealing with major civil engineering projects like building a bridge, complex calculations must be done to ensure the bridge is stable and able to support the roadway with vehicles on it.

Most of the forces involved in a suspension bridge design are those of tension and compression, illustrated with pictures and diagrams below for any budding young engineer!

The Humber Bridge over the Humber estuary, north-east England, walked over and photographed in November 2018.

When it was completed in 1981 it was the World's longest single span suspension bridge and in 2018 it still holds 8th place!


Despite the obvious complexity of the bridge structure most of the contact forces involved are 'in tension' or 'in compression' forces which I've indicated by yellow arrows.

Each of the two vertical towers of the Humber Bridge, consist of a pair of hollow vertical concrete columns.

These support the weight of the two main 'sagging cables' - which actually comprise thousands of parallel steel wires (~15,000) bound together in a black casing.

These cables in turn support the slightly angled (from the vertical) steel rods that support the roadway.

Therefore there is a tremendous vertical force of compression acting in the two load bearing vertical concrete towers, as they must hold up the whole weight of the bridge!

Within the 'sagging' main cable, and the almost vertical steel rods holding up the roadway, are the forces of tension.

These are pulled downwards by the weight of the roadway and of the cables themselves.

However, unlike the two towers which bear the whole weight of the bridge, the total force of tension is split between the main cable and nearly vertical steel rods.

For this reason, the rods supporting the roadway can be relatively thin (62 mm diameter), but there are a lot of them!

For more pictures see Humber Bridge in the 'doscpics' section of my otherwise science website!


(4) The bow and arrow! (the physics of 'Robin Hood' and the 'Battle of Agincourt' in 1415!

English longbow (picture 1 above), and other bows are classified by the tension in lbs on pulling the bow string to its maximum tension.

In old 'weight' units this equates to typically 50 lb (23 kg) to 150 lb (68 kg).   (1 lb = 0.454 kg).

Imagine these 'weights' hanging from the bow string.

You can measure this with a spring balance by attaching it to the bow string and pulling it back and noting the reading (picture 2).

So the tension in the bow string is equivalent to hanging on it 'weights' of typically 23 kg to 68 kg.

In terms of weight equalling the tension force in the bow string you multiply the mass by 9.8 N/kg (due to gravity).

weight (N) = mass (kg)  x  gravitational field constant (g)

Therefore the tension in the string is typically 224 to 667 N, and the bow and arrow is now an elastic potential energy store!

To reduce the force of friction (drag force) between the arrow and air, the arrow shaft is thin, as are the flight feathers, and a sharp metal point at the front end (picture 3).

When the bow is bent by drawing back the string, the string is in tension, AND there are forces of both tension and compression in the bow structure.

On the inner curved surface of the bow you get compression as the layers are pressed together.

On the outer curved surface of the arched bow you get the tension as the outer layers are stretched.

When you let go of the arrow. all this stored potential energy is released and converted into kinetic energy of the fast moving arrow.

The drawn bow and arrow are an elastic potential energy store.

The arrow becomes a kinetic energy store, much of it is retained on flying back down to Earth.

As the arrow flies upwards it loses kinetic energy and increases its gravitational potential energy store.

As it falls, the GPE is converted back to kinetic energy, and the arrow can be as penetrating as when first fired.

History note (not required for GCSE physics, not sure about GCSE history!):

The rapid firing of many longbows was a major factor in the English winning the Battle of Agincourt (1415) in the 100 years war between France and England. The French archers uses a crossbow that fires a bolt - deadly and very effective, BUT the bolt must be drawn back by a mechanical winding system to build up the store potential energy in the bolt mechanism. With no mechanism to deal with, just muscle power, a skilled English archer could pull back his bow and fire arrows at ten times the speed of the French crossbow men. No contest! Shakespeare built the whole thing up in his play "Henry the Fifth", he didn't know much about physics but he was pretty good with words!


(5) Normal contact forces

Any object standing on a surface in a gravitational field involves normal contact forces.

The object has weight due to gravity, so the object presses down on the surface e.g. a stationary book lying on a table or you standing on the floor or sitting on a chair.

The atoms of the table are compressed and push back on the surface against the weight of the book.

The two normal contact forces are equal, but acting in opposite directions - no change in motion!

If the they were not equal, the book would either move upwards or sink into the table!

Its the same argument for you standing on the floor.

Your body weight force acts down on the floor and the compressed atoms of the floor push back up with a force equal to your weight - otherwise you would move up or down!


(6) Impact forces

Collisions between objects involve an impact force from two colliding objects, both of which have kinetic energy.

Particle collisions in gases and liquids are examples of impact force effects and gas pressure is caused by particle impacts on a surface.

INDEX of my physics notes on FORCES Section 1. An introduction

Keywords, phrases and learning objectives for normal contact forces

Be able to describe and examples of normal contact forces between objects touching each other including friction - air resistance, but also the compression and tension forces acting in large structures.


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INDEX of my physics notes on FORCES Section 1. An introduction