Forces & Their Effects

7/27/2019 Forces & Their Effects

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Can be defined as a pushor pull in a specificdirection.

The force exerted can be

affected by differentvariables such as mass,shape etc.

Some forces can be definedby how they behave or the

situation in which theyoccur e.g. Gravitationalforce, friction or contactforce.


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Gravitational force is the pull exerted onobjects by the Earth.

Gravity is related to the mass of the object i.e.

The greater the mass, the stronger the effect ofgravity. This is known as weight and calculatedas W = m x g, where g (gravitational force) hasthe given value of 10 Newtons (N).

Because gravitational force has a direction, it isknown as a vector. Mass has only a physicalquality so it is known as a scalar.


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Friction is the force exerted when one objectmoves against another object in a specificdirection e.g. pushing a book along the surface

of the table. In other words the force thatopposes relative motion.

Friction is also affected by variables, such asthe texture of the table surface and the weight

of the book.

A by product of friction is thermal energy i.e.heat. This can be reduced by altering a variable.


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Contact force is the force exerted by two objectson each other when they are in contact e.g. Theforce of your body on your chair and the force

of the chair on your body. In the case of contact force there will be some

compression of the objects as they interact witheach other.

Even though there is no movement, this is stilla vector as the force is being exerted in aspecific direction.


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The direction that a force is exerted in will beits vector.

More than one force can be exerted on an object

in more than one direction. Vectors moving in the same direction will be

added together and give an increased value,whereas vectors working in different directionscould cancel each other out and give a reducedvalue. To equal forces working in oppositedirections will result in equilibrium.


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There are different effects of forces includingvelocity, acceleration, pressure (compression),resistance (density), stress, torsion, rotation and

equilibrium. Materials will behave in different ways under

these different forces.

The selection of a material must consider itsproperties in relation to the types of forces itwill experience.


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Stress is defined as the normalforce per unit area or = F/A.

Strain is defined as extension perunit length or = e/l

Stress and strain are not affectedby the dimensions of the sampletested so two wires of differentthickness could have the sameamount of strain exerted bydifferent forces.

The strength of a material ismeasured in terms of its ultimatetensile strength ( when itbreaks).


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The Young modulus is used to measure therelative stiffness/rigidity of a material by usingstress and strain.

Young modulus = stress /strain orE = / = F/A e/l = Fl/eA

The greater the value of the Young modulus, thestiffer the material.


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The elasticity of a material refers to the abilityto stretch out of the original shape and thenreturn to that original shape after stretching.

Plasticity refers to the ability to be stretched outof shape but to be unable to return to theoriginal shape after stretching.

Hookes Law states that the extension of a sample

is proportional to the stretching force or e F. Metals and springs can obey Hookes Law up

to the limit of proportionality, at which pointthey break.


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No material is absolutely rigid. They will allexperience some change of shape is a force isexerted on them, although this force may be

too small to detect with the naked eye. Certain materials are more resistant to stress

than others. It does depend on the shape andthickness of the material, as to how resistant it

will be. Compare aluminium used in a softdrink can and in the superstructure of anaeroplane.

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Forces & Their Effects