Forces and Mechanical Properties of Materials: Newton's Laws Explained

Force

Force is anything that can cause a body to accelerate, decelerate, rotate, produce deformation or increase pressure.

Forces are represented graphically by means of arrows. The length of the arrow is proportional to its value, and the direction of the arrow shows the direction in which the force is applied.

A force produces different effects on an object depending on the place where the force is applied. The unit of force is the newton (N). Examples of forces include weight, gravity force, friction force and normal force.

The apparatus used for measuring forces is the dynamometer.

The sum of the forces acting on an object is called the total force or the net force.

The sum of two forces can be calculated by applying either the triangle law or the parallelogram law.

Newton's Laws of Motion

Newton's First Law of Motion

Every body continues in its state of rest or of uniform speed in a straight line unless acted on by a non-zero or net force.

Inertia is the tendency of a body to maintain its state of rest or of uniform motion in a straight line.

Objects in motion have the force of friction acting in the opposite direction of the movement.

Newton's Second Law of Motion

The acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass. The direction of the acceleration is in the direction of the net force acting on an object.

Newton's Third Law of Motion

Whenever one object exerts a force on a second object, the second exerts an equal and opposite force on the first. (To every action there is an equal and opposite reaction.)

Types of Stress

Compression takes place when an object is pushed with two forces acting in opposite directions (pillars, columns).

Flexion takes place when one of the object's sides is under compression and the other side is under tension (beams, girders).

Torsion occurs when an object is twisted by the action of two opposite forces; it is said to be under torsion.

Shear (cisallament) occurs when two opposite forces act in the same part of the object, trying to cut it. We say the object works under shear stress. Riveted and bolted joints work under shear stress.

Mechanical Properties of Materials

Mechanical properties of materials are those properties that act when forces are applied to them.

Strength (resistència) is the capacity of a material to support forces without deforming, breaking or bending (metals, woods and many plastics are strong materials).

Toughness / Fragility (tenacitat / fragilitat): toughness is a characteristic of a material that does not break when receiving a sudden shock (hammer). The opposite of toughness is fragile (glass).

Elasticity: an elastic material deforms when a force is applied but then returns to its original shape when the force is removed. The greatest stress that can be applied to a material without causing permanent deformation is called the elastic limit (rubber).

Plasticity is the ability of a material to change shape permanently in response to an applied force (clay or Play-Doh).

• Ductility: the ability of a material to deform under tensile stress; the material can be shaped into wires (steel).
• Malleability: the ability of a material to deform under compressive stress; the material can be shaped into thin sheets (aluminium, gold).

Hardness (duresa) is the ability of a material to resist scratching. The hardest material is diamond. The opposite of hard is soft, like plaster, for instance.