STATICS AND DYNAMICS

4TH ESOChemistry and PhysicsIES AMES

LOOKING AT FORCES

“Peace cannot be kept by force; it can only be achieved by understanding.”

Albert Einstein

  “A mind at peace, a mind centered and not focused on harming others, is stronger than any physical force in the universe.”

Wayne Dyer

LOOKING AT FORCES

“FORCES ARE INVOLVED WHENEVER OBJECTS INTERACT”

FORCES OF NATURE: GRAVITATIONAL: All objects with mass attract each other with a

gravitational force. WEIGHT= m x g ELECTROMAGNETIC FORCE: They use attraction and repulsion

between positive and negative charges Atomic structure. NUCLEAR FORCES:

WEAK NUCLEAR FORCE: These are the forces involved in the radioactive decay of atoms.

STRONG NUCLEAR FORCE: Positive protons in the nucleus are held together by the strong nuclear force.

LOOKING AT FORCES

What about simple pushing and pulling forces?All the forces involved when objects are in direct contact are electromagnetic forces?

FRICTION FORCES BETWEEN SOLID SURFACES The size of the frictional force depends on:

The type of surfaces in contact How hard the surfaces are pressed together

The force of friction acts in the opposite direction to the motion FLUID RESISTANCE (DRAG)

Drag forces depend on: The faster the relative motion is The viscosity of the fluid

Drag forces try to prevent motion between objects and fluids NORMAL CONTACT FORCES

This is a force that exists wherever two solid surfaces are in contact. Normal means at 90º to the surfaces.

TENSION A stretched wire or a string will exert a tension force that pulls on the object it is

connected to. The tension force acts along the wire.

LOOKING AT FORCES

FORCES are caused by the interaction of two objects.

They are vector quantities, so they are drawn as vector arrows.

A free-body diagram shows all the forces acting on just one object.

Balanced beams Suspended objects Objects resting against rough and smooth surfaces.http://www.wisc-online.com/objects/index_tj.asp?objID=TP1502

LOOKING AT FORCES

ADDITION OF FORCEShttp://www.walter-fendt.de/ph14e/resultant.htm

http://www.nhn.ou.edu/~walkup/demonstrations/WebTutorials/HeadToTailMethod.htm

DIFFERENT DIRECTIONSAME DIRECTIONIf two vectors have the same direction, their resultant has a magnitude equal to the sum of their magnitudes and will also have the same direction.

LOOKING AT FORCES

RESOLUTION OF FORCES A single force can be formed by combining two or more forces. It can be replaced by, or resolved

into two components.http://www.walter-fendt.de/ph14e/forceresol.htm

LOOKING AT FORCES

THE EFFECT OF A FORCE Force is an action or agency that causes a

body of mass m a deformation, a rotation, an acceleration, or an increase in pressure for the body.

LOOKING AT FORCESstatics

STRETCHING A SPRING

Elasticity is the property of an object or material which causes it to be restored to its original shape after distortion.

A spring is an example of an elastic object- when stretched, it exerts a restoring force which tends to bring it back to its original length.

Hooke’s Law: the restoring force is generally proportional to the amount of stretch.

Measuring forces: Spring balance

LOOKING AT FORCESstatics

TURNING EFFECT OF FORCES CONDITIONS FOR EQUILIBRIUM:

There is no net force acting in any direction: F=0 There is no turning effect about any point The size of the turning effect is called the MOMENT OF THE

FORCE: M=0 CALCULATING MOMENTS:

The turning effect or moment depends on: The size of force The distance from the force to that point MOMENT= FORCE x DISTANCE.

COUPLES: When two forces (equal in size, and opposite in direction but not along the same straight line) are acting, we say they form a couple.

A Couple has no resultant force. It only produces a turning effect M= F x d

PRINCIPLE OF MOMENTS sum of the clockwise moments= sum of the anticlockwise moments

LOOKING AT FORCESdynamics: forces and motion

What is the link between force and motion?

Force is needed to start things moving

If an object is not moving there is NO RESULTANT FORCE acting on it.

But, does motion stops if the force is removed? ARISTOTLE (384-322 BC): a continuous force is needed to keep things

moving

GALILEO (1564-1642): forces causes changes in an object’s motion. Forces are needed:

To start and stop motion To change an object’s speed To change an object’s direction

NEWTON (1642-1727): LAWS OF MOTION (1687)

LOOKING AT FORCESdynamics: forces and motion NEWTON’S FIRST LAW: law of inertia

If there is no resultant force acting on an object:

If it is at rest, it will stay at rest If it is moving, it keeps on moving at a

constant velocity (constant speed on a straight line)

INERTIA: It is the reluctance to change the velocity.

The inertia depends on its mass: a bigger mass needs a bigger force to overcome its inertia and change its motion

MOMENTUM: p= m . v The greater an object’s momentum, the more

force needed to stop. Momentum is a vector quantity: it has the

same direction as the velocity of the object.

LOOKING AT FORCESdynamics: forces and motion NEWTON’S SECOND LAW:

The rate of change of momentum of an object (acceleration) is directly proportional to the resultant force acting on it.

F= (mv)/t= m(v-u)/t

Resultant force= mass x change in velocity/time taken

If the mass is constant, F= m . a

The change in momentum takes place in the same direction of that force.

1 N= it is the resultant force needed to give a mass of 1 kg an acceleration of 1m/s2.

LOOKING AT FORCESdynamics: forces and motion

NEWTON’S THIRD LAW: action and reaction principle.

For every action, there is an equal and opposite reaction

Forces always act in pairs. If an object A exerts a force on

an object B, then B exerts an equal but opposite force on A.

These forces don’t cancel out because the two forces are acting on different objects.

LOOKING AT FORCESdynamics: forces and motion

Identifying Newton’s third law pairsEach force has the same magnitudeEach force acts along the same line but

in opposite directionsEach force acts at the same timeEach force acts on a different objectEach force is of the same type

LOOKING AT FORCESdynamics: forces and motion

IMPULSEThe quantity ‘force x time taken’ is know

as impulse. It measures the effect of a force. It is measured in N s.F.t = m(v-u) Impulse = Change in momentum