FORCE

FORCE

A push or pull is called a force. The SI unit of force is the Newton(N).

A force acts in a particular direction and may have any of the following effects on an object:

Make a stationary object start moving or increase the speed of a object.

Slow down or stop a moving object.Change the direction of a moving object.Change the shape of an object.

Some forces are small while others are larger. Forces therefore have size(magnitude).

A force is represented by a line with an arrow showing the direction in which it acts ,thus:

TYPES OF FORCESGravitational forceTension Upthrust forceMagnetic forceCentripental forceCohesive and Adhesive forcesSurface tensionMolecular forceElectric forceNuclear forceElectrostatic force.

Gravitational force

This is the force of attraction between two bodies of given masses(m1 and m2 ),see figure.

When objects are thrown up from or bear the earth’s surface, they always fall downwards towards the groun.This is because of the force of attraction which the earth exerts on any body near its surface.

This force which pulls the body towards the centre of the earth is called the gravitational force of the earth.

The pull of gravity on the body towards the centre of the earth is called weight.the weight of an object varies on diffrent planets because planets have different gravitational pull.

Tension

The pull or compression of a string at both of its ends is called its tension.

Tension is as a result of two opposing forces applied,one at each end of a body,see figure 3.3

EXP3.1:to study the relationship between mass and extension of a spring.

The length of a spring increases when loading since the weight of the load acts on the spring,forcing it to strectch.

The graph of extension against mass is a straight line, see figure 3.5.

Some forces compress bodies and are called compression forces.

Materials that can be compressed or extended without breaking are called elastic materials.

Upthrust Force

There is always an upward force acting on an object immersed in a fluid(liquid or gas).This upward force is called upthrust.an object in a vacuum will not experience upthrust.

EXP3.2:To illustrate upthrust in liquids.

If the medium is air,the reading is larger than when the medium is a liquid.The reading also varies from one liquid to another.

The reading of the balance is highest when the object is in air and lowest when the object is completely submerged in water.

The difference between the readings when the object is immersed in liquid and when the object is in air is due to upthrust force.

Cohesive and Adhesive Forces

The forces of attraction between molecules of the same kind is known as cohesive force,e.g,between a water molecule and another water molecule .

while the forces of attraction between molecules of different kinds is called adhesive force,e.g.,between water molecules and glass molecules.

EXP.3.3:to investigate the behaviour of water on different surfaces

Water on the clean glass slide spreads on the glass surface.However ,small drops of water collect into small spherical balls on the waxed surface.

If mercury is used,small mercury drops in a clean glass dish surface collect into spherical balls.Larger mercury drops form oval balls, as in figure 3.10.

Water wets the glass surface because the adhesive forces between the water molecules and the glass molecules are greater than the cohesive forces between water molecules.

Water does not wet the waxed glass surface because the cohesive force is greater than the adhesive force.

The stronger cohesive forces in mercury form spherical drops of mercury even on clean glass surface.The weak adhesive force between mercury and glass makes mercury not wet the glass.

EXP3.4:to demonstrate cohesive and adhesive forces of liquids on narrow tubes

Apparatus ; narrow tubes with different size of bore,beaker,water,glycerol,kerosene,and methylated sipirit.

*the level of the water inside the tubes is higher than that outside the tubes.

Different liquids rise by different heights,depending on the diameter of the glass tube.

The level of mercury in the tube with a smaller bore will be lower than in the tube with a larger bore.

Since the adhesive force between the water and glass molecules is greater than cohesive force between water molecules,the water rises up the tube so that more water molecules can be in contact with the glass.

The force of cohesion within the mercury is greater than the force of adhesion between mercury and glass.the mercury sinks downthe tube to enable mercury molecules to keep together.

Frictional force

Friction is a force that opposes relative motion between two surfaces in contact.

Friction is caused by the interlocking of the surfaces and attractive force between the surface molecules.

Frictional force can be reduced by using rollers,oiling or smoothening.

As it moves through liquid,it rubs against the liquid ,molecules.Due to the movement of the body ,there is opposing force between the layer of the liquid molecules on the body and the layer of the liquid molecules in the measuring cylinder.the opposing force (frictional force) involving a fluid is called viscous drag (viscosity).

Magnetic force

The force which causes attraction or repulsion by magnet is called magnetic force.

A magnet has two types of poles, anorth and a South pole.like poles repel while unlike poles attract.

Which get attracted are called magnetic materials while not attracted are called non-magnetic materials.

Electrostatic Force

The rubbing creates static changes .The force of attraction or repulsion due to these charges is called electrostatic force.

When glass window is wiped with a dry cloth on a dry day, dust particles are attracted on it.

Centripental Force

Centripental force is a force which contrains a body to move in acircular path or orbit.This force is directed towards the centre of the orbit.

QUICK CHECK

Q:Give a reason why the weight of body varies on different planets.

A;weight depends on gravity-the force of gravity varies from one place to another.

Q:what do you understand by upthrust force?

A:upthrust is the upward force exerted by a liquid on an object placed in it.

KCSE-QUESTION

Surface Tension

It is commonly observed that liquids form drops,water wets some surfaces but runs off others,some insects like pond skater manage to rest on the surface of water without sinking,water rises up a narrow glass tube but mercury is pushed down to alower level in the same tube and stell or razor blade floats on water,even though steel is denser than water.

Floating a needle

If carefully placed on the surface, a small needle can be made to float on the surface of water even though it is several times as dense as water. If the surface is agitated to break up the surface tension, then needle will quickly sink.

The force which causes the surface of aliquid to behave like a stretched elastic skin is called surface tension.this force is due to the force of attraction between individual molecules of the liquid(cohesion).

Molecular Explanation of Surface Tension

A molecule ,say C deep in the liquid is surrounded by molecules on all sides so that the net force on it is zero.However,molecules of the surface, say A and B will have fewer molecules on the vapour side and hence will experience a resultant inward force,causing the surface of the liquid to be in tension.

To study the behaviour of soap bubbles

Observation :the bubble flattens to a film and the film slowly rises up the funnel.

Explanation: the soap bubble behaves as if its surface is tightly stretched.As it tries to make its surface as small as possible ,it rises up the funnel.

To study the behaviour of soap films

When the film is broken on one side,the thread assumes a perfect curve.

Explanation:

The thread lies in any position in the soap film because the thread is being pulled on both sides by equal forces of surface tension.

When one side of the thread is broken ,the surface tension acts only on ane sode of thread.

As the water tries to make its surface as small as possible,it pulls the thread in such a way that it forms a perfect curve.The soap film exhibits surface tension.

Examples of Surface Tension

Washing with cold water

The major reason for using hot water for washing is that its surface tension is lower and it is a better wetting agent. But if the detergent lowers the surface tension, the heating may be unneccessary.

 

Don't touch the tent!

Common tent materials are somewhat rainproof in that the surface tension of water will bridge the pores in the finely woven material. But if you touch the tent material with your finger, you break the surface tension and the rain will drip through.

Soaps and detergents

help the cleaning of clothes by lowering the surface tension of the water so that it more readily soaks into pores and soiled areas.

Walking on water

Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface.

When a brush is in water ,the bristles spread but when it is taken out,they cling together,see figure 3.22(b).

Surface Tension of Different Liquids

The surface tension of soap is less than that of water

A match stick or small toy boat rubbed at one end with soap and placed on the surface of water starts moving immediately.It moves in one direction only and in such a way that the end that is not rubbed with soap is always in front.Any attempt to make it move in the opposite direction fail,see figure 3.22(a).

The soap at the end of the stick immediately dissolves in water,therby lowering the surface tension at that end of the stick .the surface tension at the other end which is now greater pulls the stick and makes it move in that direction.the movement gradually weakens and ultimately ceases.This happens when the whole surface of water is covered with soap solution.

Factors Affecting Surface TensionImpurities

Impurities reduce the surface tension of a liquid.Detergents,weaken the cohesive force between liquid molecules.

Temperature

With rise in temperature ,the kinetic energy of the molecules of a liquid is increased.The intermolecular distance increase and the force of cohesion is decreased.So,the surface tension is lowered.

Consequences of surface Tension•Water insects can rest on the surface of water

without breaking the surface.•Mosquito larvae float on water surface

Action and Reaction

Forces are interactions between objects.

A pushes/pulls B --- B pushes/pulls A.

One of these forces is called the action force, and the other one is called the reaction force - it doesn't matter which is which. For example, if:

You push book. (Action force)

Book pushes you. (Reaction force)

Action and reaction are equal and opposite, i.e,when one force acts on a body, an equal and opposite force acts on another body.

HOMEWORK:EXERCISE 3.1

MASS AND WEIGHT

While mass is the quantity of matter present in an object,weight is a measure of the pull of gravity on the object.This pull of gravity is always directed towards the centre of the earth.So, weight has both direction and size.

The SI unit of weight is the Newton(N).Weight is measured by a spring balance calibrated in newtons.

The weight of an object varies from place to place .So,a body weighs more at the poles than the equator.

Relationship between Mass and Weight

Exp :3.12:To determine the gravitational field strength `g` of a place

Apparatus

Six 20 g masses ,spring balance calibrated in newtons ,retort stand.

Result:

The value of the slope is called the gravitational field strength(intensity).This is the gravitational force on a unit mass at that place on the earth.

On the earth`s surface, an object of mass m has a gravitational pull of mg on it,where g is acceleration due to gravity(free fall).

Thus ,the weight W=mgThe earth pulls each kilogram of mass on its

surface with a force of about 9.8 N,which is approximately 10 Nkg-1

Example :

Calculate the weight of each of the following:

A cat of mass 2.5 kg

A pencil of mass 10 g

A lorry of 8 tonnes.(use g=10N/kg)

Differences between Mass and Weight

Measuring Force

A spiral spring extends when a stretching force is applied.The larger the force,the more the spring extends.The extension of a spring can therefore be used to measure an applied force.

Such an instrument that uses the extension of a spring to measure forces is called a spring balance,see figure 3.24.

SCALAR AND VECTOR QUANTITIES

A scalar quantity is one that has magnitude only but no direction.

Vector quantities have both magnitude and direction.

A vector quantity is represented by a straight line with an arrow –the length of the arrow represents the magnitude of the vector.

Vectors can be added or subtracted.

The sum of the two or more vectors is called the resultant vector.

HOMEWORK:

EXERCISE 3.2 AND EXERCISE 3.3