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P5/6 science

Forces At Work

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Taken from ScieNce PartNer: a complete Guide to Upper Block Science

Destructive Forces

Although forces can help us to do work such as cutting fruit or pushing the trolley in the supermarket, there are some forces which can harm us.

The force of a hurricane can uproot trees and damage houses.

A house is damaged when a tree is uprooted.

In 2004, a massive underwater earthquake off the coast of Sumatra generated massive tsunami waves. It inflicted massive damage along the coastal lands of ten countries including Indonesia and Sri Lanka. More than 280,000 people were killed in the natural disaster.

People running for their lives away from the tsunami

Different Types Of Forces

The effects of forces can be felt in our everyday life. There are four different types of forces.

1 GravityGravity is the invisible force that pulls two things towards each other. It acts from a distance.

Anything that has mass has a gravitational pull.

Heavier objects are able to exert a larger force of gravity on surrounding objects.

Since the sun is much larger and heavier than the earth, it exerts a force of gravity on the earth which is strong enough to keep the earth in its fixed orbit around the sun.

earthmoon

sun


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Forces At Work

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The earth itself possesses its own gravitational field. It is this force of gravity that attracts all falling objects to earth.

The apples fall downwards to the ground due to gravity.

The moon also has its own gravitational field but due to its smaller mass, its gravitational field is only 1__6

that of the earth.

The force of gravity of the earth acting on an object is known as the object’s weight. In other words, objects have weight because of the gravitational force between them and the earth.

Do not get confused between the mass of an object and the weight of an object. Mass and weight refer to two different things.

Mass is the amount of matter that is present in a body. The mass of an object remains the same at any place.

Weight, on the other hand, depends on how much gravity is acting on an object at that moment. The unit of measurement for weight is Newton.

A person would weigh less on the moon than on earth. However, his mass remains the same whether he is on the moon or on earth.

2 Elastic Spring ForceAn elastic spring force causes a spring or rubber band to stretch when pulled.

When the force is removed, the spring or rubber band returns to its original shape and length.

When the rubber band on the catapult is pulled backwards, it stretches. When the rubber band is released, it suddenly springs forward, propelling the stone far away.

The stretched rubber band exerts a force on the stone.

stone


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Forces At Work

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The springs attached to the baby’s hammock repeatedly stretch and go back to its original position, bringing about a gentle up and down rocking motion which lulls the baby to sleep.

The stretched springs exert a force on the hammock, and the baby in it.

A pulling force is exerted to stretch a spring. A pushing force is exerted to compress a spring.

Wound up springs can be found in some watches. The small knob at the side of the watch face is twisted to wind up the spring. The compressed spring then unwinds very, very slowly, pushing the hands of the watch to show the passage of time.

A mechanical spring watch

When springs are stretched or compressed, they exert a force on whatever is stretching or compressing them.

Extension

A pulling force can make a spring stretch. The original length of the spring becomes longer when it is stretched.

The larger the force exerted on the spring, the longer the spring extends.

hammock

knob

s

Pushing forcePulling force


P5/6 science

A spring before and after different loads were attached to it.

3 FrictionFriction is a force that opposes motion.

Frictional force occurs between the surface of the road and the base of the moving box.

As the box is being pushed across the surface of the road, there is a force of friction between the two surfaces — the road and the bottom of the box.

Friction acts in the opposite direction of motion. As the box moves forward, friction is acting backwards.

As a result of the friction, the force exerted by the hand to push the box must be greater than the weight of the box. The additional force is needed to overcome the frictional force.

The rougher the surface, the greater the frictional force.

Disadvantages Of Friction• Causes wear and tear

The tyre threads (patterns carved onto the wheel surface) on a tyre wear out after some time andthe tyre becomes bald. It is then time to change to a new tyre.

The soles of shoes wear out after constant walking.

4 cm

2 cm

4 cm

road surface

box

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Forces At Work


P5/6 science

Forces At Work

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• Causes moving objects to slow down

It requires more energy to push a heavy load along a rough road surface.

• Produces heat

As the moving parts of machines rub against each other, some energy is lost as heat.

If the metallic parts of a machine rub against each other excessively, sparks may be produced anda fire may start.

Advantages Of Friction

• Enables two surfaces to grip onto each other to prevent slipping

Try walking on a wet marble floor. The lack of sufficient friction will make you slip and fall easily.If friction between your feet and the ground is not present, you will not be able to walk steadilywithout slipping. In fact, holding on to any object without friction will be almost impossible as theobject will slip right out of your hands!

Following the same principle, car tyres have patterns carved into them so that the uneven surfaceof the tyres will have a better grip on the road surface to prevent it from skidding.

The tyre threads on a car tyre

Likewise, the soles of your shoes have grooves and patterns cut into them.

The grooves and patterns on the soles of shoes

• Produces heat

In the old days, people rubbed two sticks together to get a spark to start a fire. Without friction, we would not be able to light a match.

When you are feeling cold, rubbing your palms together produces a small amount of heat to keepyou a little warmer.

tyre threads

grooves and patterns


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Forces At Work

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As we have seen through the examples, friction is a double-edged sword, i.e. there are both advantages and disadvantages to its presence.

Sometimes, a disadvantage in one circumstance can be an advantage in another circumstance.

While friction becomes a negative thing when it slows us down too much, having some friction between our feet and the ground is necessary to slow us down just a little so that we do not slip.

Ways To Overcome Friction

Friction can really become a nuisance at times. Therefore, we find ways to reduce friction.

Lubricants such as oil or even water are applied to surfaces that are constantly rubbing against each other in machines. This allows the parts to glide more smoothly across each other to reduce wear and tear.

Rollers or ball bearings (small metallic balls) will also work in cases when you do not want to have liquid lubricants messing up the machines.

4 The Magnetic ForceMagnets come in different shapes and sizes.

Some types of magnets

Natural magnets can be found buried in the ground. They are called lodestones. However, lodestones are rare.

Most magnets are man-made from magnetic material like iron, steel, cobalt or nickel.

Only magnetic materials can be made into magnets and be attracted by other magnets.

A magnet has two poles at its ends – a North pole (or North-seeking pole) and a South pole (or South-seeking pole).

If a magnet is cut into two pieces, it will become two separate magnets, each with its own North and South poles.

lodestone(natural magnet) horseshoe magnet

button magnet U-shaped magnet

ring magnet bar magnet

rod magnet


P5/6 science

Types Of Forces

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Taken from Nail thoSe McQs! PriMary ScieNce UPPer Block 5/6

1. Which of the following forces are able to act from a distance?A. elastic spring forceB. frictional forceC. gravitational forceD. magnetic force

(1) A and B only(2) B and C only(3) C and D only(4) A, C and D only ( )

2. Which of the following activities does not require any force?(1) Admiring a painting on the wall(2) Taking the dog out for a stroll(3) Writing a letter(4) Folding a shirt ( )

3. Under which of the following circumstances will a person not experience gravitational force?(1) a person on a roller coaster(2) a person in an elevator(3) a person in a space station(4) a person up in a plane ( )

4. Which of the following actions show(s) that friction is useful?A. to stop a moving car at a traffic junctionB. to push a wardrobe across the roomC. to unscrew a bottle cap

(1) A only(2) C only(3) A and B only(4) A and C only ( )

5. Which of the following statements about gravity are correct?A. Gravity allows all objects on earth to have mass.B. Gravity pulls everything in the air towards the ground.C. Gravity is a kind of force which acts from a distance.D. Gravitational force varies with the distance of the object from the earth.

(1) A and B only(2) B and C only(3) C and D only(4) B, C and D only ( )


P5/6 science

Types Of Forces

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Taken from Nail thoSe McQs! PriMary ScieNce UPPer Block 5/6

6. The diagram below shows a man throwing a javelin.

ground

The javelin stays midair for a few seconds before landing on the ground. Which of the following statements about the moving javelin is true?(1) No force is acting on the javelin when it is first thrown.(2) Gravitational force is acting on the javelin throughout its motion.(3) Gravitational force only acts on the javelin when it is at its highest point.(4) The javelin travels at a constant speed throughout its motion. ( )

Answers: 1. 3 2. 1 3. 3 4. 4 5. 4 6. 2

forces at work - sapgrp.com · forces at work sapscsp5&6_w25. page 1/8 taken from. science...

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