Types of forces © 2009 Marshall Cavendish International (Singapore) Private Limited

A question to think about… What are the different types of forces? © 2009 Marshall Cavendish International (Singapore) Private Limited

Different types of forces friction elastic spring force gravity magnetic force My Pals are Here! Science © 2008 Marshall Cavendish (Singapore) Private Limited © 2009 Marshall Cavendish International (Singapore) Private Limited

Friction What happens when you push a book on your desk with your finger? The distance in which the book will move depends on how hard you push it. The book will slide along the surface of you desk, slows down and comes to a stop. Why does this happen? © 2009 Marshall Cavendish International (Singapore) Private Limited

Friction Whenever two surfaces are rubbed together, friction is produced. Friction between the surface of the book and the desk causes the book to slow down and stop. Friction is a force that opposes motion. Friction acts in an opposite direction to the movement of an object. Thus, friction can cause moving objects to slow down and stop. © 2009 Marshall Cavendish International (Singapore) Private Limited

Friction There is more friction on a rough surface compared to a smooth surface. An object moves more slowly on a rougher surface. A marble floor has a smooth surface. A gravel-covered floor has a rough surface. © 2009 Marshall Cavendish International (Singapore) Private Limited

Friction Friction also produces heat. When the surfaces of objects are rubbed against each other, heat is produced. Try doing a foot rub using your hands. Do your hands feel warm soon after? © 2009 Marshall Cavendish International (Singapore) Private Limited

Friction can be useful Friction is useful in our daily lives as it provides grip. Friction allows us to hold on to objects without them slipping and sliding away from each other. For examples, friction allows us to hold a pencil and write. © 2009 Marshall Cavendish International (Singapore) Private Limited

What are the other uses of friction? Friction can be useful What are the other uses of friction? sole Friction between the soles of your shoes and the ground allows you to run fast without slipping. Friction between the soles of your shoes and the ground stops you from slipping when you walk. Friction is very helpful when the ground is slippery or wet. © 2009 Marshall Cavendish International (Singapore) Private Limited

Friction can be useful The patterns on the tyres of a vehicle help the vehicle to stay on the road. They prevent the vehicle from skidding under wet, slippery conditions. They also allow the vehicle to slow down and stop when the brakes are applied. © 2009 Marshall Cavendish International (Singapore) Private Limited

Friction can be a problem Although friction is useful, it can also be a problem as it causes objects to: move slowly overheat wear out For example, there is friction between the moving parts of a machine because the parts are always rubbing against each other. Machines need extra energy to work against friction, causing heat to be produced. After some time, the parts become worn out. © 2009 Marshall Cavendish International (Singapore) Private Limited

Friction can be a problem When the soles of your shoes are worn out, you cannot walk properly and may slip on a smooth floor. © 2009 Marshall Cavendish International (Singapore) Private Limited

Friction can be a problem Old and worn-out tyres give very little friction between the tyres and the road. This is dangerous as the car can skid more easily. © 2009 Marshall Cavendish International (Singapore) Private Limited

Ways to overcome the problems of friction Lubricants Lubricants like oil or grease on the moving parts of a machine help to reduce friction. With less friction, machines last longer. Oil to be placed in between the gears. A can of grease © 2009 Marshall Cavendish International (Singapore) Private Limited

Ways to overcome the problems of friction Rollers and wheels Rollers and wheels reduce friction between the surfaces of objects and the floor. This allows us to move heavy objects easily. wheel © 2009 Marshall Cavendish International (Singapore) Private Limited

Even though friction gives use problems at times, life would be difficult without it. Things would slip off from tables. Shoelaces would unfasten themselves. Objects would slip from your hands, and you would not even be able to walk! © 2009 Marshall Cavendish International (Singapore) Private Limited

Elastic spring force Elastic materials are those that have the ability to return to their original shapes after a force is applied to them. Clothes such as t-shirts and socks are elastic. You stretch them to put them on, but they return to their original shapes after that. © 2009 Marshall Cavendish International (Singapore) Private Limited

Elastic spring force Plasticine and dough are not elastic. You can change their shapes but they will not return to their original shapes on their own. © 2009 Marshall Cavendish International (Singapore) Private Limited

Spring A spring is an object that is elastic. It returns to its original shape after it is stretched or compressed. When we pull a spring, we apply a force that causes the spring to stretch. spring is stretched © 2009 Marshall Cavendish International (Singapore) Private Limited

Spring When we push a spring down, we apply a force that compresses the spring. When springs are stretched or compressed, they exert a force called elastic spring force. spring is compressed © 2009 Marshall Cavendish International (Singapore) Private Limited

Spring When a force acts on a spring, the spring stretches. The larger the force, the longer the springs gets. spring A spring balance © 2009 Marshall Cavendish International (Singapore) Private Limited

Objects with springs A trampoline A stapler A pen spring spring spring © 2009 Marshall Cavendish International (Singapore) Private Limited

Gravity There are forces that act at a distance. They make an object move without actually touching them. Gravity is such a force that pulls everything down towards the Earth. Gravity is acting on you all the time, pulling you downwards even when you stand still. Without gravity, you would be floating in the air! © 2009 Marshall Cavendish International (Singapore) Private Limited

Gravity Gravity pulls all objects towards the Earth. An object will fall to the ground unless it is supported or held by another force. If the force supporting or holding the object is removed, the object will fall to the ground. What will happen if the string is cut? My Pals are Here! Science © 2008 Marshall Cavendish (Singapore) Private Limited © 2009 Marshall Cavendish International (Singapore) Private Limited

What gravity does Gravity makes things in the air fall to the ground. Gravity pulls you back as you try to climb uphill Gravity causes water to flow downwards. Gravity speeds you up as you go downhill. © 2009 Marshall Cavendish International (Singapore) Private Limited

Weight All objects on Earth have weight. They have weight because of the gravitational force between them and the Earth. Spring balances can be used to measure weight. The spring is stretched when an object is hung on it. The greater the weight, the longer the spring stretches. © 2009 Marshall Cavendish International (Singapore) Private Limited

Weight Weighing machines such as bathroom scales and kitchen scales can also measure weight. The springs in bathroom scales and kitchen scales are compressed by the weight placed on top of them. kitchen scale bathroom scale © 2009 Marshall Cavendish International (Singapore) Private Limited

Weight Weight is not the same as mass. Weight is a force, while mass is the amount of matter an object has. Weight Mass Weight is a force the force of gravity acting on an object Mass is not a force the amount of matter in an object Weight is different at different places. The weight of an object is different on the Earth, the Moon and other planets. Mass is constant. The mass of an object is the same wherever the object is. © 2009 Marshall Cavendish International (Singapore) Private Limited

Weight Moon If a person weighs himself on the Moon, his weight would be only one-sixth of what it is on Earth. His mass, however, remains the same whether he is on the Earth or the Moon. Earth © 2009 Marshall Cavendish International (Singapore) Private Limited

Magnetic force Magnetic force is another type of force that can act at a distance. Magnetic forces are caused by pushing and pulling actions of magnets. A magnet pulls or attracts objects made of iron or steel. These materials are known as magnetic materials. magnetic objects © 2009 Marshall Cavendish International (Singapore) Private Limited

Magnetic force A strong magnet can attract magnetic materials even when they are separated by a non-magnetic material. © 2009 Marshall Cavendish International (Singapore) Private Limited

Magnetic force A magnet can also push or pull another magnet. Every magnet has a north pole (N) and a south pole (S). Two north poles of two magnets, when brought together, will push or repel each other apart. We say the like poles of two magnets repel each other. This push is also known as the force of repulsion. force of repulsion S N N S © 2009 Marshall Cavendish International (Singapore) Private Limited

Magnetic force A north pole of a magnet and the south pole of another magnet, when brought together, will pull or attract each other. We say the unlike poles of two magnets attract each other. This pull is also known as the force of attraction. force of attraction S N S N © 2009 Marshall Cavendish International (Singapore) Private Limited

The end © 2009 Marshall Cavendish International (Singapore) Private Limited