2. Forces are actions which make objects move, stop or change shape. They can be
When you throw a ball, it moves. The movement is caused a push or a pull
by the force you applied to the ball.
Types of forces
There are different types of forces.
Gravity: this force make objects fall to the ground. If we throw an object into the air,
gravity pulls it to the ground.
Electric force: this force is found between electrical charged objects. Electric force
can attract or repel.
Magnetic force: this force affects magnets and some metallic objects. Magnetic
force can attract or repel.
Friction: when an object touches another object, it experiences a frictional force.
This force acts in the direction opposite to the direction of movement.
It slows or stops moving objects. For example, friction between a ball and
the ground stops the ball rolling after a few metres.
Forces
3. Forces and motion
Forces can distort some objects temporarily or permanently.
Elastic objects, such as a spring, return to their original shape when the force stops acting
upon them. They are distorted temporarily.
Non-elastic objects, such as a piece of plasticine, remain permanently distorted when the
force has stopped.
Forces can affect motion in many different ways. They can:
Make something move: Stop a moving body: Change the direction
for example, when you for example, when a of a moving object:
open a door, you push basketball player for example, when a
or pull to move the door. catches a ball, the tennis player hits a ball,
ball stops. it goes to the other side
of the court.
Increase the speed
of a moving body: Reduce the speed of a moving body:
for example, the force when the skateboarder puts both
a skateboarder uses feet on the skateboard, the wheels rub
to push the skateboard against the ground andf riction makes
makes it go faster. the skateboard go more slowly.
4. Machines
Machines are devices which use the action of a force or convert one type of energy
into another. For example, a bicycle is a machine which transforms human energy into
mechanical energy. Machines save us time and physical effort.
Types of machines
Some machines, such as pulleys, are very simple because they have very few components.
Other machines, such as computers, are complex because they have many components.
Machines can be classified depending on the energy source. There are machines which
work with
Human energy, such as water or wind, such a Fuel, such as Electricity, such as a
a bicycle or a hammer. watermill or a windmill. a car or a gas washing machine or
stove. a mobile phone.
Machines which produce movement
Many machines produce movement, such as escalators, cars or food blenders. These
machines have a motor or an engine. The motor converts fuel or electrical energy into
movement.
5. Thermal machines
Thermal machines, such as heaters and air conditioners, produce heat or cold.
Information-processing machines
Information-processing machines are used for communication and calculations. For
example, we can communicate with others using mobile phones and we can make
calculations on computers.
Computers can process texts, images and
numbers. They also enable us to communicate
through the Internet and by e-mail.
