20040 AC Science Year 4 Physical sciences
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How are objects moved by forces? - I<br />
Read the text.<br />
Objects cannot move by themselves. They need some kind of force to make them<br />
move. Forces cause objects to speed up, slow down or change direction. The<br />
greater the mass of an object, the more force is needed to start or stop it moving.<br />
An object can be made to move by pushing, pulling, lifting, stretching, twisting or<br />
spinning forces acting on it.<br />
Forces move in a particular direction and are usually shown on a diagram with<br />
arrows. The arrows show the direction in which the force is acting.<br />
Forces such as pushes, pulls and collisions are applied directly and transfer<br />
energy to an object. When a ball is kicked, energy from the foot is transferred to<br />
the ball, sending it in the direction and at the speed the player wants. When you<br />
throw a ball, a similar transfer of energy from your arm to the ball occurs.<br />
Some substances can be stretched or squeezed when a force is used to pull or<br />
push them. But these substances spring back into their original shape when the<br />
force is removed. Many balls are made from materials that stretch like this. So<br />
when a ball hits a wall, it is squashed a bit then it springs back into shape. As it<br />
springs back, the energy stored in the squashed ball is released. This is the force<br />
that causes the ball to bounce back again.<br />
Friction is another directly applied force. A rolling ball would continue to roll along<br />
without stopping if it wasn't for the force of friction acting on it. A ball rolling along<br />
a rough surface slows down and stops quicker than one rolling along a smooth<br />
surface. This is because the force of friction between the two surfaces is greater.<br />
Gravity is a natural force, exerted from a distance, which causes a mass to attract<br />
and be attracted by any other mass. Because the Earth has the greatest mass, all<br />
other objects with a smaller mass are pulled downwards towards it. The famous<br />
scientist Sir Isaac Newton used the example of an apple dropping from a tree and<br />
falling to the ground, to explain this theory. Objects have weight on Earth because<br />
gravity pulls them down.<br />
Forces are said to be balanced (or equal) if they are the same size but acting<br />
in opposite directions. If balanced forces are acting on an object, the object will<br />
not change its motion. If it is still, it will remain still. If it is moving, it will continue<br />
moving in the same direction and at the same speed. Unbalanced forces change<br />
the way an object is moving by making objects start to move, speed up, slow<br />
down, or change direction.<br />
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AUSTRALIAN<br />
CURRICULUM SCIENCE (<strong>Year</strong> 4)
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