# Forces MIX

Created 2 years ago

Duration 0:06:05
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Slide Content

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2. ### Slide 2 - Forces around you

• Forces are pushes or pulls. They can start objects moving, and they can stop, speed up, slow down, or change the direction of moving objects. They can lift things, or cause them to turn, bend or twist, they can also prevent motion.
• Some of these forces act by contact and are called contact forces. For example, when you push something by hand, or pull it with a rope, you are using contact forces. Other examples are the wind blowing the trees and ocean waves crashing on rocks.
• Some forces do not need contact, and can act at a distance. These are noncontact forces. For example, two magnets exert a force on each other without even touching. Other examples of non-contact forces are gravitational and electrostatic forces.
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5. ### Slide 5 - Balanced and unbalanced forces

• In a tug-of-war there are two equal forces acting in opposite directions. There is no motion until one force becomes greater than the other. You can use arrows to show the direction and strength of the forces.
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8. ### Slide 8 - Bicycle forces

• To start off riding your bike, you use your muscles to push on the pedals. This force then turns the back wheel, which pushes on the road causing the bike to start moving. There are also frictional forces that tend to slow you down.
• However, your pushing force is greater than the frictional forces and the bike speeds up. The forces are unbalanced, causing an increase in speed.
• When you reach a constant speed the forces are balanced. If you stop pushing on the pedals, the forces are again unbalanced, and the bike soon stops (unless you are going down a hill!).
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11. ### Slide 11 - Measuring forces

• A spring stretches when a pulling force acts on it, and is squashed or compressed when a pushing force acts on it. The bigger the force, the more it is stretched or compressed. For this reason, a spring can be used to measure the strength of forces. A pointer attached to the spring moves as the spring changes its length, and the force can be read on a scale. To measure larger forces you use a stronger spring. Spring balances measure pulls, and kitchen or bathroom scales measure pushes.
• The unit used to measure force is the Newton (N), named after Sir Isaac Newton. The force required to lift a 1 kg weight is approximately 10 Newtons.
• You can also use force measurers in a horizontal position. For example, you can use a spring balance to measure the force needed to pull a door open.
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14. ### Slide 14 - Friction

• Friction is an example of a contact force. It occurs whenever two surfaces in contact
• move past each other. Friction always opposes motion. Even when you do move an object, friction still opposes the motion. Stop pedalling your bike and the frictional forces soon bring you to a stop.
• Friction occurs because objects are never completely smooth. The roughness of the two
• surfaces means there are many points which catch and stick together. Friction also
• depends on the weight of an object.
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19. ### Slide 19 - Friction in everyday life

• We use friction every day. Sometimes we need friction, and at other times we try to reduce it. Imagine trying to walk if there was no friction. This would be like walking
• on ice or a highly polished floor. Friction also prevents knots from coming undone, and holds nails and nuts and bolts in place.
• Friction produces heat. This can be useful when you rub a match on the side of a matchbox, but it can cause a car engine to overheat.
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20. ### Slide 20 - The pull of gravity

• You are sitting at the top the Giant Drop, the 135 metre tower at Dreamworld. The catch is released and you plummet towards the Earth. You and everyone else in the car are being pulled towards the Earth by the force of gravity.
• Gravitational force is a non-contact force because it exists between objects even when they are not touching. The gravitational force between the Earth and the Moon keeps the Moon in orbit. Similarly, a gravitational force keeps satellites in orbit around the Earth, and all the planets in orbit around the Sun.
• The Moon has less mass than the Earth. This is why gravity is less on the Moon than it is on Earth. Similarly, larger heavier planets like Jupiter have more gravity than smaller lighter planets like Mars.
• In the 17th century Sir Isaac Newton came to the conclusion that gravity is the force of attraction between objects, and that the size of this force depends on the mass of the objects.
• Spring balances and scales actually measure the force of attraction between an object and the Earth. This is what weight is. Because it is a force it is measured in Newtons.
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25. ### Slide 25 - Isaac Newton

• Isaac Newton was born in 1642. His parents lived on a farm in England, and his father died the day before Isaac was born. Newton had a good imagination. For example, he imagined a very tall mountain from which he could fire a bullet. His idea was that if you could fire the bullet fast enough, it would continue to circle the Earth, just as the Moon does.
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