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FORCE. By M iss B uicke. What we must know from the syllabus OP4 Appreciate the concept of force, recall that the newton is the unit of force , and describe forces and their effects. OP5 Investigate examples of friction and the effect of lubrication.
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FORCE By Miss Buicke
What we must know from the syllabus OP4 Appreciate the concept of force, recall that the newton is the unit of force, and describe forces and their effects. OP5 Investigate examples of friction and the effect of lubrication. OP6 Investigate the relationship between the extension of a spring and the applied force.
Can you think of a force? Pushing Pulling lifting Throwing squeezing
What is happening in the picture? What force is being demonstrated?
What is a force? A force is anything which causes an object to move or change its velocity. The word force is used quite often in everyday conversations. Even though we use the word force regularly, it can be difficult to explain its exact meaning. In physics, we are very precise with the meaning of words and we must use the proper definitions when answering questions.
A famous scientist, Isacc Newton, wrote a very • exact meaning of force: • A force can cause a stationery object to move. • A force can cause a moving object to move more • quickly, to move more slowly or to change the • direction in which it is moving. Unit of force The unit used to measure force is the newton and the symbol is N.
Different types of force and their effects. Force of gravity Magnetic force 1. 4. 3. Electric force 4. Force of friction 2. 3.
1. The force of gravity If a pen rolls over the edge of a table, it falls to the ground. This is because the earth pulls objects towards it. The pull of the earth is due to the force of gravity. Gravity is pulling the apple to the ground
The story of Newton and the apple. The scientist and physicist Sir Isaac Newton made one of the greatest discoveries while sitting down under an apple tree when an apple fell on his head. Besides giving him a headache, it made Newton think about why the apple fell down straight to earth, and later realised that it didn't exactly “fall”, but rather it was drawn to the Earth's center. This is because the earth pulls objects towards it. The pull of the earth is due to the force of gravity.
2. Magnetic force When a magnet is brought near a nail, you will notice that the magnet will attract the nail. The force that moved the nail is the magnetic force of the magnet. If you bring two magnets near each other, they may attract or repel. The force of gravity always attracts objects towards the ground, while the magnetic force may attract or repel.
3. Electric force When you rub a plastic pen against the sleeve of your jumper, you can lift small pieces of paper with the pen. The pen has become electrically charged and the electric force can attract the small pieces of paper.
4. Force of friction How are brakes able to slow down a bicycle? When you pull the brake handle the brake pad pushes against the wheel of the bicycle. The friction between the wheel and the brake pad causes the bicycle to move more Slowly.
Definition of friction Friction is a force which prevents easy movement between two objects in contact. The rougher the two surfaces in contact, the greater the friction. All surfaces are rough even though they do not seem to be. If you could see a close-up photograph of any surfaces, it would appear rough and jagged. The rough surface is what causes the force of friction.
How Does friction cause a matchstick to light? The red part of the match on the end of the stick along with the red strip on the side of the box have rough surfaces which, when rubbed together quickly, create friction. The friction causes heat, which ignites the tip of the match.
What happens as a block of wood is pushed along a table? When a block of wood is placed on a table the rough surface of the block is caught in the rough surface of the table. When we push the block, some of your blushing force will move the block and some of your pushing force will be needed to overcome the friction between the table and the block. The amount of your pushing force needed to overcome the friction will depend on how rough the surface in contact are.
Advantages of friction: • Friction is needed to stop a bike. Friction between the • brake pad and the wheel is what stops the bike. • When driving a car around a corner friction is needed. • If the tyres on a car are worn there is a risk that a lack of • friction could cause the car to skid. An accident while • turning a corner is more likely on a wet day because there • is less friction between the tyres and the road.
Friction allows grip, so a person can walk, climb and run. • When we walk there is friction between our shoes and • the ground, which stops us from slipping. • If you step onto an oily patch on the ground, you will • most likely slip. The oil patch reduces the friction between • your shoes and the ground.
Disadvantages of friction • Friction can also be a nuisance. • Friction causes wearing of moving surfaces in contact eg. wear and tear of tyres and footwear. • Getting a blister. The blister is caused by skin rubbing against the shoe.
Heating effect • When a space shuttle returns to earth there is a lot of friction • between the surface of the shuttle and the gases in the • atmosphere. This could cause the shuttle to overheat, • with disastrous consequences.
Friction wastes fuel • When a car is moving there is friction between the air • and the surface of the car. This friction is overcome by the • force of the engine. • However, this means that some of the fuel is needed • to overcome friction. Therefore wasting fuel.
Reducing the force of friction • To reduce friction between two surfaces you could try to • make the surface smoother. • There is no perfectly smooth surface so we often • Use special materials to reduce friction • These materials are called lubricants. • Common examples are oil, liquid polish and soap.
There are many moving parts in a car engine. Friction would lead to wearing. Friction would also generate heat which would damage the engine. Oil and grease are used as lubricants in car engines.
The bones at the joint in the human body are surrounded by a fluid that acts as a lubricant. This greatly reduces friction.
Experiment: to investigate friction and the effects of lubrication • Equipment: Spring balance, four identical blocks of wood, sheet of sandpaper, • piece of carpet, oil. • Method: • place one block of wood on the table attached to a spring balance • Pull the block along the table and record the force in neutons • Take the second block and attach a piece of sandpaper. • pull across the table and record the force in neutons • Take the third piece of wood and attach the carpet to it • Pull across the table and record the force • And finally apply oil to the last piece of wood • Pull across the table and record the force in neutons.
Result: The pulling force was different in each case. Conclusion: The reading on the spring balance will be different for the four blocks. This shows that the force of friction depends on the type of surfaces that are in contact
Gravity as a force: weight It is important to understand the difference between the words mass and weight. In everyday usage, these words often have the same meaning. In physics, they have Very different meanings.
Calculations There is a simple formula to convert mass to weight. Weight (in newtons) = mass ( in kilograms ) x 10 Sample question 1 What is the weight of an object of mass 6.5kg? Weight = mass x 10 Weight = 6.5kg x 10 Weight = 65 N
Sample question 2 Calculate the weight of an object of mass 200g Remember to change the unit of mass to kilograms Weight = mass x 10 Weight = .2kg x 10 Weight 2 N
Sample question 3 What is the mass of a person whose weight is 620 N ? Weight = mass x 10 Weight/10 = mass 620/10 = mass 62 kg
The effect of a force on a spring The scientist Robert Hooke discovered the relationship Between the force applied to an elastic body (spring) and the extension of the elastic body (spring) caused by the force. This discovery is known as Hooke’s Law Definition of Hooke’s law The extension of an elastic body (a spring) is directly proportional to the force causing the extension.
This means that if the force on the string doubles so will the extension; if the force triples then the extension will also triple, etc. • When a spring is being extended as in the Hooke’s law experiment, • a problem may arise • This problem happens if the force used to extend the spring is too • large. • If the force is too big, the spring will be over-stretched and lose • its elasticity. • Hooke’s Law will be no more valid. We say that the spring has • exceeded its elastic limit
Experiment: • To investigate the relationship between the extension of a spring and the force that is stretching it. • Use a newton-meter to stretch a spring and note the extension and the force used. • Repeat for lot of different forces. • Plot a graph of force used against extension.
Result: Your graph should be a straight line. • Conclusion: because the points gave a straight line through the origin, it can be concluded that: the extension of an elastic body is directly proportional to the force causing the extension.
