UNIT 6 – FORCES
WHAT IS A FORCE?
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A force is a push or a pull that changes a body’s natural state. It can take on any of the following forms: a) twisting b) lifting c) stretching d) squeezing All of these can be classified as pushes or pulls. e)
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kicking turning slamming bending TYPES OF FORCES
1. Stretching or tension force -­‐ Forces can stretch objects e.g. rubber band, spring. 2. Compression force -­‐ Forces can squeeze or compress objects e.g. compressing plasticine. 3. Pushing force -­‐ e.g. pushing a trolley to make it move forward. 4. Turning force -­‐ e.g. turning a door to open it. 6. Magnetic force -­‐ Force that causes attraction and repulsion e.g. causes compass needle to deflect (move). 5. Gravitational force -­‐ Force that pulls object to the earth e.g. fruit falling off its tree. 7. Frictional force 8. Other forces include: -­‐ Force that exist when two surfaces rub against each other. e.g. friction between a book and a surface. LSS Year 7 Notes 1 of 5 Unit 6 -­‐ Forces EFFECTS OF A FORCE
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A force is invisible, but its effects can be observed. A force can produce: A force makes a body at rest (stationary) begin to move. A force can change speed of moving object. If a body is already moving, a force acting on it can change its speed. The body will move faster if the force is applied in the direction of its motion. A force can change the direction of moving object. However, if the force is applied in a different direction, this will cause the moving body to change its direction of motion. A force can also slow down the moving body or eventually stop it from moving. A force can also change the size and the shape on an object. For example, when we apply a force to some plasticine, its shape changes. The size of an object can be changed by force as well. LSS Year 7 Notes SOMETHING EXTRA!
2 of 5 Unit 6 -­‐ Forces FRICTIONAL FORCE
1. Friction is a force produced by two surfaces in contact with each other. 2. It slows down and stops the motion of an object. 3. Friction is a nuisance (troublesome) in machine parts because extra work has to be done to overcome it. 4. This work done is energy lost as heat due to frictional force. 5. However, friction can also be important to us. 6. We are able to walk without slipping because of frictional force that exists between the floor and our shoes. 7. If friction did not exist, we would not be able to stop our moving vehicles as there would not be any grip at all between the wheels and the ground. Advantages of friction Disadvantages of friction 1. Friction prevents slipping during movement, thus 1. Friction slows down movement. enabling us to walk or run. 2. It wears away material. 2. It helps vehicles to slow down or stop when we apply 3. The soles of our shoes wear out after some time due the brakes. to friction. 3. We make use of friction when we polish a surface to 4. It may produce unwanted heat in machines. smoothen it. 4. It wears away the rough surfaces to make them smooth. 5. It helps us to grip on objects. For example, it is friction between the wood and the nail that holds them together and enables us to fasten two pieces of wood together. Reducing friction 1. Friction between moving parts can be reduced using rollers or ball bearings. 2. For example, engine shafts are mounted on bearings to help rotation of the shaft. 3. Friction is reduced because the area of contact between the surfaces is reduced. 4. Another way of reducing friction is to apply lubricating oil between the surfaces in contact. 5. A thin film of oil between the surfaces reduces the contact between them and therefore lessens the rubbing. MEASURING FORCES
1. Forces can be measured using forcemeters. 2. An example of a forcemeter is the spring balance. 3. There are different types of spring balance: the extension spring balance and the compression spring balance. 4. The amount of extension or compression of the spring is proportional to the force acting on it. 5. Spring balances measure forces in units of Newton (N). Unit of force
1. The SI unit for force is the Newton (N). 2. Look at the object hanging on the spring. The spring stretches. There must be a force pulling the object down. 3. The force acting on the object is the force of gravity that pulls things towards the 4. The force pulling on an object is called weight. 5. On earth, an object with a mass of 1 kg has a weight of 10 N. 6. Therefore, to convert kilogram to Newton, we just multiply it by ten. earth. 1 kg = 10 N WORK DONE
LSS Year 7 Notes 3 of 5 Unit 6 -­‐ Forces 1.
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Work is done when a force is applied to move an object over a distance in the direction of force. Work is not done when force applied does not move the object. For example, pushing a wall and the wall is not moving or holding a book stationary, no work has been done. Therefore, work will be done only when a) there is a force acting on an object, b) the object moves, and c) the object moves in the direction of the applied force. The amount of work done can be calculated using the formula: Work done (J) = Force (N) x Distance (m) 1 Joule is the amount of work done when a force of 1 N moves an object through a distance of 1 metre in the direction of force. More work is done when a) more force is applied, b) the distance moved by the object increases. Below is an example of calculating work done: a) Amirah is pushing a trolley over a distance of 5 m. If she pushes it with a force of 20 N, what is the amount of work done by Amirah? b) Mahdi has a mass of 46 kg carries a box of mass 4 kg up a flight of stairs 3 m high. Calculate the amount of work done by Mahdi? Solution: Work = force x distance = 20 N x 5 m = 100 Nm = 100 J Solution: Work = force x distance = [(46 + 4) x 10] x 3 m = 500 N x 3 m = 1500 Nm = 1500 J PRESSURE
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Pressure is defined as the force per unit area exerted over a surface of a body. The formula for calculating pressure is Pressure = 3.
Force
Area
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The unit for pressure is Newton per square metre (N/m ) which is also known as Pascal (Pa). 2
In other words, 1 N/m = 1 Pa 2
4. Another smaller unit for pressure is Newton per square centimetre (N/cm ). 5. Pressure depends on two factors: a) the size of force, and b) the area of contact. 6. The larger the force exerted, the greater the pressure. 7. The smaller the area of contact, the greater the pressure. Which will damage the floor more – the elephant or the heel of the lady’s shoes? Answer: the heel of the lady’s shoes! LSS Year 7 Notes 4 of 5 Unit 6 -­‐ Forces WHY?! Even though the elephant exerts a greater force as it is heavier, it doesn’t damage the floor as much as the heel of the lady’s shoes. This is because the area of heel of the lady’s shoes is less than the area of the elephant’s feet, therefore it exerts more pressure on the floor. The following calculation will prove that the heel of the lady’s shoes exert more pressure than the elephant’s feet. Elephant Lady’s shoes Weight of elephant = 40 000 N 2
Area of each foot = 1 000 cm Weight of lady = 400 N 2
Area of each foot = 2 cm Force
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= 10 N/cm Force
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= 2x2
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= 100 N/cm Pressure = Pressure = So the pressure of the heel of the lady’s shoes is ten times greater than the elephant’s feet. Therefore, the heel of the lady’s shoes will damage the floor. EXAMPLES OF PRESSURE INVOLVING SOLID OBJECTS A bulldozer The feet of a duck A bulldozer has special wheels. The wheels have a large area. Although the bulldozer is heavy, its weight is spread over a large area, so the pressure on the ground is small. Therefore, the bulldozer does not sink into mud or sand. The duck has large webbed feet. The weight of the duck is spread over a large area. As the pressure is small, the duck does not sink into the mud. Bag shoulder strap Football boots The string is thin and therefore has a small area of contact with hands or shoulders. The pressure is high because all the weight of the box is exerted on this small area. Thus, it hurts to hold heavy load with a string. Use a big strap bag so that it won’t leave marks due to less pressure. The studs on football boots increase the pressure on the ground. The studs sink easily into the ground and the player does not fall over. LSS Year 7 Notes 5 of 5 Unit 6 -­‐ Forces