forces and motion...1.distance-time graphs slope = y-axis/x-axis = gradient = distance (m)/time (s)...

PSBS PHYSICS 2012/2013

H a n y E l ‐ G e z a w y “ F o r c e s a n d M o t i o n ” C h a p t e r 2 Page 1

Forces and Motion

Exceed the speed limits.



Forces and Motion In this unit we will study the idea of speed and the way forces affect how things move. We will also look at how streamlining is used to reduce the effect of water and air resistance. We will use the idea of balanced and unbalanced forces to explain how falling objects move. Describing how fast something moves When we want to know how fast something moves we measure its speed. A unit tells you what something is measured in. The speeds of the car and the lorry have been measured in the unit 'Meters per second'. We use the symbol m/s. For example, the speed of the car is 15 m/s.

The car and the lorry go past the lamp-post at the same time. the next picture shows where they are one second later

One second later. The car has travelled 15 meters. So, it has a speed of 15 meters per second. In the same time the lorry has only travelled 10 meters per second. The car has a higher speed.

Questions 1) Is the car or the lorry moving at a lower speed? Give a reason for your answer. 2) What is the unit used to measure the speed of the car and the lorry? 3) What is the difference between the speed of the lorry and the speed of the car? Explain

how you worked out your answer. Different units Meters per second can be a very useful unit for speed, but sometimes it is more useful to use a different unit. Road signs in Britain use the unit 'miles per hour'. Similar signs in France use the unit 'kilometers per hour'. The shorthand symbol for 'miles per hour' is 'mph'. The Symbol for the unit 'kilometers per hour' is 'km/h'. The speed of a tortoise walking across the floor is probably about 1 centimeter per second. We would write this as 1 cm/s. Questions 1) What is the symbol for the unit 'kilometers per hour'? 2) Both photographs show speed limits of 40. The top photo is from

Thailand and the bottom photo is from Britain. 3) What is the important difference between the two? 4) Which speed is higher, 2 mm/s or 2 cm/s? Give a reason for your

answer.



Working out the speed Sometimes you can get a direct measurement of speed from a measuring instrument that has been designed especially for the job. Police use radar guns and speed cameras to measure the speed of cars. People who study the weather use an instrument called an anemometer, which measures the wind speed. 1) Name two devices that give a direct measurement

of speed. 2) What does an anemometer measure? Timing things If you want to compare the speeds of things that travel the same distance, you only need to measure the time they take. This happens in races since everyone runs the same distance. Why can time alone be used to compare the speeds in an athletics race? The timing method in the picture is the one used about 50 years ago. Today, the starting signal sounds from a small speaker behind each athlete. A clock is started electronically at the same time as the signal sounds. When the athletes cross the finish line, a camera linked to the clock registers the times. If two athletes pass the finish together, then the photograph from the camera is used to work out the winner. It is sometimes called a 'photo finish'. Questions 1) What is different about the starting signal for an athletics race

today and one about 50 years ago? 2) The person with the stopwatch used to start it when they saw the

smoke from the gun, and not when they heard the bang. Why do you think this is more accurate than starting the watch when you hear the gun go off?

3) Suggest two reasons why electronic timing is better than using a stopwatch.

Calculating the speed You calculate the speed of something from the distance it travels and the time it takes to travel that distance. The swift has travelled 240 m in 6 s. assuming the swift travels at a constant speed; you can work out its speed like this: Speed = distance travelled time taken Questions 1) What formula do you use to calculate speed? 2) Which two measurements are needed so you can calculate speed? 3) If the pupil calculated the correct speed for the giraffe, what answer did she get?



Speed

Speed is the distance travelled per unit time. S.I. unit of speed is m/s

Speed (v) = distance (d) / time (t)

Calculating distance and time from the speed formula You can change the formula round to get other versions of it. Time taken = distance travelled /speed Distance travelled = speed x time taken

Average speed Speed of a car will never be a constant speed all over the trip, so we can calculate the average speed

Average speed = total distance travelled / total time taken.

Example A cyclist covers 20 m in 2 seconds then it covers another 20 m in 4 second.

Solution:- Average speed = total distance / total time. = 40 m / 6 s = 6.66 m/s

Question:- Calculate the average speed in meters per second of a runner who runs 1500 m in 5 minutes?



MOTION GRAPHS (Plotting graphs)

1. Distance-time graphs

Slope = Y-axis/X-axis = gradient = distance (m)/time (s) = speed (m/s)

Slope = velocity = distance/ time Slope = velocity = 0

Body at uniform velocity

Body at Rest Question The figure on the right shows the distance/time graph for two trucks, A and B, on an expedition across the Mongolian desert. a) i) How far did truck A move in the first hour of its journey? ii) What was its speed during this time? b) How did the speed of truck A change in the second hour of its

journey? c) Was truck B moving faster or slower than truck A in the first

hour of its journey? d) What do you think might have happened to truck A in the third

hour of the journey?

Distance Time 0 9:45 10 10:00 20 10:15 30 10:30 40 10:45 50 11:00 60 11:15



Showing a bike ride on a graph If you plot a graph of the distance travelled against the time you take you get something called a “distance/time” graph. It gives you a picture of what is happening on the journey. A horizontal part of the line means you have stopped. A steep part of the line represents a high speed. Questions

1) On which part of the journey is the speed of the cyclist the highest?

2) On which part of the journey is the speed of the cyclist zero?



2- Speed-time graphs Slope = gradient = Y-axis/X-axis= velocity (m/s)/time(s) = [a] acceleration (m/s2)

Slope = a

= velocity/ time (+ve a)

Slope = a

= velocity/ time (-ve a)

Slope = a = 0 (no increase in

velocity)

Slope = a = 0 (no increase in

velocity)

Body at uniform

acceleration

Body at uniform deceleration

Body at uniform velocity

Body at Rest

Questions

1) The table gives some data for a Ferrari racing car at the start of a Grand Prix race; Plot a speed-time graph for the car

2) Describe in as much details as possible, the motion of a car which has this graph:



Forces Pushing and pulling, stretching and turning these are some of the things a force can do. • You use a force to push a broken-down car. “Motion forces” • You use a force to pull a drawer open. “Motion forces” • You use a force to stretch a rubber band. ”Stretching Force” • You use a force to turn a door handle. “Moment” Push, pull, stretch and turn - these are some of the ways in which a force can act. (We say that a force 'acts' on an object.)

Forces cannot be seen Our bodies allow us to feel forces. There are nerve endings in our skin which can detect pressure.

• Press gently with your finger on the tip of your nose; you will feel the force of your finger pushing on your nose.

• Sit on a chair; you can feel the upward push of the chair. • Put your hand on the chair and sit on it. Your hand is squashed by two forces: the

force of your body pushing downwards and the force of the chair pushing upwards. We can't see these forces but we can feel their effects. In the drawings, the forces will be represented by arrows. A force arrow is a good way to represent a force because it shows the direction in which the force is acting. Labeling force arrows A force arrow shows us the direction of a force. We label the arrow to show two things: the object that the force is acting on and the object that is producing the force. The picture shows an example. The woman is pushing the shopping trolley. The force arrow is labeled to show which object is doing the pushing, and which object is being pushed. This helps us to understand where forces come from. Forces appear when two objects interact with each other.



Labeling forces A magnet can attract an iron nail. The magnet and the nail interact. The magnet is pulling. The nail is being pulled. The picture shows the force of the magnet on the nail.

Questions While they are playing together, Sam picks up his little brother Joe. Think about the force that acts on Joe.

a) In which direction does this force act?

b) What are the two objects that are interacting?

c) Draw a diagram to show the force that acts on Joe. Take care to label the force arrow correctly.

d) Draw a line below Sam represent the ground below Sam, and the forces acting at his foot, his weight “W” and Push of the ground “UP”

Comparing Forces “big and small” Forces can make things move. You have to push a shopping trolley to start it moving around the shop. You have to pull on a handle to open a drawer. Question The pictures show some forces making things move. Which of these things needs the biggest force? Look at the pictures. Put the forces in order, from smallest to biggest. (Write 1, 2 and 3)



Forces act in pairs When things touch there are two forces acting in opposite directions, one force on each object. When the legs of a stool touch the floor there are push forces on the legs and push forces on the floor. When you carry a bag, the handles and your hand touch each other. Two forces are produced where your hand and the bag touch. There is a pull force downwards from the bag on your hand and a pull force upwards from your hand on the bag. You can show these forces on a diagram with arrows. The length of the arrow shows how big the force is. Look at the pictures of the person and the book. Questions 1) Does the up or down arrow show the force of the person on the

floor? 2) What force is pushing on the bench?

3) What is its direction?

4) What can you say about the size of the forces that happen where the book touches the bench?



Measuring forces In science, if we want to know if one force is bigger than another, we don't simply guess. We make measurements. How can we measure forces? We use an instrument called a forcemeter to measure a force. (Another name for this is a newtonmeter.) The picture shows one type of forcemeter. This is how you use it to measure the force needed to pull a block of wood along the bench. Notes:-

• Check that the forcemeter reads zero before you start.

• Attach the hook of the forcemeter to the block. • Hold the ring at the other end of the forcemeter and pull the block. • Read the value of the force from the scale.

How a forcemeter works There is a spring inside a forcemeter. The pulling force stretches the spring and this moves the indicator along the scale. The bigger the force, the further the indicator moves. The unit of force We measure forces in newtons (N). This unit is named after Isaac Newton, an English scientist who explained how forces affect the way things move. To make it easy, we can write N instead of 'newton'. How much is a newton? If you hold an apple on the palm of your hand, it presses down with a force of about 1 N. If you hold 5 apples, that's about 5N. Palm of your hand, it presses down with a force of about I N. If you hold 5 apples, that's about 5N. Questions Look at the picture of the laboratory stool hanging from the forcemeter. 1) What is the biggest force this forcemeter can measure?

2) How big is the force lifting the stool?



Measuring pushing forces If you stand on weighing scales, you press down on the scales and the reading on the dial increases. You can use scales like this to measure pushing forces. You need a set of scales which measures in newtons. If it gives readings in kilograms, you need to know that 1kg means 1O N, 2 kg means 20 N, and so on. The pictures show three ways of using weighing scales to measure forces. • You can stand on the scales. This measures the

downward force of your weight. • You can use your hands to press the scales against the

wall; this measures the pushing force of your arms. You can use your feet instead, this measures the pushing

force of your legs.

Weight “the pull of gravity” We live on the Earth. It is difficult to get away from the Earth. If you jump upwards, you fall back down again. The Earth's gravity pulls you downwards. The Earth's gravity causes a force that pulls any object downwards. This force is called weight. Like any other force, weight is measured in newtons (N). Gravity always pulls you towards the center of the Earth. It doesn't matter where you are on the surface of the Earth. When we draw a force arrow to represent an object's weight, the arrow points towards the center of the Earth. Questions

1) Draw a diagram to show yourself, standing on the ground. Add a force arrow to show your weight.

2) Draw a diagram to show the Earth. Mark the center of the Earth. Show yourself, standing on the Earth. Add a force Arrow to show your weight.



Falling through the floor The Earth's gravity is pulling on us all the time. It pulls us downwards, but we don't fall through the floor. Why not? The floor pushes upwards on us with a force. This force is called the contact force. Any object that you push on pushes back with a contact force. Usually the force is big enough to balance the pull of gravity. But if you stand on something that isn't very strong, its upward push may not be enough to support you. Question

3) Go back to the diagram you drew for Question 1, Add a contact force arrow, to show the force of the ground acting on you.

Mass and weight Again; already you studied mass and weight in past chapter; so when you weigh yourself at home, the scales show the value in kg. You might say, 'I weigh 50 kg.' However, in science, we would say that your: mass is 50 kg. The mass of an object is measured in kilograms (kg).It tells you the amount of matter the object is made of The Earth's gravity pulls on each kg with a force of about 1O N. So, if your mass is 50 kg, your weight on Earth is about 500 N. Weight-the pull of gravity Imagine going to the Moon. The Moon's gravity is weaker than the Earth's. You weigh a lot less up there. You can jump much higher on the Moon - but you still fall back downwards. If you go far out into space, far from the Earth, Moon or any other object, your weight is zero. Your mass stays the same, however - you are still made of 50 kg of matter. Questions 1) Look at this table.

Quantity Description Units a force caused by gravity an amount of matter

In the first column, write the words 'mass' and 'weight' in the correct spaces. Add the correct units in the last column.

2) A set of weighing scales gives values in kilograms. Are the scales measuring mass or weight?

3) When astronauts went to the Moon, they found it much easier to lift heavy objects than on Earth. Explain why.



Friction Forces

Press your hands together. Rub them together. You should be able to feel the force of friction that each hand exerts on the other. Rub your hands together hard and they will start to get warm. You have observed the heating effect of friction. That's useful on a cold day. Friction is a force than can appear when two objects are in contact with each other. ('In contact' means 'touching'.) The picture shows a heavy box lying on the floor. Imagine that you try to push it. If you try to push it to the right, the force of friction pushes back in the opposite direction - to the left. Eventually, if you push hard enough, the box will move. Your pushing force is greater than the force of friction. Questions

1) When will the box move?

2) If you try to push the box to the left, in which direction will friction act? Draw a diagram to show the two forces.

The direction of friction We say that friction acts to oppose motion. To draw a force arrow to represent friction, you must ask yourself: Which way is an object moving or trying to move? For example, the heavy weight in the picture is trying to slide down the slope. This tells us that friction acts up the slope Question

3) Omar is sliding along the school corridor. Here is a picture of Omar sliding along the floor. Draw a force arrow to show the force of friction acting on Omar.

4) Which of these names represents the friction with air “air resistance” for this airplane?

Air resistance is “……………..”



Investigating friction 1) Where does the friction force occur? 2) What is the direction of the friction force

compared to the pushing force? 3) What happens to the box when the pushing

force is bigger than the friction force?

Investigating friction again You can u e a forcemeter to measure the force of friction. The diagram show how. Place a wooden block on the bench and pull it with a Forcemeter. When the block just starts to move, the forcemeter will show you the value of the force. You can investigate the different factors that affect the size of the force of friction. Here's how.

a) Add weight on top of the block to make it heavier.

b) Turn the block so that a different face is in contact with the bench, this changes the area of contact.

c) Use a material such as paper or cling film to cover the surface, making it rougher or smoother.

Factors affecting friction

1) You are going to investigate how friction depends on two of the factor mentioned above. Start by changing the weight of the block.

2) First, make a prediction. If you increase the weight of the block, will friction increase, decrease or stay the same? Give a reason for your prediction. Carry out an experiment to test your prediction.

3) Now investigate how friction depends on the area of contact between the block and the bench.

Question Press your hands very gently together and rub them. Now press much Harder and rub again. Describe what you observe. What does this tell you about the force of friction?

Friction is a force that acts when two surfaces in contact with each other. Friction acts to oppose motion.



Air resistance If you drop something, it falls to the ground. Its weight - the pull of the Earth's gravity – makes it fall. The photograph shows some parachutists falling. Eventually, they will reach the Earth's surface. The parachutists will not be travelling very fast when they hit the ground. This is because they are falling through the air. This means that there is force acting on them. This extra force is the force of air resistance. This slows them down to a safe speed. Balanced forces As the parachutist falls, air pushes upwards on the inside of the parachute. We can represent this force using a force arrow, pointing upwards, There are two forces acting on the parachutist. They are equal in size but point in opposite directions, so they cancel each other out. The parachutist falls at a safe speed. When forces cancel each other out like this, we say that the forces are balanced. Moving through air It is easy to wave your hand through the air. Air is a very 'thin' substance, so we can move easily through it. That's why a parachute must have such a big area - a small parachute would be useless. Question

1) Name the two forces that act on a parachutist who is falling towards the ground. Give the direction of each force.

2) Explain why a parachute would be useless if you went to the Moon. The air resistance produced by a parachute is also used to bring sky divers safely to the ground. The resistance of the gases in the atmospheres of other planets in the Solar System is used to slow down space probes so they can land safely and the devices on board are able to carry out their investigations. Question How would the size of parachute required on a space probe to allow it to land safely differ on:

a) A planet such as Venus which has a thick atmosphere

b) A planet such as Mars which has a thin atmosphere?

Explain your answers.



Water resistance When an object moves through water it pushes the water out of the way, and the water moves over the object's sides and pushes back on the object. This push on the object is called water resistance or drag. Objects that can move through the water quickly have a streamlined shape. A fish such as a barracuda, which moves quickly through the water, has a much more streamlined shape than a slow-moving sunfish.

Flying fish and squirrel have shapes help them to fly



Reducing friction Friction is a problem when surfaces need to move over each other. Anything with moving parts has a problem with friction. Friction will wear away a surface where that surface meets another one. Friction also uses up energy. You can show this by rubbing your hands together. The friction soon warms them up! Movement energy is converted to heat energy through friction. There are three main ways to reduce friction: • You can make the surfaces smooth; • You can put a lubricant like oil on them; • You can design the moving parts so they roll on each other rather than slide. You can judge how much friction there is between an object and a surface by tilting the surface. If the friction force is big, the object will not slide until the surface is quite steep. With a tin of beans standing on its flat end, the slope can get to 400 before the tin slides down. If you put the tin on its side, it rolls as soon as there is any slope at all.

It takes a steep slope for the tin to slide. The tin rolls much more easily. Rollers can be used to reduce friction when you are trying to move large blocks of stone. The same idea is used on a small scale in the wheels of a bike. The wheels of a bike turn on sets of ball bearings: These are designed to reduce friction because they roll rather than slide. You can also reduce the friction by using oil on the moving parts. We say that the oil lubricates the moving parts. 1) What does friction do to surfaces? 2) Name a substance that can be used as a

lubricant. 3) What three things can you do to reduce the

friction between surfaces?



Friction can be useful If there were no friction you could not walk and your shoelaces would be impossible to tie. When you walk you push your foot backwards. The effect of the friction forces between your foot and the floor is to move you forward. This happens because the floor does not move. If you try the same thing off a skateboard, which can move, then you won't get very far because the skateboard moves back instead of you going forward. This happens because the wheels of the skateboard act like rollers and reduce the friction to a very low level. Questions 1) What is the effect of the friction force between your foot

and the floor when you walk? 2) Why is it difficult to step forward off a skateboard? 3) Why is it difficult to walk on ice? Another friction force The friction force when something slides through air or a liquid is called drag. It slows things down and makes speeding up harder. People who design racing cars spend a lot of time working out what shape the car has to be to make the drag as low as possible. The same idea is also used to design the shape of cars and vans so they do not use as much petrol. A car with a shape that moves through the air easily does .not use as much petrol as a car of the same size going at the same speed with a shape that gives more drag. We say that the shape that goes through the air with less drag is more streamlined. Drag can also be very useful. If you want to slow down the fall of an object you can increase the drag with a parachute. Some plants use the drag of the air to allow

their seeds to be dispersed by the wind. The seed can travel a long way on the wind before it hits the ground. This means the new plant growing from the seed will not be competing with the original plant for food and space. Questions 1) What is drag? 2) How can you reduce the drag on something moving

through the air? 3) Describe some situations where drag is useful.

forces and motion...1.distance-time graphs slope = y-axis/x-axis = gradient = distance (m)/time (s)...

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