Unit 6P.3: The effects of forces

Transcription

1 Unit 6P.3: Physical Processes What forces can do Water and air resistance Science skills: Investigation Classifying By the end of this unit you should : Know that forces are push or pull, and forces can move or stop objects or change the shape. Know that air and water resistance are formed by friction. Unbalanced forces causes objects to speed up or slow down Represent forces acting on a body with arrows. 98

2 6.15.1, , Physical Processes What do you remember about forces? Write as many things about as you can: (one has been done for you!) PUSH FORCES Are there forces involved in the following pictures? discuss in your groups! 99

3 What forces can do Forces are pushes and pulls, they can make things move, stop moving, change direction, change shape and fall to the ground. Effects of forces: If we apply force on an object, it is likely that it will do one of the following. 1. On a stationary objects: Cause a stationary object to start moving. (Stationary: not moving) 2. On moving objects: a. Change the shape of an object b. Cause an object to slow down 100

4 c. Change the direction in which an object is moving. d. Cause an object to speed up Key Words Force, Push, Pull, Stationary Key Ideas Forces can change the shape of an object, change the speed of an object, change the direction of an object or cause an object to start moving. 101

5 Drawing Force: Big Forces are shown by a BIG arrow! Small Forces by a small arrow! Arrows show the direction and size of a force. The shopper provides the push to start the trolley moving. We can show the force by drawing an arrow. The arrow shows us the direction in which the force is pushing or pulling. 102

6 The boot is pushing the ball in the direction of the arrow. The bike is slowing down. The force from the brakes is slowing it down. The force of gravity is pulling on the case. The drawing shows the four forces on an aircraft during flight. 103

7 Activity: The pictures show people using forces. Are they push or pullforces? Use an arrow to indication the direction of the force for each picture. The first one has been done for you. A B C D E F 104

8 Key questions 1.Draw an arrow to show the direction of push on the tennis ball. 2.This car is slowing down with its brakes. Draw an arrow to show the direction of the force. 3. Draw the pull of gravity on this apple. 105

9 Balanced forces: Activity: Look at the picture above and answer the following questions: 1. What is the state of motion of the yo-yo? 2. What happens if the string is cut? 3. Why does this happen? 4. What are the forces acting on the object 5. Draw the forces acting on the yo-yo and label them. Look at the pictures below: Even though the ball and the ship are stationary (still) there are several forces acting upon it, why don t these objects sink into the water? 106

10 The weight of this ship is pulling it down (due to gravity). A second force called up thrust of water is pushing it up. These two forces are equal and cancel each other so the resultant force acting on the object is zero and we call these forces balanced forces, so the ship floats. It is the same with the football. Another example.. Upward Force from the table Downward Force due to gravity The two forces cancel each other and the mug remains still. The forces are balanced. Balanced force: equal forces acting in opposite directions. 107

11 Activity: What will need: wooden block (with a hook), a Newton balance, 1 kg mass, thick rubber bands Steps: 1- Put the mass on the wooden block. 2- Pull the block along the bench with the Newton balance. Force needed to pull the block = N. 3- Wrap some rubber bands around the block and do the experiment again. Force needed to pull the block with rubber bands = N. By using rubber bands what force did you increase between the wooden block and the bench? 108

12 If the friction force is greater than the motor bike, it will stop. Draw an arrow to show the direction and size of the friction force needed in order to stop the bike. Force from the motorbike engine Think of some examples where friction can be useful and not so useful. Fill in the table use the pictures for some ideas. 109

13 More balanced forces: Upthrust from water Gravity Upthrust from water push from table Gravity Gravity Gravity 110

14 If several forces are applied on an object, they may cancel each other out. When this happens we say that the forces are balanced. If two tug of war teams pull with the equal forces in opposite direction, the forces are balanced and there is no movement of the object. What happens if these forces are not equal? If one of the teams pulls with a force which is greater than that of the opposition, the forces are unbalanced and there is movement. 111

15 The movement will be in the direction of the greater force. More unbalanced forces: In order to lift the bags the man must use a greater force upwards than the weight of the bag. The gravity force is greater than air resistance force, so the climber will fall dawn. If the weight of the object is greater than the up-thrust of water, then the object sinks. 112

16 Slow down You ll cause friction! How do cars and buses slow down? Why are cars more likely to skid on a wet road than they are on a dry road? The answer is because of friction. What is friction? Friction is the force involved when objects rub together. It is friction that makes it difficult for us to push large objects across the floor. Friction is a force that slows the motion of moving objects. 113

17 Getting the resultant force. Forces acting horizontally The forces acting horizontally must be balanced. So force P and force F are equal in size. Calculating net force: Look at the direction of the forces acting on an object. If the forces are in the same direction we add them. If the forces acting on an object are in opposite direction we subtract them. We can calculate the net force (resultant force) acting on the object. Example: The block shown has two opposing forces applied on it: 7 N to the right and 4 N to the left. 4 N 7 N 114

18 To calculate the effect, subtract one from the other: 3 N 7N 4 N = 3 N to the right. The movement will be in the direction of the greater force Balanced and unbalanced forces: Unbalanced force Air resistance and friction are greater than driving force. Balanced force Driving force, air resistance and friction all are equal forces. Unbalanced force Driving force is greater than air resistance and friction. 115

19 Key words Balanced forces, Unbalanced forces, Up thrust. Key ideas An object that is not moving (stationary) or moving in constant speed has balanced forces applying on it. An object that is moving ( slowing down or speeding up) has unbalanced forces applying on it. Key Questions 1-Which of the following are forces? A. The movement of a boy on a skateboard. B. The push of the air coming from a jet engine. C. The weight of a girl. D. The pull of a car on a trailer. E. A ball falling through the air. 2- Jassim is cycling at a constant speed (same) along a level road. a- Explain whether the forces acting on him are balanced

20 b- Write down the names of two forces that can slow Jassim down When a ball rolls on grass, it stops after a time. Explain why the ball slows down and stops What is the resultant force acting on each object? Resultant Force 117

21 Physical Processes Air and water resistance Water resistance (friction in liquids) It is more difficult for us to walk through water than it is to walk through air. This is because the water rubs against our legs and cause friction (water resistance). We can try to walk fast in water but that can be very difficult. We usually change our shape in order to move faster, we swim. If we try to move our legs quickly we find that it becomes very difficult and very tiring! People move in a special way when in water, because it can be a better method of pushing through the water when it is deep. 118

22 Activity: You will need: Small ball bearing, 3 large measuring cylinders or large plastic bottle, different thick liquids, stop watch. Steps: 1-Take three bottles/cylinders. 2- Add different liquids. in each bottle/cylinder.(here is a suggestion) (water/oil/wall paper paste glue) 3- Measure the time takes for each ball to fall to the bottom of the liquid. Liquid Time taken to fall down (sec) Does the ball fall down in three liquids at same time? Explain: 119

23 Air resistance Air is all around us, even though we tend not to think a lot about it, it still has an effect on us. When we move, the air rubs against us. The friction caused by air resistance is much less On a windy day you can see the effects! than the friction caused by water resistance. The larger the surface area, the greater the friction. Activity: You will need: Two paper sheets with same type and size (A4 paper is fine). Steps: 1. Crumple one sheet of paper. 2. Take the plain sheet and the crumpled sheet and let them fall to the ground from the same height. What do you observe?

24 Conclusion: Air resistance slows the open sheet down so it drops much slower than the crumpled sheet. Parachutes use air resistance to stop us from flying through the air too quickly. Parachutes An interesting use of air resistance is a parachute. A parachute has a large surface and traps a lot of air. This makes it very difficult to move quickly through the air. Parachutes are designed to slow things down. Hand gliders use the same principle of air resistence! The earliest version of a parachute was created in a very long time ago! by a scholar from Cordoba in Andalusia عباس بن (Spain) called Abbas Ibn Firnas In an attempt to fly, he made a.فرناس glider and tried using it like wings. He jumped off a tower in Cordoba - Spain, but injured himself. So though his attempt at flying did not succeed, it is considered the earliest parachute made. 121

25 A parachute slows this jet down quickly as it lands. Free fall parachutists know all about air resistance. If they want to fall faster, they turn themselves head-first so that there is little air resistance to slow them down. If they want to go slower, they spread their arms and legs out horizontally, so that there is more air resistance. Air resistance Air resistance Gravity Gravity When they are approaching the ground, they open their parachutes. The area of parachute is very large, so there is a lot of air resistance and they slow down before they hit the ground. 122

26 Air resistance Physical Processes Gravity Jumping from an airplane is a different story. As you fall your speed increases and the air resistance acting on your body gets greater and greater. Eventually you will reach a speed where the upward and downward forces are then balanced and you carry on going at a steady speed called the terminal velocity. Terminal velocity is when the air resistance is equal to your weight. Increasing the air resistance by opening a parachute greatly reduces your terminal velocity, enabling you to land safely. 123

27 Activity: Parachutes - How will the area of the canopy affect the time it takes to fall to the floor from a given height? Ahmad and Mona were investigating parachutes. They made one from a plastic bag and some string and tested it to make sure it worked. What do you think? will the toy fall more slowly with a smaller or bigger parachute? plan an investigation to find out! (see next page) which prediction is correct. Make your parachute like this: 124

28 Activity: Parachute Investigation I will need: Our question: What will happen When I change My prediction. I think that I will keep the following the same through the experiment:, I will measure the for each parachute to fall and calculate the average. Table of Results. SIZE Diameter of canopy (cm) 1 st attempt 2 nd attempt 3 rd attempt Average Conclusion

29 Activity: what falls faster? You will need: Two objects of similar size, but different mass, such as (cricket ball and a tennis ball). Steps: 1. Hold two balls of a similar size, but different mass. 2. At the same height above and release them to the ground. Prediction: Which ball do you think would hit the ground first? Why do you think so? Observation: Explain what happened? Why?

30 Reducing air and water resistance: If we want to travel quickly or easily through air or water it would help us if we could reduce the resistance that causes us to slow down. This can be done by making a shape that is easier to push through air or water. This is called streamlining Streamlining means to make a shape that can push through air or water easily. 127

31 Look at the shapes of the following animals and think which ones are able to move rapidly. Some of these animals have features which help them to move quickly. Their shapes make it easier for them to cut through the air or water and this has helped them survive over the years. Describe Investigating Paper planes! Which is the most streamlined? 1. Make a simple paper plane. 2. Predict: What would happen if I changed my paper plane design? (choose one of the following words long, short, card, fold)

32 Procedure: This is what we will do First Then I will measure: I will keep the same: Paper plane designs: (draw here) Materials: This is what we need Results: This is what happened

33 Project How does the number of paper-clips affect the time it takes the spinner to fall to the ground? Plan your experiment fully and write a conclusion! Here is a table to help you Number of 1st attempt 2nd attempt 3rd attempt Average paper clips Key words Friction, Water resistance, Air resistance, Streamlined shape, Terminal velocity, Parachute Key ideas Water and air resistance slow things down. The speed of an object when forces are balanced is called terminal velocity. 130

34 Key Questions 1. Saleh is sledging down a snow-covered hill when he hits a patch of grass. What happens to the frictional force on the sledge when it hits the grass? a. it increases b. it decreases c. it stays the same 2. What is streamlining? How do parachutes slow something down? Draw your invention to stop this falling egg from smashing! 131