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# Forces

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A set of slides created to teach Forces to learners at Bishops Diocesan College in Cape Town.

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### Forces

1. 1. Forces G9 Keith Warne www. Teach Bomb .com
2. 2. Effects of Forces <ul><li>Identify a force acting in each picture. </li></ul><ul><li>what effect the force is having. </li></ul>Force: …………………..…. Effect: …………….………. Force: …………………..…. Effect: …………….………. Force: …………………..…. Effect: …………….………. Force: …………………..…. Effect: …………….………. Force: …………………..…. Effect: …………….……….
3. 3. Effects of Forces <ul><li>Identify a force acting in each picture. </li></ul><ul><li>what effect the force is having. </li></ul>Force: GRAVITY Effect: …………….………. Force: PRESSURE Effect:DEFORM Force: Elastic Effect: ACCELERATE Force: THRUST Effect: ACCELERATION Force: GRAVITY Effect: TURN
4. 4. Effects of Forces <ul><li>Applying a force to an object can have a number of effects </li></ul><ul><li>………………… .... </li></ul><ul><li>…………………… </li></ul><ul><li>…………………… </li></ul>
5. 5. Effects of Forces <ul><li>Applying a force to an object can have a number of effects </li></ul><ul><li>Change shape </li></ul><ul><li>Change speed </li></ul><ul><li>Changing Direction </li></ul>
6. 6. Types of Motion <ul><li>Investigation </li></ul><ul><li>Aim: To investigate the four different types of motion. </li></ul><ul><li>Apparatus: </li></ul><ul><ul><li>Wooden block </li></ul></ul><ul><ul><li>Spring balance or elastic band </li></ul></ul><ul><ul><li>Flat surface. </li></ul></ul>Pull Gently Pull Harder No movement Constant speed Pull a lot Harder ACCELERATE! Pull ?? DECELERATE!
7. 7. Types of Motion Pull ……….? Movement ? Force? <ul><li>Method: </li></ul><ul><li>Connect the spring balance to the block using the string as shown in the diagram. </li></ul><ul><li>Apply a force to produce one of the four types of movement. </li></ul><ul><li>Read the size of the force on the spring balance. </li></ul><ul><li>Note this force down in a suitable table. </li></ul><ul><li>Repeat steps 2-4 with each of the other of the four types of movement. </li></ul>
8. 8. Types of Motion - Results Pull Gently Pull Harder No movement Constant speed Pull Harder ACCELERATE! Pull ………. DECELERATE! Expt. Motion Force (N) 1 No movement 2 Constant speed 3 Acceleration 4 Deceleration
9. 9. Types of Motion - Analysis Pull Gently Pull Harder No movement Constant speed Pull Harder ACCELERATE! Pull ………. DECELERATE! Analysis The largest force is needed to accelerate the block. Constant speed requires a small force. Deceleration needs the least force. Expt. Motion Force (N) 1 No movement 8.4 2 Constant speed 6.5 3 Acceleration 14.3 4 Deceleration 5.2
10. 10. Types of Motion - Conclusion Pull Gently Pull Harder No movement Constant speed Pull Harder ACCELERATE! Pull ………. DECELERATE! Conclusion: A force seems to oppose the motion. This force is greater when the object is stationary. Expt. Motion Force (N) 1 No movement 8.4 2 Constant speed 6.5 3 Acceleration 14.3 4 Deceleration 5.2
11. 11. FRICTION <ul><li>Friction is a force that acts …………………. to the ……………………… whenever objects that touch each other are in ………… ………. </li></ul><ul><li>Friction is …………………. when the object is ………. MOVING!! </li></ul>……………… …………
12. 12. FRICTION <ul><li>Friction is a force that acts …………………. to the ……………………… whenever objects that touch each other are in ………… ………. </li></ul><ul><li>Friction is …………………. when the object is ………. MOVING!! </li></ul>……………… …………
13. 13. FRICTION <ul><li>Friction is a force that acts opposite to the direction of motion whenever objects that touch each other are in relative motion. </li></ul><ul><li>Friction is GREATER when the object is NOT MOVING!! </li></ul>Applied force Friction
14. 14. Balanced Forces <ul><li>Two forces act on an object </li></ul><ul><li>Forces are …………………………….. </li></ul><ul><li>OPPOSITE in DIRECTION. </li></ul><ul><li>Forces are said to be ………………………… </li></ul><ul><li>The object’s motion will ………………………... (It ill be at ………….. or have a …………………… velocity.) </li></ul>Newton’s First Law - ………… ” A body continues in its ……………….. or of ………… motion in a ……………….. , unless acted on by a ……………… force .“
15. 15. Balanced Forces <ul><li>Two forces act on an object </li></ul><ul><li>Forces are EQUAL in SIZE </li></ul><ul><li>OPPOSITE in DIRECTION. </li></ul><ul><li>Forces are said to be BALANCED </li></ul><ul><li>The object’s motion will not CHANGE. (It ill be at REST or have a CONSTANT velocity.) </li></ul>Newton’s First Law - Inertia ” A body continues in its state of rest or of uniform motion in a straight line , unless acted on by a resultant force .“ (Inertia.)
16. 16. Unbalanced Forces <ul><li>Two forces act on an object </li></ul><ul><li>Forces are ………………………….. in SIZE </li></ul><ul><li>Forces are said to be ……………………… </li></ul><ul><li>The object’s motion ……………. (……………...) </li></ul>Newton’s Laws Second Law A ………………….. force on an object causes it to ………………… in the ………………. of that force.
17. 17. Unbalanced Forces <ul><li>Two forces act on an object </li></ul><ul><li>Forces are NOT EQUAL in SIZE </li></ul><ul><li>Forces are said to be UNBALANCED </li></ul><ul><li>The object’s motion WILL CHANGE (Acceleration). </li></ul>Newton’s Laws Second Law A resultant force on an object causes it to accelerate in the direction of that force.
18. 18. Inertia <ul><li>A body has a ............................................ any changes to its state of motion. </li></ul><ul><li>This resistance is known as ....................... . </li></ul><ul><li>If the card in the picture is flicked ................. ....................................... . Inertia keeps the peg stationary when the card is moved quickly . </li></ul><ul><li>The peg’s Inertia is overcomes .................. ................ which try to keep it’s position on the card. </li></ul><ul><li>The moon was moving past the earth in a straight line but became ................. by the ............................ </li></ul><ul><li>Gravity does not act against the direction of motion (90 o ) so the motion continues because ............................................... to .................... . </li></ul><ul><li>(The question is who threw it in the first place!) </li></ul>Every object in a state of uniform motion tends to remain in that state of motion unless an external (unbalanced)force is applied to it. Gravity Motion Motion Friction Inertia Earth 1. 2.
19. 19. Inertia <ul><li>A body has a RESISTANCE TO any changes to its state of motion. </li></ul><ul><li>This resistance is known as INERTIA </li></ul><ul><li>If the card in the picture is flicked THE PEG DROPS INTO THE BOTTLE . Inertia keeps the peg stationary when the card is moved quickly . </li></ul><ul><li>The peg’s Inertia is overcomes FRICTION FORCES which try to keep it’s position on the card. </li></ul><ul><li>The moon was moving past the earth in a straight line but became TRAPPED by the EARTH’S GRAVITY. </li></ul><ul><li>Gravity does not act against the direction of motion (90 o ) so the motion continues because NO FRICTION OR FORCE to STOP IT. </li></ul><ul><li>(The question is who threw it in the first place!) </li></ul>Every object in a state of uniform motion tends to remain in that state of motion unless an external (unbalanced)force is applied to it. Gravity Motion Earth 1. 2. Motion Inertia Friction
20. 20. Newton’s Laws Second Law <ul><li>A resultant force on an object causes it to accelerate in the direction of that force. </li></ul>F res = m x a These are the two most basic scenarios. F m a F m T W
21. 21. <ul><li>&quot;For every action there is an equal and opposite reaction .” </li></ul>Newton’s 3rd Law ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ..............................................................................................................................................................................................................................................................................................................................
22. 22. Newton’s 3rd Law <ul><li>&quot;For every action there is an equal and opposite reaction .” </li></ul>The gasses experience a force backwards out of the rocket, this has an equal but opposite reaction force which drives the rocket forward ! The force exerted by the hand on the head is the same magnitude as the force exerted by the head on the hand! The head hits the hand just as hard as the hand hits the head!
23. 23. Pairs of forces
24. 24. Force examples <ul><li>An object on a table. </li></ul>• � <ul><li>Gravity </li></ul><ul><li>Exerted by the earth on the apple </li></ul>Normal or Reaction force
25. 25. An apple on a table. <ul><li>Non contact forces Gravity </li></ul><ul><li>Exerted by the earth on the apple </li></ul><ul><li>Reaction by the apple on the earth. </li></ul>Contact forces: Table Apple exerts a force down on the table. Table exerts a Normal or Reaction force on the apple. • <ul><li>All these forces are equal so there is no resultant force. </li></ul><ul><li>Forces on apple: gravity (down) & reaction (up) </li></ul>
26. 26. Moving Car <ul><li>What forces act on the car when it moves with constant speed? </li></ul>
27. 27. Moving Car <ul><li>What forces act on the car when it moves with constant speed? </li></ul>• <ul><li>Weight </li></ul><ul><li>The force of gravity exerted by earth on the car. </li></ul>Road holding car up Thrust (engine pushing forward) Friction (Drag)
28. 28. Mass & Weight <ul><li>Mass (kg): </li></ul><ul><li>This is a measure of the …………………. (stuff) that makes up a body. </li></ul><ul><li>Weight (N): </li></ul><ul><li>This is the ……….. of …………… exerted on a body’s mass by the earth (planet). </li></ul>F g 1 kg
29. 29. Mass & Weight <ul><li>Mass (kg): </li></ul><ul><li>This is a measure of the amount of matter (stuff) that makes up a body. </li></ul><ul><li>Weight (N): </li></ul><ul><li>This is the force of gravity exerted on a body’s mass by the earth (planet). </li></ul>F g 1 kg
30. 30. Mass & Weight <ul><li>Mass (….): </li></ul><ul><li>Measured in …………….. using a ………………. </li></ul><ul><li>Weight (….): </li></ul><ul><li>Measured in …………….. using a ……………….. or …………….. meter. </li></ul>0.5 kg
31. 31. Mass & Weight <ul><li>Mass (m): </li></ul><ul><li>Measured in kilograms using a balance. </li></ul><ul><li>Weight (F g ): </li></ul><ul><li>Measured in Newton's using a spring balance or force meter. </li></ul>0.5 kg
32. 32. Mass & Weight - table Investigation: To determine the relationship between an object’s mass and weight. Method: We are going to weigh a number of different masses on a spring balance. We will record the mass and weight in each case. (In a table.) We will then analyse the results to see if any patterns (relationship) can be seen. Mass/kg Weight/N
33. 33. Mass & Weight - table Investigation: To determine the relationship between an object’s mass and weight. Method: We are going to weigh a number of different masses on a spring balance. We will record the mass and weight in each case. (In a table.) We will then analyse the results to see if any patterns (relationship) can be seen. Mass/kg Weight/N 0.05 0.3 0.10 0.8 0.15 1.3 0.20 1.8 0.25 2.3 0.30 2.8
34. 34. Mass vs Weight - Graph From the graph we can see that the weight rises as the mass is increased. The straight line indicates a direct proportion between mass and weight. (Double the one the other will also double). Line corrected for systematic error scales were not zero’d correctly. Graph should pass through (0;0)
35. 35. Mass & Weight - Results <ul><li>From the table we can see: </li></ul><ul><li>As the mass increases the weight increases. </li></ul><ul><li>The weight is (roughly) equal to the ten times the mass of the object. </li></ul><ul><li>W = mass x 10 </li></ul><ul><li>Conclusion: The weight of an object in Newtons is equal to the object’s mass in kilograms multiplied by ten . </li></ul>Mass/kg Weight/N 0.05 0.3 0.10 0.8 0.15 1.3 0.20 1.8 0.25 2.3 0.30 2.8
36. 36. Visit my site for more presentations & resources. Keith Warne www. Teach Bomb .com
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