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Understanding Forces and Newton's Laws of Motion

Learn about forces, Newton's laws, friction, lubrication, gravity, and more. Explore how to investigate and describe different forces effect on objects. Practice Newton's laws through hands-on investigations.

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Understanding Forces and Newton's Laws of Motion

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  1. At the end of this unit you should: • Understand force and know that the newton (N) is the unit of force. • Be able to describe forces and their effects. • Know how to investigate examples of friction and the effect of lubrication. • Know how to investigate the relationship between the extension of a spring and the applied force. • Understand that weight is the force of gravity and that it varies with location; recall that mass in kilograms multiplied by 10 is approximately equal to weight in newtons on the surface of the earth. 6. Be able to explain and give the unit for work.

  2. acceleration direct proportion extension force friction gravity lubrication mass newton weight weightlessness work

  3. LIGHTBULB QUESTION Force is something being pulled/pushed. It is anything which causes the motion of an object to change.

  4. Force: Anything which causes the motion of an object to change. The unit of force is the newton (N). Acceleration Due to Gravity: The downward force experienced on the earth. The value of gravity on the earth is approximately 10 m/s2.

  5. Newton’s First Law of Motion: A body at rest will remain at rest, and a body in motion will remain in motion unless it is acted upon by an external force. Newton’s Second Law of Motion: The force is directly proportional to the acceleration and mass of an object. Newton’s Third Law of Motion: For every action there is an equal and opposite reaction.

  6. (a) Copy and complete Table 12.01.01, establishing what caused the change (mass/acceleration), the approximate size of the force (small/large) and in what direction the force acted (up/down/left/right, etc.). Add some more examples of your own.

  7. (a) Copy and complete Table 12.01.01, establishing what caused the change (mass/acceleration), the approximate size of the force (small/large) and in what direction the force acted (up/down/left/right, etc.). Add some more examples of your own.

  8. (b) What does Newton’s Second Law of Motion state? The force is directly proportional to the acceleration and mass of an object.

  9. (c) What is the mathematical statement of Newton’s Second Law of Motion? F = ma

  10. (d) An object of mass 10 kg is accelerated at 1 m/s2. What is the resultant force? 10 N

  11. (e) A person has a mass of 55 kg. What force would be required to accelerate this person at 15 m/s2? 825 N

  12. (f) An apple falls from a tree under an acceleration of 9.8 m/s2, and hits you on the head. What force would it exert on a person’s head if the apple has a mass of 0.0125 kg? 0.1225 N

  13. (g) A toy car of mass 5 g gains a force of 4.5 N. What acceleration did it obtain to achieve this force? 900 m/s2

  14. (h) A force of 100 N is required to accelerate a boulder 0.5 m/s2. Find the mass of the boulder. 200 kg

  15. Investigation 12.01.01: Action and reaction forces Equipment: You and a classmate (though you won’t be using ice skates!).

  16. Investigation 12.01.01: Action and reaction forces Instructions: 1. Before carrying out this investigation complete Question 1 in the ‘What did you learn?’ section of this investigation. 2. With a partner, re-enact the scene given in Fig. 12.01.06. 3. Examine what you think happened, compare this with your drawings in Step 1, and mark up any corrections you need to make. 4. Next, have one of you push much harder and discuss what the force arrows would look like now.

  17. 1. Before carrying out the investigation, copy Fig. 12.01.06 and draw what you think would be the force arrows. • Have the arrows been drawn equal in size? Also have they been drawn from the same point in each body and in the same/different directions?

  18. 2. Now re-enact the diagram with your partner. Do your force arrows accurately represent what happens in the investigation? If not, redraw the force arrows in a different colour pen. • If you’ve made a mistake, how can you rectify it?

  19. 3. Now get one person to push harder. What happens? Redraw the arrows again using a different colour pen, this time indicating which force was greater. • You should now have one arrow much larger than the other.

  20. Investigation 12.01.02: The forces acting on a spring balance Equipment: Students in pairs, Spring balances.

  21. Investigation 12.01.02: The forces acting on a spring balance Equipment: Spring balances. Instructions: 1.With a partner, hook together a pair of spring balances. 2. Pull gently away from each other. 3. Take a note of the readings.

  22. 1. Draw a diagram of the spring balances and the relevant • force arrows. • You should draw a diagram with two arrows of equal size pointing in opposite directions.

  23. 2. What do you notice about the weight in each balance? • The balance should be equal on both spring balances.

  24. 3. Can you draw a conclusion about the relationship between the forces involved in the spring balances? • The forces are equal and opposite.

  25. Mass: The amount of matter in an object. Mass is constant for a particular object. This means it is a property of that object. Weight: The force of gravity acting on an object. The unit of weight is the newton (N). Weight = mass x gravity.

  26. (a) Copy and complete Table 12.01.02 by calculating the weight of each astronaut on each planet.

  27. (a) Copy and complete Table 12.01.02 by calculating the weight of each astronaut on each planet.

  28. (b) On which planet/moon do the astronauts weigh the least? Phobos.

  29. (c) Which planets/moons can the astronauts actually land on? Justify your answer with reference to your calculations. They can land on all planets except Jupiter, where the force of gravity is so strong that if they were to land on it, they would be crushed under their own weight.

  30. Friction: The force that opposes motion. Lubrication: A method of reducing friction.

  31. LIGHTBULB QUESTION Since the car is moving at a constant speed, this means it is neither speeding up nor slowing down. This tells us that the forces are equal and opposite. So the force of friction is 2770 N. Since both forces are equal, this means the force of the engine is also 2770 N.

  32. Investigation 12.01.03: The effects of forces Equipment: Retort stand and clamp, slotted weights, spring, metre stick.

  33. Investigation 12.01.03: The effects of forces Instructions: 1. Set up the apparatus as shown. 2. Take an initial reading with just the spring as well as just the mass hanger. 3. Add 1 N weight and take note of the new length. 4. Subtract this length from the initial length to get the extension. 5. Repeat Steps 3 and 4 up to 6–8 N.

  34. 1. Draw a table of the force (N) and the extension caused by that force. • The table should look similar to the table below. • Note the figures are made up and are not the expected answer.

  35. 2. Did you notice a pattern in the data obtained? • You should notice that the extension increases by a fixed amount each time.

  36. 3. Now draw a graph of the table you made. • The graph should look similar to Fig. 12.01.01 here. Note the figures are made up and are not the expected answer.

  37. 4. Can you describe the pattern you found (if any) in your graph? • You should notice a straight line graph through the origin.

  38. 5. Is this pattern consistent with the pattern (if any) you found in your table? • Yes, it should be.

  39. 6. What do you think will happen if you use a different spring for this experiment? • You will get a different set of results for the extension. BUT that new spring will extend by a fixed amount each time (it won’t be the same amount as the spring you are using now).

  40. 7. Could you use another of your class group’s graphs to determine an unknown force for your spring? Why? • No – each spring is different, so you would not be able to use someone else’s spring on your data. • However, you could use their friends spring AND data to calculate an unknown force.

  41. Direct Proportion: When one physical quantity changes, the other physical quantity changes at the same rate. Work: Work is the ability to move an object; measured in joules (J). Work=Force (N) x Distance (m)

  42. Copy and Complete In this unit I learned that a force causes something to move. To find force we multiple mass and acceleration due to gravity. I also learned that Newton’s Third Law states that for every action there is an equal and opposite reaction. Mass is the amount of matter in a body. Mass is always constant. I also learned that mass and weightare not the same. Weight is the force of gravity acting on a body. This is why astronauts experience weightlessness in space. Frictionis the force which opposes motion. There are useful examples of friction such as in car brakes. However, if we want to reduce friction we use a technique called lubrication. I also learned a key relationship between the force applied to an elastic object and the extension of this object. This is also known as Hooke’sLaw. Finally I learned that work is done when a forcemoves an object over a certain distance.

  43. 1. What is a force and what is its unit of measurement? A force causes something to move and its unit is the Newton (N).

  44. 2. What tool is used to measure forces? Spring balance.

  45. 3. State Newton’s Second Law of Motion. The force is directly proportional to the acceleration and mass of an object.

  46. 4. What is the mathematical formula for Newton’s second law? F = ma

  47. 5. State Newton’s Third Law of Motion. For every action there is an equal and opposite reaction.

  48. 6. Give three examples of action/reaction pairs of forces. Two pool balls colliding; firing a bullet from a gun (bullet goes forward, gun recoils backwards); two people pushing against each other.

  49. 7. Define friction. Friction is the force that opposes motion.

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