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Explaining motion (along a line) Newton’s laws 1 & 2

Explaining motion (along a line) Newton’s laws 1 & 2. Learning outcomes. use the relationship between net force and motion (Newton 1 & 2) to explain familiar and unfamiliar situations apply velocity and acceleration to the motion of real objects, including free fall

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Explaining motion (along a line) Newton’s laws 1 & 2

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  1. Explaining motion(along a line) Newton’s laws 1 & 2

  2. Learning outcomes • use the relationship between net force and motion (Newton 1 & 2) to explain familiar and unfamiliar situations • apply velocity and acceleration to the motion of real objects, including free fall • account for the terminal velocity of a falling object in terms of forces acting on the object • establish concepts qualitatively (using proportional reasoning) before introducing quantitative relationships (equations) • using ticker timers, light gates and timers, record and analyse motion • translate information about objects, forces and motions between words, pictures, graphs and kinematic equations • develop a strategy for solving quantitative problems

  3. The story so far • Newton’s law 1 (static equilibrium) • Newton’s law 3 (forces come in pairs) • describing motion as constant speed or constant acceleration • equations of 2 kinds: definitions and derived • graphs: s - t (gradient is speed), v - t (gradient is acceleration, area under graph is distance), a - t (area under graph is velocity) • solving quantitative problems, including a general approach (using algebra)

  4. Johnny von Neumann Hungarian-born American mathematician and computer pioneer (1903 – 57) In mathematics you don’t understand things. You just get used to them.

  5. Common misconceptions how forces act • only animate objects can exert a force • forces only affect objects through contact • there is ‘more gravity the higher up you go’ … but beyond the atmosphere there is no gravity forces and motion • if an object is moving, there must be a force acting on it • no motion means no forces acting • an object stops moving when its force ‘runs out’ (force is a property of objects)

  6. Teaching challenges • You cannot see forces; they are an abstract construction, especially forces that act at-a-distance. • The laws of motion are mostly counter-intuitive. Newton himself struggled for many years to produce the consistent account given in Principia.

  7. Physics modelling Look through ‘force spectacles’, to make a ‘free body force diagram’. 1 Isolate the object. 2 Attach force arrows. The arrow direction shows the direction in which the force acts. The length of the arrow indicates the size of the force. Each arrow starts from the point where the force acts.

  8. Using labelled arrows Questions to ask: Where is the force? What is the force acting on? What provides the force? What kind of force is it? Tell the force story: ‘the______ force of the ______ on the _____’

  9. A forces circus TASK 2 (in pairs) • mug hanging on 1 string • mughanging from 2 strings • ping-pongballwith hairdryer • paper clipattracted to a magnet • floating orange • sunk orange • shoeon slope

  10. Law 1, with an object at rest Consider all forces acting on a single object. If the object is in static equilibrium, the net force acting is zero (i.e. forces are balanced).

  11. Friction-free motion A rare thing! • air track • spacecraft in interplanetary space (no rocket firing) • experiments conducted in a vacuum Name some experiments, devices or techniques involving motion in vacuum conditions.

  12. Friction, air resistance, etc solid surfaces fluids

  13. Galileo’s thought experiment

  14. Thought experiment 2 Where will the ball land when Jim releases it? 14

  15. Real-world motion, with friction Starting to move, a C21 video [IP4.6] Forces in cycling, a C21 video [IP4.14] In pairs: • Do C21 activity 4.23 Forces in cycling • Sketch and label any forces acting on these objects, when moving at a constant speed: swimmer, 100 m sprinter, car at 30 mph, car at 50 mph, parachutist

  16. Newton 1 Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it. … either static equilibriumor dynamic equilibrium • a universal law

  17. Inertia ‘the tendency of masses to resist changes in motion’ It is difficult to • start a mass moving • change its direction of motion • stop a mass moving In other words, it is difficult to accelerate any mass. So, what has inertia to do with Newton’s laws? An inertial balance. Does inertia have units?

  18. Unbalanced forces What can happen to the object’s motion? • Steady force in the direction of motion: speeds up (accelerates – increases speed at a constant rate) • Steady force against the direction of motion: slows down (accelerates – decreases speed at a constant rate) • Steady force at right angles to direction of motion: moves in a circle (changes direction at a constant rate)

  19. Newton’s 2nd law The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed. • a universal law

  20. Newton’s second law Experiments show that … This means that SI units: • metre(distance travelled by light in free space - in a very short time!) • second(the duration of 9,192,631,770 periods of the radiation produced during a transition between 2 electron levels of Cs-133) • kilogram(a lump of stuff kept near Paris)

  21. Law 2 - standard version or The newton is a derived unit. 1 N force gives a 1 kg mass an acceleration of 1 m/s2. Summary The concept of force: something that changes motion. If an object’s motion changes, then a force must be acting on it. The bigger the force, the bigger the change of motion. Any body (mass) is treated as passive, with external forces acting on it. A moving object does not carry force (or ‘impetus’) with it.

  22. Practical session • Inertia with pendulums • Inertia on a low friction surface • More inertia experiments • ultrasound sensor + software: making s - t graphs • Investigating Newton’s 2nd law of motion • Relationships between acceleration, F and m • The effects of F and m on motion • Large trolley investigations of acceleration

  23. Free fall and terminal velocity Free fall: Falling through a fluid: A drag force … … and a buoyant force

  24. or … story in a v-t graph or … story in a v-t graph

  25. Two experiments • Ball-bearings fall through a viscous liquid • Cake cups fall through air How can you tell when a falling object reaches a terminal velocity?

  26. Inertial frames of reference In pairs: One of you, standing on a railway bridge, sees an object on a table inside a passing railway carriage. By coincidence, the other person is sitting at the same table! Discuss whether a force is needed - • to make the object move with the table, when the carriage is moving at constant speed (in a straight line)? • to slow the object, if the carriage slows down? • to speed up the object, if the carriage speeds up?

  27. Support, references talkphysics.org SPT 11-14 Forces Ep2 Contact forces EP3 Non-contact forces EP5 Balanced forces and steady speed Ep6 Unbalanced forces: speeding up and slowing down David Sang (ed., 2011) Teaching secondary physics ASE / Hodder PhET simulationsMotion Practical Physics Guidance pages e.g.Newton’s laws of motion

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