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Forces and motion


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A lightning tour, qualitatively, of the physics of forces and motion. Intended as a backgrounder for middle years teachers.

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Forces and motion

  1. 1. MOTIONWhat does is mean for an object to be stationary?Is there any such thing?If an object is in motion, we can describe its position, and how fast it is changing its positionWe can also describe the direction it is movingAll motion is described in relative, not absolute termsUsually relative to the earth‟s surface in the local area, but not for rockets, planets, stars
  2. 2. CONCEPTS - MOTIONSpeed – rate of change of position (distance unit per time unit, e.g. metre per second)Velocity – same as speed but with a direction specified (e.g. 30 m/s North)Distance – total distance travelled relative to the ground (distance unit, e.g. metre)Displacement – current distance from point of origin, including direction (e.g. 25 m West)
  3. 3. CONCEPTS - ACCELERATIONAcceleration – rate of change of velocity (not speed)Unit of distance per unit of time per unit of time (e.g. metre per second per second, m/s/s, ms-2)Can involve a change of speed or a change of direction (or both)Can be in the same direction or the opposite direction compared to motion, or at an angle to it
  4. 4. CONCEPT - FORCESA push or pull that acts on an objectMeasured in an SI unit called the newton (N) which is a kilogram metre per second per second, kgms-2)How we arrive at this unit will become obviousCan involve contact (e.g. a tug pulling a ship, a bulldozer pushing dirt) or not (e.g. earth pulling on the moon)
  5. 5. EXAMPLES OF FORCESA golf club hitting a ballPeople pushing a carMagnets attracting or repellingA book pushing down on a desk and the desk pushing upward on the bookGravity pulling downward on a flying seagullPositively charged protons in the nucleus attracting negatively charged electrons
  6. 6. GRAVITYGravity is a particular kind of forceIt is a result of the nature of matter in this universe(related to the Higgs field and the Higgs boson)It causes all masses to be attracted to all other massesIt is a very weak force (compared to electrostatic force, for example), which means it becomes important when masses are very large, but it exists for all massesIt decreases with the square of separation distance
  7. 7. CONCEPTS - MASS AND WEIGHTMass – measures the amount of matter in something in mass units (e.g. kg)Weight – the force that gravity, at a particular place, exerts on a particular massMass of an object is the same everywhere in the universeWeight of an object depends on the local gravitational fieldObjects have the same mass but about 1/6 the weight on the moon
  8. 8. NEWTON’S FIRST LAW (OF MOTION)First Law – “An object remains at rest, or in motion in a straight line at constant speed, unless acted on by an unbalanced external force”(since motion is relative, „rest‟ and „uniform straight line motion‟ only depend on your point of view!)This is not intuitive to us, since things tend to slow down – but this is because friction is acting (see below)Think about the situation in outer space…
  9. 9. NEWTON’S SECOND LAWSecond Law – “The acceleration of an object is directly proportional to the (external, unbalanced) force acting on it and inversely proportional to its mass”Usually written as an equation for calculations: a = F/mAlternatively, F = maIt makes sense that the bigger the force the bigger the acceleration„Inertia‟ is the name we give to the idea that bigger mass will lead to smaller acceleration with the same force
  10. 10. NEWTON’S THIRD LAWThird Law – “For every force there exists another force of the same magnitude (size), that acts in the opposite direction, on a different object”People talk about „action‟ and „reaction‟ forces, but that can be confusing – it‟s only a point of viewEarth pulls the moon, and the moon pulls back with the same force: but because the earth has more mass, the moon accelerates more (i.e. changes its direction of motion, moving in (nearly) a circle)There are no exceptions to the Third Law
  11. 11. FRICTIONFriction is a kind of force that occurs when objects are in contact with each other, either stationary or moving relative to each otherFrictional forces always oppose the motion that causes themEnergy (which is a topic for another day) is lost to frictional forces, causing heatDifferent kinds of frictional: static (stationary relative to each other), kinetic (sliding past each other), rolling
  12. 12. AIR RESISTANCEWhen an object is in „free fall‟ in an atmosphere, as it gains speed the air will oppose the motion – this is a form of frictionThe gravitational force remains constant and the air resistance increases with velocity – „terminal velocity‟ is reached when the two forces balance
  13. 13. BUOYANCYWhen an object is placed in water it displaces waterThe weight force due to that displaced water acts upward on the body – the force of buoyancyIf buoyancy balances the weight of the object, it will float, if not, it will sink