Chapter 11- Out Into Space

A summary of Advancing Physics chapter 11

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Forces and Momentum

  • Momentum is dependent on mass and velocity. It is a vector quantity
  • Momentum is conserved (if no external forces act). Total momentum before two objects collide = total momentum after collision
  • Rocket propulsion is explained by momentum. The forward direction momentum of the rocket = the momentum of the exhaust gases backwards
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Newton's Laws of Motion

  • 1st Law- The velocity of an object will not change unless a resultant force acts on it eg. a body will remain at rest or will move with a constant velocity and direction unless a resultant force acts.
  • If forces aren't balanced, the overall resultant force will cause the body to accelerate (a change in speed, direction or both)
  • 2nd Law- The rate of change of momentum of an object is directly proportional to the resultant force which acts on an object. F = ma
  • 3rd Law- If an object A exerts a force on object B, then object B exerts an equal but opposite force on object A
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Work and Energy

  • Work- the amount of energy transferred from once form to another when a force causes movement. Work = Force x Distance
  • The force is not always in the same direction as the movement
  • Energy cannot be created or destroyed. It can be transferred from one form to another, but the total amount of energy in a closed system will not change
  • Kinetic energy is the energy of anything moving
  • Gravitational potential energy is the energy something gains when you pick it up. Depends on the mass of the object, the height it's lifted and the value of g
  • Elastic potential energy is the energy in an elastic band (for example). Can be found as the area under a force-extension graph
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Circular Motion

  • Angles can be expressed in radians. Angle in radians = arc-length / radius
  • Angular speed is the angle an object rotates through per second
  • For circular motion, frequency is the number of complete revolutions per second, and period is the time taken for a complete revolution
  • If travelling in a circle with constant speed, an object is still accelerating as its velocity is changing with the changing direction
  • This is centripetal acceleration and it acts towards the centre of the circle
  • According to Newton's laws, in order for centripetal acceleration to occur there must be a centripetal force acting towards the centre of the circle
  • The centripetal force keeps the object moving in a circle
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Gravitational Fields 1

  • An object with mass will experience an attractive force when it's in the gravitational field of another object. The attractive force is a vector- it depends on mass of objects and the distance between them
  • As the law of gravitation is an inverse square law, as r increases F will decrease, and if the distance r doubles the force will be 1/4 of the original strength
  • Gravitational lines of force are arrows showing the direction of force the masses will feel. The earth's grav field is radial. As you move a mass away from the earth, the force experienced will decrease as lines of force are further apart
  • Close to the earth's surface, the gravitational field is uniform, so the field lines are parallel
  • Gravitational field strength g is force per unit mass. It is a vector quantity, and is negative as it is always pointing towards the centre of the mass with the field being described
  • g is a constant, and is the acceleration due to gravity
  • A mass in a uniform gravitational field will experience a constant force, mg
  • The potential energy of a mass in a gravitational field is given by mgh. It increases with height in a uniform gravitational field
  • ΔPE is positive when distance increases, and negative when distance decreases
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Gravitational Fields 2

  • The strength of a radial gravitational field decreases as you move away from the centre of mass. g is inversely proportional to r2.
  • Gravitational potential energy of a mass is the work that needs to be done to move it against gravity. A mass on the earth's surface has -ve gravitational PE. As it moves away from the earth, it gains PE, but is 0 at an infinite distance from the earth
  • The gradient of a tangent to an Egrav against r graph gives the value of gravitational force at that point
  • Gravitational potential is potential energy per unit mass
  • The speed of an orbit of a body depends on its radius and mass of the larger body
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