# Chapter 11- Out Into Space

A summary of Advancing Physics chapter 11

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• Created by: R_Hall
• Created on: 02-03-14 15:56

## 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

• 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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