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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