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# Mechanics

### AS91171 - 2.4

Question | Answer |
---|---|

Forces | A push or a pull measured in Newtons. A vector quantity. |

Net Force | When forces are balanced there is no net force (speed will remain constant or object will be stationary). When they are unbalanced the object will accelerate or decelerate. |

Friction/Drag | A force occuring when two objects move against one another or trhough a fluid. Can appose movement, is increased by making surfaces rougher or by pressing them together and is decreased by lubrication. |

Scalars | Measurements will magnitude only (distance & speed). |

Vectors | Measurements with both magnitude and direction (displacment, velocity & acceleration). |

Distance/Time Graphs | Direction isnt consider, distance cannot decrease and the slope gives speed. |

Slope of Distance/Time Graphs | Gives speed, the steeper the gradient the faster the object, curved lines mean acceleration or deceleration. |

Displacement/ Time Graphs | Direction is important, displacement may increase, decrease or remain the same, gradient gives velocity (+ve means forward movement, -ve means backward movement). |

Speed/Time Graphs | No direction, all date points are positive, the line is always above x-axis, gradient shows acceleration and area under the curve shows direction. |

Velocity/Time Graphs | Shows direction, data points above x-axis show forward motion, data points below show backward motion. Gradient give acceleration and area between graph line and x-axis gives displacement. |

Acceleration/Time Graphs | Change in direction of an object in motion is shown. |

Adding Vectors | Trig may have to be used, vectors are shown with arrows. |

Vector Subtraction | Used to find the difference between final and inital velocity - draw vector arrow in opposite direction to find quantity. |

Relative Velocity | Movement of 2+ objects, subtract the velocity of your object from the ‘other object’ to show overall relative velocity. |

Vertical Motion under Gravity | Air friction is disregarded, gravity is acceleration/deceleration. |

Object's in Free Fall | Objects under the influence of gravity. |

Objects Falling Only | Acceleration due to gravity is positive. |

Objects Thrown Up then Fall Down | Acceleration due to gravity is negative. |

Horizontal Projectile Motion | Objects are at angles to surface and move along symmetrical curved paths, air friction and acceleration is disgrearded. |

Speed of Objects in Horizontal Projectile Motion | Initial speed = final speed, constant horizontal velcotiy while accelerating downwards, vertical velocity decreases when object moves upwards and increases when moved downwards, vertical velocity = 0 at peak of parabola due to pure horizontal velocity. |

Terminal Velocity | Rate at which an object falls, determined by gravity, time and friction. |

Vertical Projectile Motion | Acceleration is considered. |

Hooke's Law | F = -kx, negative sign represents the spring trying to return to equilibrium position, spring constant (k) is the stiffness of the spring (a large value is a spring that is hard to strech or compress). |

Newton's First Law of Motion | ‘An object at rest will remain at rest unless acted on by an unbalanced force’. |

Newton's Second Law of Motion | ‘If an object with mass is acted on by a force it will accelerate’. |

Newton's Third Law of Motion | 'For every action there us an equal and opposite reaction’. |

Forces Acting at Angles | According to Newton’s second law, if the object is accelerating horizontally, then the net force must be horizontal. |

Centripetal Acceleration | Direction and velocity are constantly changing, object is constantly accelerating towards the centre while maintaining a constant speed. |

Centripetal Vector Diagram | Velocity at any instant is tangent to the circle. vector triangles are utilised - vf is drawn first, then vi is drawn backwards and Δv is used to finish the circle. Vector subtraction shows a change in velocity that points to the centre of the circle. |

Centripetal Force | Caused by the force keeping the object in a circular motion. |

Centrifugal Force | Not a real force, the outward senstation when turning corners, explained by newtons first law and related to momentum. Objects will only turn if there is an unbalanced force with enough friction. |

Period | Symbol - T, measured in seconds. The amount of time it takes for something to happen. |

Frequency | Symbol - f, measured in Hz. How many times something happens in one second. |

Torque | Angular or turning force. Related to the size of the linear force and perpendicular distance if the force from the fulcrum or pivot point (fulcrum). Symbol - τ and measure in (N m). |

Equilibrium | When all forces are balanced. |

A Couple | Pair of equal but opposite forces acting independently of one another, causes the object to rotate. The rotation of a couple is midway between them. |

Centre of Mass | Where there is no net torque due to weight, symmentrical objects will have a centre of mass in the middle of them. An object's mass will create a torque if a fulcrum is not placed at the centre of mass. |

Fulcrum | ‘Point against which a lever turns or is supported’. |

Angled Support Forces | The principle of balanced torques can be used to find the tension force in angled supports. For a torque to exist the distance and force must be perpendicular. |

Kinetic Energy | Energy an object possesses due to its motion. |

Gravitational Energy | The energy stored in an object due to its height above ground. |

Elastic Energy | Energy an object posses in a strech or compressed spring or elastic. |

The Conservation of Energy | Energy cannot be created or destroyed, only transformed or transferred. |

Energy in Isolated Systems | Will remain constant due to freedim from external forces. |

Energy in Dynamic Systems | Energy loss will be definite due to resistance or frictional losses. |

Work | The ability to transfer energy. If an object us moved perpendicular to the force then no work is done by the force on that object. |

Power | The rate of doing work or how much energy is transformed or transferred per second. |

Momentum | The resultant of mass and velocity or 'mass in motion'. Measure of difficulty stopping an object in motion. Symbol - p, units kg m s^{-1}. |

Impulse | The objects cahnge in momentum, the time taken to chnge and the force creating the momentum change. The measure for how long a force is placed on an object. |

Conservation of Momentum | “The total linear momentum of an isolated system remains constant in magnitude and direction unless acted upon by an external net force". |

Conservation of Momentum - Sums of Momentum | The sum of the initial momentum is equal to the sum of their final momentums. |

Elastic and Inelatic Collisions | Momentum is always conserved. |

Elastic Collisions | Ek before and after collision is conserved. |

Inelastic Collisions | Ek is greater before collision. |

Momentum in Two Dimensions | Use conservation of momentum because there has been no impulse/external force acting on the system. In an isloated system there is no momentum change. |