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# Forces and Motion

### Physics notes

Term | Definition |
---|---|

What is a scalar quantity? Give 3 examples. | A scalar quantity has only magnitude. Three examples are mass, temperature and time. |

What is a vector quantity? Give 3 examples. | A vector quantity is a quantity which has both magnitude and direction like momentum, velocity and current. |

What is the moment of a force? Give formula and units. | The moment of a force is the product of the magnitude of the force f and the perpendicular distance d from the pivot to the line of action. M=fd - newton metre, Nm |

What is the law of moments? | When an object is in equilibrium the sum of the clockwise moments about any point acting as a pivot is equivalent to the sums of the anticlockwise moments about the same point. |

Define centre of gravity. | The centre of gravity of an object is the point where its entire weight acts for any change of the object. |

What is Aristotle's law of motion? | He assumed an object needed a force to move and the speed of an object depends on the force. His law states that the speed of an object is proportional to the force applied. |

What was Aristole's error? | Aristotle could not explain why objects fell without any visible force and why planets moved for no apparent reason so his ideas were not scientifically based. |

What were Galileo's achievements? | He invented the telescope and was the first to discover that earth was not the centre of the universe and it circled around the sun. He also studied the pendulum, motion and falling ojects and the effect of friction on objects |

What were Sir Isaac Newton's achievements? | He studied the nature of light and proved that white light was made of various colours, he produced Newton's Law of Gravitation, Newton's New Law of Calculation, Newton's Laws of Motions and invented the first reflective telescope. |

What is Newton's first law of motion? | The 1st law states an object at rest remains at rest or a moving object continues in a uniform motion unless acted on by an external force. |

What is inertia? | Inertia is the laziness or tendency of matter to reduce changes in its motion. Inertia makes it hard for objects to start and stop moving, change direction or accelerate. |

What is Newton's 2nd law of motion? | When a force acts on a body the rate of change of momentum is directly proportional to the applied force and takes place in the direction on the force. |

What is the formula associated with Newton's 2nd law and state and define the units. | F= ma, measured in Newtons (N). 1N is the force needed to give a mass of 1kg an acceleration of 1m/s² |

What is Newton's 3rd law of motion? | If body A exerts a force on body B, body B exerts an equal but opposite force. |

Give the formula and units for momentum. | p= mv and kilograms metre per second (Kgm/s) |

List the ways the force applied can be found. | f= m(final velocity - initial velocity)/t (f= m(v-u)/t. |

Define impulse and provide the formula and units. | Impulse is equal to the change in momentum, it is the product of the force applied and time. ft = mv-mu and newton second (Ns) is the unit. |

What is the law of conservation of momentum? | The total momentum before a collision is equal to the total momentum after a collision unless there is an external force. |

Define terminal velocity and provide the formula. | When an object falls it gains a velocity, a frictional force (Fr) opposes its weight (W) and the resultant downward force (F) is obtained. F = W - Fr |

How is acceleration calculated from this formula? | F = ma and F = W - Fr so ma = W - Fr and a = W - Fr/m |

Describe the process during free fall. | As velocity increases magnitude of the frictional force increases also, until frictional force is equal to the weight The resultant force is 0 since it opposes the direction of the frictional force. W=Fr, W-Fr=0 and F=0. a=0 object falls at constant speed |

What is kinetic energy and its formula and units? | Kinetic energy is the motion energy a pushed object gains. Also it is the energy a body possesses due to its motion. KE = 1/2mv² measured in Joules (J) |

What is potential energy and its formula and units? | The stored energy an object possesses due to its condition and position is called potential energy. PE = mgh |

What is the link between kinetic and potential energy? | Change in kinetic energy= change in potential energy= loss of energy= gain of energy |

Define centripetal force. | This is the external force needed for an object to flow in a circular path. It always acts perpendicular to the direction of motion of the object |

What is the formula for motion in a cirlce? | F= mv²/r |