【国际物理】Unit1force&motionChapter2Force&shape

Unit 1 FORCE AND MOTION different kinds of forces01 力的分类 how they affect the way objects move02 运动状态的改变 how they can affect the shape of objects03 形状的改变 OBJECTIVES You will learn about: • 从力的性质来分类有： • weight重力、elastic force弹力、friction摩擦力、 gravitational force万有引力、electrostatice force静 电力、electric force电场力、magnetic force磁场力 • 从力的作用效果来分类有： • push推力、pull拉力、squash压力、 support支持力、 driving force动力、drag阻力、 attraction 吸引力、 repulsion排斥力、centripetal force向心力、 upthrust浮力 1. different kinds of forces Unit: Newton(N) A force of one newton will make a mass of 1kg accelerate at 1m/s2 Foce is another example of a vector. 1. different kinds of forces Weight mgWm Wg  Centre of gravity •It is the point where the whole of the weight of the object appears to act. •The center of gravity is related to the shape and mass distribution of the object. • A diagram which shows all the forces acting on a body in a certain situation is called a free-body diagram. • It represent both magnitude and direction on the diagram. • These forces application point is the centre of gravity. Friction • friction is the force that causes moving objects to slow down and finally stop. • energy...... 生活中的摩擦力 Company Logo Drag and air resistance •The force on an object that resists its motion through a fluid is called drag. •Air resistance is an upward force exerted on an object as it falls by air exercise page 21 1,4,7 More than one force How many forces are acting on the following objects? 静止的电灯 匀速下降的跳伞运动员匀速上升的潜水艇 静止的轮船 当物体静止或运动时，不止受到一个力的作用，一般要受到多个力的作用。 • 当物体受到平衡力时，原来静止的保持静止，原来运动的保持匀速直线运动 • balanced force： the force are the same size but act in opposite directions along the line. •when the forces acting on the objects are unbalanced the objects will start to move in the direction of the greater force. •There will be an unbalanced force in that direction. •Unbalanced forces acting on an object cause it to change the way it is moving. • The unbalanced forces acting on it caused it to accelerate. •We have seen that forces can make things start to move, accelerate or decelerate. •The other effect that forces can have—— they can change the shape of an object. 暂时的temporary changes of shapes •elastic or resilient: materials do not break easily and tend to return to their original shape when the forces acting on them are removed, like rubber, metal wires, springs. •brittle materials: do not change shape easily, breaking rather than stretching,like glass, chips,ceramic.陶瓷 •plastic materials: change shape permanently when forces removed, like plasticine.橡皮泥 •Springs change length when a force acts on them and return to their original length when the force is removed. But do ont overstretch them. •The 伸长量 extension of the spring is proportional to the force. This relationship is known as Hooke's Law. For force-extension graph, it is a straight line passing through the origin. •elastic limit (弹性极限): Once you have stretched a spring beyond this limit it has changed shape permanently and will not return to its original length. •Rubber bands do not obey Hooke's Law- the extension is not directly proportional to the force cauing it. force extension exercise page 22 9