长江大学大学物理双语教学第二章.pptVIP

Measuring the weight of a body: ★The weight of a body must be measured when the body is not accelerating vertically relative to the ground. ● apparent weight : elevator (lift) cab ★The weight of a body is not the mass of the body. 3.Frictional Force（摩擦力） ⑴ Kinetic friction between two bodies ⑵ Static friction force exits between two surfaces at rest. If external force is increased, static frictional force will automatically increase until the external force is sufficiently large to overcome （克服）the frictional force and to accelerate the body. The maximum of fs (the breakaway（突破） is is coefficient(系数） of static friction which depends on the materials and the contacting surface(接触面）. In general： ＞ It exerts on the body which moves through a fluid（流体）, such as air or water…. The experiments have shown（证明，表明） that the resistance（阻力） acted on the body by the surrounding（周围） fluid is given approximately(近似地） by at low velocity at great velocity ⑶ Fluid friction ( Drag拖 force ) Skydiving Group Skydiving 4. Newton’s Law of Universal Gravitation Every particle of matter in the universe attracts（吸引） every other particles with a force which is directly proportional to（正比于） the product（乘积） of the masses of the particles and inversely proportional to（反比于） the square（平方） of the distance between them: m1 m2 or in vector form Acceleration due to gravity on the ground 地球表面的重力加速度 Acceleration due to gravity is a vector, its direction is towards the center of earth. Acceleration due to gravity varies with altitude重力加速度g随高度的变化 §2-3 The Fundamental Forces of Nature 1. Gravitational Force: 2. Electromagnetic force 3. Strong nuclear force:强力 It holds the nucleons(核子) together. It’s the strongest of all. 4. Weak nuclear force: 弱力 Short-range nuclear force that tends to produce instability in certain nuclei. Most radioactive decay reacti