《牛顿第定律》

1.What is a force?2.What can forces do?Newtons First Law of Motion 牛顿第一定律牛顿第一定律牛顿第一定律v一切物体在没有受到外力作用的时候，一切物体在没有受到外力作用的时候，总保持匀速直线运动状态或静止状态。总保持匀速直线运动状态或静止状态。一切物体没有受到外力作用总保持An object maintains its state of rest or constant velocity motion unless it is acted on by an external unbalanced force.Newtons First Law of Motion 力和运动的关系：力和运动的关系：力不是维持物体运动的原因，力不是维持物体运动的原因，而是而是改变物体运动状态改变物体运动状态的原因。的原因。What can forces do?一切物体都具有保持静止一切物体都具有保持静止(at rest(at rest )或匀速或匀速(constant velocity)(constant velocity)直线运动直线运动状态的性质。状态的性质。牛顿第一定律表明：牛顿第一定律表明：物体具有保持原来匀速直物体具有保持原来匀速直线运运动或静止状或静止状态的性的性质，我，我们把把这种性种性质叫做叫做惯性性 inertia。牛顿第一定律也称惯性定律牛顿第一定律也称惯性定律law of inertia情境模拟情境模拟InertiavMore massive objects are harder to move than less massive objectsvAll matter has mass,inertia,gravityvMore mass=more gravity=more inertiavLess mass=less gravity=less inertia 牛顿第二定律牛顿第二定律Newtons Second Law of Motion Explain how change in motion relates to unbalanced forcesNewtons Second Law of Motion Balanced ForcesUnbalanced ForcesBalanced ForcesvWhen all forces equal each other,we have balanced forces.vMoving objects wont accelerate or deceleratevMoving objects wont change directionvNot moving objects will remain at restUnbalanced ForcesvWhen one force is greater than the other forcesvA moving object can accelerate or deceleratevMoving objects can change directionvMoving objects can stop movingvNot moving objects can start to move作用在一个物体上的加速度与作用在物作用在一个物体上的加速度与作用在物体上的力成正比，与物体的质量成反比。体上的力成正比，与物体的质量成反比。Newtons Second Law of Motion The acceleration of an object varies in direct proportion to the external unbalanced force applied to it and inversely proportional to its mass.Mathematically:a F and a 1/m An unbalanced force of 48 N west is applied to a 6.0 kg cart.Calculate the carts accelerationA car of mass 2000 kg decelerates from 30 m s1 to rest in a distance of 100m.Calculate the retarding force required to stop the car.