高一物理课件：牛顿第二定律(英文版).ppt

One day ,Stephen Su was driving a motorcycle on the street. Meanwhile, David Sun was driving a trunk at a fast speed. Stephen Su did not stop in time as soon as the traffic light turned red. David sun could not control the van and collided with Stephen. The remaining question is ,even though Stephen slammed on the brake, the accident happened anyway. What caused this tragedy? tendency resist inertia motion absence According to Newtons first law, an object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force. Newtons first law of motion牛顿第 一定律: Every body continues in its state of uniform motion or rest unless acted up on by an external force Application of the Newtons first law Safety belts Any passengers in the car will also be deccelerated to rest if they are strapped to the car by seat belts. Being strapped tightly to the car, the passengers share the same state of motion as the car. As the car accelerates, the passengers accelerate with it; as the car decelerates, the passengers decelerate with it; and as the car maintains a constant speed, the passengers maintain a constant speed as well. If two objects, a uniform disk and a uniform sphere, have the same moment of inertia about their axes of rotation and the same angular velocity, then the disk has the larger rotational kinetic energy. (T) True (F) False If two ends of a rope are pulled with forces of equal magnitude and opposite direction, the tension at the center of the rope must be zero True False A disk and a hoop of the same mass M and radius R roll without slipping across a horizontal floor. Both the disk and the hoop are moving with velocity v when the floor starts to slope upward. Which statement is correct? (B) (a) The disc makes it to a greater height than the hoop. (b) The hoop makes it to a greater height than the disk. (c) Both the disk and the hoop make it to the same height. Newtons Second Law of motion pertains to the behavior of objects for which all existing forces are not balanced. The second law states that the acceleration of an object is dependent upon two variables - the net force acting upon the object and the mass of the object. The acceleration of an object depends directly upon the net force acting upon the object, and inversely upon the mass of the object. Newtons Second Law tells us that rate of increase (or decrease) in the speed of something which is moving is proportional to the force acting on it. Imagine that you are riding a bicycle along a perfectly smooth and level road and you decide to stop pedaling. If there is a strong wind pushing against you, you will stop completely in a shorter time than you would if the wind was light. Now that we know Newtons Laws of Motion, how do we apply them? How can they let us predict the motion of an object if we know all the forces acting upon it? How can they let us predict the forces on an object if we know its motion? FORCE = MASS times ACCELERATION or Kobes car, which weighs 1,000 kg, is out of gas. Kobe is trying to push the car to a gas station, and he makes the car go 0.05 m/s2. Using Newtons Second Law, you can compute how much force Kobe is applying to the car. Answer = 50 newtons Example 1: Consider a crate being pulled along a horizontal, frictionless floor. A rope is tied around it and a man pulls on the rope with a force of T. T is the tension in the rope. What happens to the crate? These forces are shown on the “free body diagram“ above. We have drawn in all the forces acting on the object. The net force is the vector sum of these forces. Fnet = F = T + n + w Fnet, x=F x=Tx+n x+ w x Fnet,y =Fy=Ty+ ny+ w y n = w Example 2: This particular example is stated in terms of weighing a fish in an accelerating elevator. It is also fun to think of weighing yourself in an accelerating elevator. When does an elevator accelerate upwards? When does an elevator accelerate downwards? The forces acting on the fish are shown in the free-body diagram. T is the tension supplied by the scale. This is the value the scale reads. We may call it the apparent weight of the fish. The net force on the fish is Fnet = T - mg The net force is (always!) equal to the mass times the acceleration. This fish is moving along with the elevator. In this diagram we have taken the acceleration to be up so it is positive. Fnet = T - m g = m a T = m g+ m a T = m (g + a) While the elevator accelerates upward, the apparent weight of the fish is greater than its true weight, mg. The forces on the fish are again shown in the free-body diagram Fnet = T - mg The net force is (always!) equal to the mass times the acceleration. This fish is moving along with the elevator. Now the acceleration to be up so it is negative. Fnet = T - m g = m ( - a ) T = m g - m a T = m (g - a) While the elevator accelerates downward, the apparent weight of the fish is less than its true weight, mg. 1. Who was the scientist who gave us the Laws of Motion? Answer: Sir Isaac Newton 2. How many Laws of Motion are there? Answer: three 3. What is another name for the first law of motion? Answer: Law of Inertia 4. Which law explains why we need to wear seat belts? Answer: First Law of Motion 5. Which law says that force is equal to mass times acceleration (F=MA)? Answer: Second Law of Motion 6. Which law says that heavier objects require more force than lighter objects to move or acce