4 Newton s Second Law of Motion

Transcription

1 4 Newton s Second Law of Motion orce causes acceleration riction Mass and weight Mass resists acceleration Newton s second law of motion ree all and Non-ree all Dr. Jie Zou PHY

2 orce Causes Acceleration Any object that accelerates is acted on by a push or pull-a force of some kind. Kick the ball and it accelerates. Acceleration is caused by force. Acceleration produced is directly proportional to the net force, or acceleration ~ net force orce of hand accelerates the brick. Twice as much force produces twice as much acceleration. Dr. Jie Zou PHY

3 riction When surfaces slide or tend to slide over one another, a force of friction acts. No friction exists on a crate that sits at rest on a flat floor. The direction of the friction force is always in a direction opposing motion. When you apply a force to an object, a force of friction usually reduces the net force and reduces the resulting acceleration. An object sliding down an incline experiences friction directed up the incline. f v Dr. Jie Zou PHY

4 An Example-riction Example: A jumbo jet cruises at a constant velocity of 1000 km/h when the thrusting force of its engines is a constant 100,000 N. What is the acceleration of the jet? What is the force of air resistance. Answer: Constant velocity acceleration is zero net force is zero the air resistance is balanced out by the thrusting force. So, force of air resistance on the jet = 100,000 N. Dr. Jie Zou PHY

5 Mass and Weight The acceleration produced to an object depends on applied forces, friction forces, and its inertia (the property of things to resist changes in motion). Mass: The quantity of matter in an object; how much inertia an object possesses depends on the amount of matter in the object, or its mass. Massis a measure of the inertia of a material object; the greater the mass of an object, the greater its inertia. Think about why it is more difficult to slow down or stop a heavily loaded truck than a Toyota Corolla. Dr. Jie Zou PHY

6 Weight Weight: The force upon an object due to gravity (gravitational attraction to the Earth). Mass and weight are directly proportional to one another, G = mg. G: gravity (weight), m: mass, g: the constant of proportionality due to gravity, also the acceleration of gravity (g = 9.8 m/s 2 10 m/s 2 on Earth). Units of weight: newtons (N), pounds Units of mass: kilogram (kg), gram (g) Think about how much you will weigh if you are on the surface of the Moon? And how much your mass will be? Dr. Jie Zou PHY

7 Mass Resists Acceleration The amount of acceleration depends not only on the force, but also on the mass being pushed. or a given force, the acceleration produced is inversely proportional to the mass, or Acceleration ~ 1/mass orce of hand accelerates the brick The same force accelerates 2 bricks ½ as much. Dr. Jie Zou PHY

8 Newton s Second Law Newton s second law gives the relationship of acceleration to force and inertia. Newton s second law: The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object. a = net /m; a: acceleration produced by the net force (m/s 2 ), net : the net force (N), m: the mass of the object (kg). The acceleration of an object is always in the direction of the net force. The force is applied in the opposite direction of the motion, speed is decreased. v The force is applied in the direction of the motion, speed is increased. v Applied at a right angle, it will deflect the object. v Dr. Jie Zou PHY

9 ree all Galileo could not explain why objects of various masses fall with equal accelerations. Newton s second law provides the explanation: A falling object accelerates toward the Earth because of the gravitational force of attraction (gravity) between the object and the Earth. The acceleration due to gravity is a constant g, for the same locality. m /m = g 2m 2/2m = g The acceleration of free fall is independent of an object s mass. Question: In a vacuum, a coin and a feather fall equally, side by side, would it be correct to say that equal force of gravity act on both the coin and the feather when in a vacuum? Dr. Jie Zou PHY

10 Non-ree all In the presence of air resistance, the net force on a falling object is less than the gravity-it is the gravity minus air drag, the force arising from air resistance. Air drag is opposing the direction of motion and decreases the net force. Thus, a < g. Dr. Jie Zou PHY

11 Homework Ch. 4, p , Exercises: #14, 29, 41, 49. Ch. 4, p. 73, Problems: #2, 6. The above homework problems are assigned from the 10 th edition of the textbook by Hewitt. Dr. Jie Zou PHY