Physics 10. Lecture 4A. "It is good to have an end to journey towards, but it is the journey that matters in the end." -- Ursula K.

Transcription

1 Physics 10 Lecture 4A "It is good to have an end to journey towards, but it is the journey that matters in the end." -- Ursula K. LeGuin

2 Mass and Weight Mass is a measure of how much inertia an object. Mass also is the quantity of matter an object has. Weight, on the other hand, is the force of gravity on an object. What is the difference between mass and weight? Weight depends on gravity, while mass does not depend on gravity. So if we take a ball to deep space, its mass remains the same while weight becomes zero.

3 Mass and Weight Mass and weight are directly proportional. If you double the mass you double the weight. Mass is measured in kilograms (kg). Weight is measured in Newtons (N). On Earth s surface, 1kg of mass is equal to 9.8N of force. Or in English units, 1kg of mass is equal to 2.2 pounds of force.

4 Friction When an object attempts to move over a surface, there will be a resistance known as friction. When an object is at rest and you are attempting to move it, the resistance is known as static friction, f s. Static friction will increase as the applied force increases until it reaches a maximum static friction.

5 Friction The static force will be in the direction opposite the applied force that is attempting the motion. Before you can get an object to move you must overcome the maximum static friction. Once you have an object moving over a surface, the friction will become kinetic friction, f k. Kinetic friction is less than the maximum static friction for a given surface. If an object moves through the air, there will be resistance due to the air.

6 Newton s 2nd Law Newton s Second Law of Motion: When a net external force (ΣF) acts on a mass m, an acceleration, a, will result per the following formula: or The unit of net force is the Newton = (kg)(m/s 2 ). Newton s Second Law of Motion basically states how an object will accelerate based on all of the external (or outside) forces that are upon it.

7 Newton s 2nd Law Newton s Second Law is the bridge between forces and motion. An external net force will lead to an acceleration on an object. Please note that Newton s Second Law is not concerned with any one particular force, but instead with the vector sum of ALL forces on an object. Newton s Second Law will also exhibit independence in horizontal and vertical directions. Forces in the x-direction will not affect forces in the perpendicular y-direction.

8 Perpendicular Directions BIG PHYSICS CONCEPT!!!!!! Horizontal and vertical motion are independent of one another. Neither motion affects the other. This means that if there is an acceleration in the y-direction; it will not affect motion in the x-direction at all. This also means that forces in the y-direction will not affect motion in the x-direction at all.

9 Vertical Motion Example A hunter tries to shoot a rare monkey hanging from a tree with a dart gun. The hunter has the monkey in his sights. But the monkey notices the hunter and drops from the branch exactly when he hears the hunter blow. Will the hunter hit the monkey or miss the target? Clicker Question 4A-2 A) The dart hit the monkey. B) The dart miss above the monkey. C) The dart miss below the monkey.

10 Projectile Motion Answer Monkey and the dart fell at the same rate, under the same influence of gravity. So the dart will hit the monkey.

11 Free Fall As we mentioned last time, an object that falls with only the force of gravity acting upon it, is said to be in free fall. Is this object in mechanical equilibrium? No, if only one force acts on an object it isn t in mechanical equilibrium. All objects that undergo free fall will accelerate toward the Earth at g (10 m/s 2 ). If you double the mass, force will double as well.

12 Air Resistance Unfortunately in everyday life, air resistance will complicate matters. Air resistance acts upon objects that are falling so that, eventually, the force will reach the same magnitude of the force of gravity. Air resistance depends on two things: 1. the shape of objects. 2. the speed of objects. An object with a large surface area will have large air resistance. An object that is moving fast will have large air resistance.

13 Air Resistance After a certain speed, air resistance will exactly balance out with gravity leading to mechanical equilibrium. At that point the object will no longer accelerate and it will attain terminal velocity. Terminal velocity is the final velocity of the object (a = 0).

14 Clicker Question 4A-3 Which encounters the greater force of air resistance: a falling elephant or a falling feather? A) The falling elephant. B) The falling feather. C) They both will encounter an equal force of air resistance as they fall.

15 For Next Time (FNT) Read Chapter 5. Finish up the homework for Chapter 3 and start the homework for Chapter 4.