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This is the note of course BNP12603 Physics for Engineering Technology
Typology: Lecture notes
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CHAPTER 3 FORCE THE LAWS OF MOTION
object within the boxed area. Some agent in the environment external to the boxed area exerts a force on the object.
Class of forces are called : contact forces - physical contact and field forces - non physical contact_._ The only known fundamental forces in nature are all field forces: (1) gravitational forces between objects, (2) electromagnetic forces between electric charges, (3) strong forces between subatomic particles, and (4) weak forces that arise in certain radioactive decay processes.
scale.
Newton’s first law of motion, sometimes called the law of inertia, defines a special set of reference frames called inertial frames. If an object does not interact with other objects, it is possible to identify a reference frame in which the object has zero acceleration. Such a reference frame is called an inertial frame of reference. In the absence of external forces and when viewed from an inertial reference frame, an object at rest remains at rest and an object in motion continues in motion with a constant velocity (that is, with a constant speed in a straight line). In other words, when no force acts on an object, the acceleration of the object is zero. From the first law, we conclude that any isolated object (one that does not interact with its environment) is either at rest or moving with constant velocity. The tendency of an object to resist any attempt to change its velocity is called inertia.
Conceptual Example 3.2 How Much Do You Weigh in an Elevator? You have most likely been in an elevator that accelerates upward as it moves toward a higher floor. In this case, you feel heavier. In fact, if you are standing on a bathroom scale at the time, the scale measures a force having a magnitude that is greater than your weight. Therefore, you have tactile and measured evidence that leads you to believe you are heavier in this situation. Are you heavier? No; your weight is unchanged. Your experiences are due to your being in a noninertial reference frame. To provide the acceleration upward, the floor or scale must exert on your feet an upward force that is greater in magnitude than your weight. It is this greater force you feel, which you interpret as feeling heavier. The scale reads this upward force, not your weight, and so its reading increases.
Conceptual Example 3.3 You Push Me and I’ll Push You A large man and a small boy stand facing each other on frictionless ice. They put their hands together and push against each other so that they move apart. (A) Who moves away with the higher speed? According to Newton’s third law, the force exerted by the man on the boy and the force exerted by the boy on the man are a third-law pair of forces, so they must be equal in magnitude. (A bathroom scale placed between their hands would read the same, regardless of which way it faced.) Therefore, the boy, having the smaller mass, experiences the greater acceleration. Both individuals accelerate for the same amount of time, but the greater acceleration of the boy over this time interval results in his moving away from the interaction with the higher speed. (B) Who moves farther while their hands are in contact? Because the boy has the greater acceleration and therefore the greater average velocity, he moves farther than the man during the time interval during which their hands are in contact. Figure 3.
