College Physics 1: Lecture 17 - Weight, Apparent Weight, and Normal Force

This paragraph introduces the concepts of weight and apparent weight, emphasizing the distinction between mass and weight. Mass, measured in kilograms, is defined as a scalar quantity that describes an object's inertia or resistance to acceleration. Weight, on the other hand, is a vector force measured in newtons, representing the gravitational pull exerted by a massive body like Earth. The paragraph also explains how an object's weight can vary depending on the gravitational force of different planets, using the example of an astronaut's experience on the Moon to illustrate the concept of weightlessness. The relationship between mass and weight is established through Newton's second law, highlighting that weight (W) is the product of mass (m) and the acceleration due to gravity (g).

The second paragraph delves into the idea of apparent weight, which is the weight that an individual feels due to contact forces. It explains that we do not directly sense gravity but rather the contact forces that support us, such as the normal force from a chair or the floor. The concept is further explored through the example of a roller coaster, where passengers may feel lighter or heavier than their actual weight due to the accelerations experienced. The paragraph also uses the example of an elevator to illustrate how apparent weight changes with acceleration, and introduces the formula for apparent weight (W_apparent = W_true + ma), where 'm' is mass, 'a' is acceleration, and 'W_true' is the true weight of the object.

This paragraph presents a scenario involving a person named Anje in an elevator to discuss the concept of apparent weight in more detail. It describes how the scale in the elevator would read 750 newtons as the elevator slows to a stop, indicating Anje's apparent weight. The paragraph uses this example to explain the difference between actual weight and apparent weight, and how the latter can be greater than the former due to the normal force being larger than the gravitational force acting on the person. The explanation involves a free body diagram and Newton's laws, leading to the conclusion that the elevator must have been moving downward when the scale reading was taken.

The fourth paragraph discusses the experience of free fall and weightlessness. It explains that when there is no support, such as in a free-falling elevator, individuals do not feel their weight due to the absence of a normal force. The paragraph clarifies that weightlessness does not mean the absence of weight; rather, it is the lack of a supporting force that results in the sensation of weightlessness. It also mentions a plane that can simulate the experience of weightlessness by performing a maneuver that causes passengers to feel as if they are floating in space.

This paragraph introduces the normal force, which is the force exerted by a surface that is perpendicular to the object in contact with it. It explains that the normal force adjusts to balance the force applied to the surface, increasing in strength as the applied force increases. The paragraph also discusses the importance of considering the normal force when dealing with inclined surfaces, as it requires the use of trigonometry to resolve the weight vector into components parallel and perpendicular to the surface. An example of a skier sliding down a slope is used to illustrate how to calculate the skier's acceleration using Newton's second law and the components of the weight vector.

The sixth paragraph continues the discussion on the skier's acceleration, focusing on the application of Newton's second law in the context of inclined surfaces. It describes how to set up the problem by drawing a free body diagram and resolving the weight vector into its components along and perpendicular to the slope. The paragraph explains that the skier's acceleration is due to the component of the weight acting parallel to the slope and that the acceleration can be calculated by canceling out the mass from the equation involving the normal force, weight, and acceleration. The example concludes with the calculation of the skier's acceleration as 4.4 meters per second squared.

The seventh paragraph presents a series of questions related to apparent weight in the context of elevator motion. It explains that when an elevator accelerates upward, the normal force, and thus the apparent weight, is greater than the actual weight. Conversely, when an elevator is in free fall, there is no normal force, and the apparent weight is zero, resulting in a sensation of weightlessness. The paragraph emphasizes the importance of understanding the relationship between acceleration, the normal force, and apparent weight, and how these concepts apply to different scenarios, such as a box in an accelerating elevator or a person in an elevator with a broken cable.

The final paragraph concludes the lecture by summarizing the key points discussed, including the concepts of weight, apparent weight, and the normal force. It also mentions that future lectures will cover other forces such as friction and drag. The paragraph ends with a note of thanks to the viewers for watching and a wish for a great day, highlighting the educational nature of the content and the intention to continue exploring physics concepts in subsequent sessions.