TRABALHO DE UMA FORÇA - DINÂMICA - AULA 18 - Prof Marcelo Boaro

Summary

In this engaging physics class, Professor Marcelo Boaro delves into the concept of work, energy, and power. He covers key topics such as the definition of work, how to calculate it, and its relationship with force, displacement, and angles. The professor explains the principles of positive, negative, and zero work, particularly in situations where forces are perpendicular or opposite to displacement. Additionally, the class introduces the concept of work for variable forces, using graphical methods to calculate work. With a strong focus on practice and exercises, the lecture aims to build a solid foundation in dynamics for students.

Takeaways

• 😀 Work is defined as the force applied to an object multiplied by its displacement and the cosine of the angle between the force and displacement.
• 😀 The unit of work is the joule (J), which is equivalent to a newton-meter (N·m).
• 😀 If the angle between the force and displacement is zero (0°), the work done is positive and equal to force times displacement.
• 😀 If the force and displacement are perpendicular (90°), no work is done, as the cosine of 90° equals zero.
• 😀 If the angle between the force and displacement is 180° (opposite directions), the work is negative and equal to negative force times displacement.
• 😀 In cases where force is not constant, work can be calculated by the area under the graph of force vs. displacement.
• 😀 The work done by a force can be positive, negative, or zero depending on the direction of the force relative to the displacement.
• 😀 The gravitational force and normal force do not do any work in horizontal displacement because the angle between them and the direction of movement is 90°.
• 😀 The centripetal force, which points towards the center of a circular path, does not do any work because it is perpendicular to the velocity and displacement.
• 😀 Work done by a variable force is calculated as the area under the force-displacement curve on a graph, with positive or negative signs based on the position relative to the horizontal axis.

Q & A

What is the definition of work in physics?

Work is defined as the force applied to an object multiplied by the displacement of the object in the direction of the force, and it is further adjusted by the cosine of the angle between the force and the displacement.

What is the unit of work, and how is it derived?

The unit of work is the joule (J), which is derived from the newton (N) times the meter (m). In other words, 1 joule equals 1 newton-meter.

What happens when the angle between force and displacement is 0°?

When the angle between force and displacement is 0°, the cosine of 0° is 1. Therefore, the work is simply the force multiplied by the displacement.

What occurs when the angle between force and displacement is 90°?

When the angle is 90°, the cosine of 90° is 0. This means there is no work done because the force and displacement are perpendicular to each other.

What is the effect of a force applied opposite to the direction of displacement?

If the force is applied opposite to the direction of displacement, the angle between them is 180°, and the cosine of 180° is -1. In this case, the work is negative, which indicates that the force is doing negative work (e.g., slowing down an object).

What is the work done by a force when it is not constant?

When the force is not constant, the work done is not calculated by the simple formula of force times displacement. Instead, the area under the force-displacement graph is used to calculate the work.

Why is work considered a scalar quantity?

Work is a scalar quantity because it does not have direction. It only has magnitude and can be positive, negative, or zero, unlike force or displacement, which are vector quantities.

What is the work done by the weight force in horizontal displacement?

When an object moves horizontally, the work done by the weight force is zero because the weight force acts vertically, and the angle between the force and displacement is 90°, making the cosine of 90° equal to 0.

What is the work done by a centripetal force?

A centripetal force does not do work because it is always perpendicular to the velocity and displacement at each point in the motion. Since work requires a component of force along the direction of displacement, no work is done by the centripetal force.

How do you calculate the work done by a variable force?

To calculate the work done by a variable force, you need to calculate the area under the force vs. displacement graph. If the area is above the x-axis, the work is positive, and if it is below the x-axis, the work is negative.