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Chapter 1: Problem 30

Which of the following forces does not do work in its given situation? (A) Normal force as a person goes up in an elevator (B) Frictional force as a box slides down a ramp (C) Centripetal force as a car drives around a circular track (D) Electrical force as a positive charge moves toward a negative charge

Short Answer

Expert verified
The forces in Option A (normal force as a person goes up in an elevator) and Option C (The centripetal force as a car drives around a circular track) do not do work in their given situations.

Step by step solution

01

Analyze Option A

The normal force is perpendicular to the direction of motion when a person goes up in an elevator. Hence, it does no work.
02

Analyze Option B

The frictional force opposes the motion of the box sliding down the ramp. It is along the direction of motion and therefore it does work.
03

Analyze Option C

Centripetal force is always directed towards the center of the circular path of the car. This force is perpendicular to the direction of motion at every point on the track, hence it does no work.
04

Analyze Option D

The electrical force on a positive charge moving towards a negative charge is along the direction of motion. Therefore, it does work.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Normal Force
Normal force is the support force exerted upon an object that is in contact with another stable object. When you stand on a floor, it is the force the floor exerts upward on you to support your weight.

The key idea about normal force is that it acts perpendicular to the surface. This means that if you are moving along the surface, the normal force doesn't contribute to the work done because its direction is not along the direction of motion.

  • Work is done when a force applied to an object moves that object.
  • For work to occur, there must be a component of force in the direction of motion.
  • Since the normal force is perpendicular, it does not cause any movement along the surface, hence no work is performed.
This is why, in the context of the problem, the normal force does not do any work as a person rides an elevator.
Centripetal Force
Centripetal force is the force that keeps an object moving in a circular path. It is always directed towards the center of the circle or curve. While it is essential for circular motion, its position perpendicular to the motion leads to zero work done by this force itself.

  • Centripetal force maintains the circular motion by constantly changing the direction of the velocity of the object.
  • The magnitude of the velocity remains constant while only the direction changes.
  • Work done is defined as the force applied along the direction of displacement.
Because the centripetal force acts orthogonally to the motion at any instant, it doesn't alter the speed of the object; it merely shifts the direction. Thus, in the context of the car on a circular track, it doesn鈥檛 perform work.
Frictional Force
Frictional force is the resistance force that one surface or object encounters when moving over another. It is directed opposite to the motion of the object and is involved in numerous activities in our daily lives.

Frictional forces, unlike normal or centripetal forces, often do perform work because of their alignment with, or opposition to, the motion of an object.

  • Frictional force does work when an object moves against friction, as it resists motion.
  • This force converts kinetic energy into thermal energy.
  • More of a dissipative force, friction reduces the mechanical energy of a system.
When a box slides down a ramp, for example, friction acts against the direction of motion, effectively performing work by slowing the box down.
Electrical Force
Electrical force arises due to the electric charges in materials. It is a fundamental force responsible for interactions between charged particles.

When charges move toward or away from each other, work is done, typically in line with the direction of the electric force.

  • Electrical forces can either attract or repel, depending on the nature of the charges involved.
  • The direction of force is along the electric field lines, meaning it has a component in the direction of motion of the charges, producing work.
  • This work can alter the potential and kinetic energies of the charges.
In the problem scenario, as a positive charge moves towards a negative charge, the electrical force does work, converting potential energy into kinetic energy.

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Most popular questions from this chapter

A block of mass \(M\) is at rest on a table. It is connected by a string and pulley system to a block of mass \(m\) hanging off the edge of the table. Assume the hanging mass is heavy enough to make the resting block move. If the acceleration of the system and the masses of the blocks are known, which of the following could NOT be calculated? (A) Net force on each block (B) Tension in the string (C) Coefficient of kinetic friction between the table and the block of (D) The speed of the block of mass \(M\) when it reaches the edge of the table

An object is resting on a platform that rotates at a constant speed. At first, it is a distance of half the platform鈥檚 radius from the center. If the object is moved to the edge of the platform, what happens to the centripetal force that it experiences? Assume the platform continues rotating at the same speed. (A) Increases by a factor of 4 (B) Increases by a factor of 2 (C) Decreases by a factor of 2 (D) Decreases by a factor of 4

Directions: Questions 1, 2, and 3 are short free-response questions that require about 13 minutes to answer and are worth 8 points. Questions 4 and 5 are long free-response questions that require about 25 minutes each to answer and are worth 13 points each. Show your work for each part in the space provided after that part. A car of known mass \(m_{1}\) will collide with a second car of known mass \(m_{2}\). The collision will be head on, and both cars will only move linearly both before and after the collision. In a clear, coherent, paragraph-length response, explain a method for determining whether the collision is perfectly elastic, perfectly inelastic, or neither. If the collision is perfectly inelastic, include at least one possible cause of energy loss.

If a ball is rolling down an inclined plane without slipping, which force is responsible for causing its rotation? (A) Normal force (B) Gravity (C) Kinetic friction (D) Static friction

A block of known mass M is connected to a horizontal spring that is sliding along a flat, frictionless surface. There is an additional block of known mass m resting on top of the first block. Which of the following quantities would NOT be needed to determine if the top block will slide off the bottom block? (A) The maximum coefficient of static friction between the blocks (B) The amplitude of the system鈥檚 motion (C) The spring constant (D) The average speed of the blocks
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