Physics - 0625 CIE

Forces

Overview

Push, pull, stretch, squeeze, turn, stop: forces are behind every change in how an object moves or what shape it is. Understanding forces is the gateway to the whole of mechanics, and it is where Newton's famous ideas about motion begin.

In this lesson you will see how a force can change an object's size and shape, read a load-extension graph and find a spring constant, add forces along a line to get the resultant, and use \(F=ma\) to link force, mass and acceleration. You will also meet the idea that a force at right angles to motion makes an object travel in a circle.

Objectives

Know that forces may produce changes in the size and shape of an object
Sketch, plot and interpret load–extension graphs for an elastic solid and describe the associated experimental procedures
Determine the resultant of two or more forces acting along the same straight line
Know that an object either remains at rest or continues in a straight line at constant speed unless acted on by a resultant force
State that a resultant force may change the velocity of an object by changing its direction of motion or its speed
[Supplement] Define the spring constant as force per unit extension; recall and use the equation F k = x
[Supplement] Define and use the term ‘limit of proportionality’ for a load–extension graph and identify this point on the graph (an understanding of the elastic limit is not required)
[Supplement] Recall and use the equation F = ma and know that the force and the acceleration are in the same direction
[Supplement] Describe, qualitatively, motion in a circular path due to a force perpendicular to the motion as: (a) speed increases if force increases, with mass and radius constant (b) radius decreases if force increases, with mass and speed constant (c) an increased mass requires an increased force to keep speed and radius constant mv 2 ( F = is not required) r

Lesson Note

A resultant force is the single cause of every change in motion in the universe, from a dropped ball to a planet in orbit. Master how forces add up, how springs respond to them, and how \(F=ma\) ties force to acceleration, and the rest of mechanics falls into place.

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Lesson Evaluation

Congratulations on completing the lesson on Forces. Now that youve explored the key concepts and ideas, its time to put your knowledge to the test. This section offers a variety of practice questions designed to reinforce your understanding and help you gauge your grasp of the material.

You will encounter a mix of question types, including multiple-choice questions, short answer questions, and essay questions. Each question is thoughtfully crafted to assess different aspects of your knowledge and critical thinking skills.

Use this evaluation section as an opportunity to reinforce your understanding of the topic and to identify any areas where you may need additional study. Don't be discouraged by any challenges you encounter; instead, view them as opportunities for growth and improvement.

A load of 10 N stretches a spring by 0.05 m. The spring constant is: A. 0.5 N/m B. 2 N/m C. 50 N/m D. 200 N/m Answer: D
A resultant force of 20 N acts on a 4 kg mass. The acceleration is: A. 0.2 m/s^2 B. 5 m/s^2 C. 24 m/s^2 D. 80 m/s^2 Answer: B
An object moves at constant velocity. The resultant force on it is: A. equal to its weight B. zero C. equal to friction times two D. increasing Answer: B
The limit of proportionality on a load-extension graph is where: A. the spring breaks B. extension is zero C. the line stops being straight D. the load is greatest Answer: C
A force acting at right angles to the motion of an object causes it to: A. speed up B. slow down C. move in a circle D. stop Answer: C