# Chapter 4 - Objectives and Summary

### for the OUP text Senior Physics - Concepts in Context by Walding, Rapkins and Rossiter

LEARNING OBJECTIVES
Knowledge & Understanding

• Explain what a force is. Describe everyday situations where forces act.
• Distinguish between gravitational mass, inertial mass and weight.
• Calculate the density of substances from their mass and volume.
• Distinguish between balanced and unbalanced forces.
• State Newton's three laws of motion and use them to solve problems (F = ma, W = mg).
• Describe experiments used to illustrate Newton's laws of motion.
• Identify examples of action–reaction forces.
• Explain what friction is and describe its effects. Solve problems involving friction.
• Explain the notion of drag force as a cause of terminal velocity .
• Describe the concept of torque and how it differs to force.

Scientific Techniques

• Determine how the acceleration of an object depends on its mass and the unbalanced force acting on it.
• Draw free-body and vector diagrams of applied forces.
• Examine graphically the result of forces on objects.
• Identify sources of errors in Newton’s Law experiments.
• Design simple experiments to test the ways of increasing or decreasing friction.
• Relate a knowledge of forces to car accidents and transport safety issues.

COMPLEX REASONING PROCESSES

• Solve problems involving Newton’s laws and frictional forces in more complex situations.
• Distinguish between relevant and irrelevant information in situations involving forces.
• Apply a knowledge of forces and torque to various sporting and mechanical situations.

### CHAPTER 4 SUMMARY

• A force is a push or a pull exerted on a body.
• Newton's First Law states: An object maintains its state of rest or constant velocity motion unless it is acted on by an external unbalanced force.
• Inertia is the tendency of an object to resist a change to its motion.
• Newton's Second Law of motion: The acceleration of an object varies directly as the external unbalanced force applied to it and inversely proportional to its mass; i.e. F = ma.
• Newton's Third Law of motion states: To every action there is an equal and opposite reaction; or: if a body A exerts a force on another body B, then body B exerts an equal and opposite force on body A.
• The force of gravity on an object is the object's weight.
• Mass is a measure of the amount of substance in an object. It is related to weight by FW = mg.
• When an object moves through a fluid, it experiences a frictional or drag force (Ff).
• When a body falls freely under gravity, it begins to accelerate at g (10 m s-2) but because of air resistance this value slowly decreases until it reaches zero. At this speed the object has reached terminal velocity.
• Normal force is the force exerted on a body by a surface against which it is pressed. It is always perpendicular to the surface.
• The types of friction are limiting, sliding and rolling.
• The ratio of frictional force to the normal contact force is called the coefficient of friction (µ); i.e. Ff = µFN.