Chapter 4 - Objectives and Summary

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.

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 F_W = mg.
When an object moves through a fluid, it experiences a frictional or drag force (F_f).
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. F_f = µF_N.