Introducing Forces

Chapter 4 (MHR Physics)

Forces

Essentially thought of as pushes or pulls on our everyday level

Are responsible for motion Cause objects to accelerate Vector quantity-has magnitude & direction Units are Newtons, (N) dynes, pounds

Common types of forces

Gravitational Frictional Electric Magnetic Mechanical

Key Terms

Inertia Kinematics Dynamics Mechanics Force Inertial Mass Gravitational Mass

Inertia (4.1)

Is the tendency of an object to remain in its current state of motion. The amount of an object’s inertia is directly related to its mass.

Examples you don’t want to get out of bed on a Saturday am

A granite boulder rests on a rocky beach

Inertia (4.1)

Galileo’s thought experiments led to our understanding of inertia-objects rolling down inclines will speed up, those rolling up inclines will slow down and those rolling along a horizontal surface will continue to roll. His work built on Aristotle’s and Buridan who believed respectively that constant force produced constant speed and that objects remained moving because they had “impetus” inside

The study of motion

Kinematics is the branch which describes an object’s motion in terms of displacement, velocity, and acceleration

Dynamics explains why objects move i.e respond to forces

Mechanics = Kinematics + Dynamics

Inertia (4.1)

Inertial mass is the property of matter causing it to resist a change in its motion

Gravitational mass is the property of matter that determines the strength of the gravitational force. The more mass, the stronger the gravitational attraction for that mass.

Einstein showed that these two are essentially the same.

Common Forces (4.2)

Forces which involve direct physical contact are contact forces e.g. pushing, pulling, and frictional

Forces which do not involve direct physical contact but which act over a distance are called non contact forces e.g. electric, magnetic, gravitational

Common Forces (4.2)

Weight (gravitational force) is the force of gravity acting on a mass. We can jump higher on the moon than on earth because the moon’s gravity is about 1/6 th that of the earth.

Fg = mg where F is force in N, m is mass in kg and g is acceleration due to gravity

Since g varies with location on the earth, your weight (not mass) will vary by location

Common Forces (4.2) Friction is a force which opposes motion. It is

measured in N and is a contact force Two kinds of friction: Static friction involves no net motion Kinetic friction involves movement The coefficient of friction is a pure number which

indicates how much friction exists between surfaces.

The symbol is: μ The formula for friction is: μ = Ff /FN or Ff = μFN

Friction

Depends on nature of the surfaces-rougher surfaces have more friction than smoother ones

Actually an electromagnetic interaction at the atomic level. Surface atoms of objects in contact interact and form attractive forces leading to a “stick and slip” process when one object slides over another e.g. marker on whiteboard, chalk on chalkboard, squealing tires

Friction

The coefficient of friction is a “stickiness” value for specific combinations of surfaces

Friction also depends on the magnitude of the forces pressing the surfaces together.

The force that a surface exerts back on an object is called the normal force. It acts perpendicular to the surface.

Friction

Force of friction is independent of velocity Force of friction is independent of the area

of contact (only if mass is evenly distributed). The surfaces must be flat.

Certain plastics and rubbers have natural properties which do not agree with the standard model of friction e.g. “ice-gripping” tires, adhesive tape

Free Body Diagrams (FBD)

Represent all the forces acting on an object.

Only contains forces and angles at which they act.

Draw them roughly to scale so a bigger force will have a bigger line.