FORCES AND

LAWS OF

MOTION

FORCE

EXAMPLES OF FORCES:

Close range Long RangePulling the handle of

the doorPushing a stroller

Hitting a tennis ball with a racquet

Bouncing a soccer ball

Force of gravity

Attraction of negative and positive particles

Magnetism

FREE BODY DIAGRAM

FREE BODY DIAGRAM

Only forces ON THE OBJECT matter

Aristotle

384 – 322 BC Believed that no motion

is possible without a force.

As soon as a force stops,motion stops.

1564 - 1642

Galileo Galilei

1564 - 1642It is as natural for an object

to keep moving once it’s

in motion as it is to remain

at rest once it’s at restA force will accelerate

an object

1643 - 1727ISAAC NEWTON

Sir Isaac Newton1643-1727

Summarized motion in his three laws.

Famous for his work on gravity, light and

chemistry. Invented calculus.

Which FBD represents the following:a) a hockey puck sliding on frictionless iceb) a hockey puck sliding on real ice

What would the third FBD represent?

An object continues in a state of restor in a state of motion at a constant speed along a straight line, unless compelled to change that state by a net force.

The net force is the vector sum of allof the forces acting on an object.

Newton’s First Law

Inertia is the natural tendency of an object to remain at rest or in motion at a constant speed along a straight line (constant velocity).

The mass of an object is a quantitative measure of inertia.

SI Unit of Mass: kilogram (kg)

An object is resting on a flat surface

An object is being pulled on a flat surface, but is not moving

The very same diagram would illustrate an object that is already in motion and keeps moving at a constant speed.

There are two forces applied to the object: force of gravity, directed downward, and the force from the surface. They are equal in magnitude and opposite in direction, so

Force of pull is equal to force of friction.

NET FORCE

NEWTON’S II LAW

When a net external force acts on an object with

mass m, the acceleration that results is directly

proportional to the net force and has a magnitude that is

inversely proportional to the mass. The direction of the

acceleration is the same as the direction of the force.

[NEWTON]= [N]

[N] = [kg ·m/s2]

A net force of 1N exerted on an object with a

mass of 1 kg will result in the object moving

with an acceleration of 1 m/s2

EXAMPLE

Space-shuttle astronauts experience accelerations of about

35 m/s2 during takeoff. What force does a 75 kg astronaut

experience during an acceleration of this magnitude?

EXAMPLE

A block with a mass of 5 kg is being pulled across a

tabletop by a force of 10 N applied by a string tied to the

front end of the block. The table exerts a 2-N frictional force

on the block. What is the acceleration of the block?

NEWTON’S THIRD LAW

When one body exerts a force on a second body, the

second body exerts an oppositely directed force of equal

magnitude on the first body.

Every action has an equal opposite reaction.

3R D LAW: ACTION-REACTION

3RD LAW: ACTION-REACTION

3RD LAW: ACTION-REACTION

MULE - CARTIf a mule is pulling on a cart with a force F and there

is a counter force –F, according to Newton’s third law,

why does the cart start moving ?

EDUCATED APPRENTICE

COMPARE FORCES ON THE OBJECTNewton’s III law is about TWO objects and TWO forces that the TWO exert on EACH OTHER.

Newton’s I (and II) law is about ONE object and ALL the forces exerted ON IT ALONE.

So, when working on problems and drawing FBD, isolate appropriate forces to determine the nature of motion.

GRAVITY

Another example of N’s III law: two bodies, two equal and opposite forces.

WEIGHT OF AN OBJECT

The weight of an object on or above earth is the

gravitational force that the earth exerts on the object. On or

above another astronomical body, the weight is the

gravitational force exerted on the object by that body.

Weight = mg, where m- mass of the body,

g – acceleration due to gravity (-9.8 m/s2)

Relation Between Mass and Weight

MASS VS. WEIGHT

Mass depends on the amount of matter. Mass can change if the amount of matter changes.Weight depends on the strength of gravitational pull and it changes from planet to planet. (g is different on different planets)

WEIGHT CAN CHANGE:

WEIGHTLESSNESS

NORMAL FORCE

Normal force is the force that a surface exerts on the object, always to the surface (normal is not ‘natural!)

EXAMPLES OF NORMAL FORCE

Normal force = weight only if an object is at rest on a flat surface. Any additional force or motion with acceleration can change (increase or decrease) normal force. Normal force is the reaction of the surface to the force applied to it by the object.

EXAMPLE

A 10.0-kg box is resting on a 20.0o incline. Determine the

normal force acting on the incline.

Find the force of friction acting on the incline.

If the incline is frictionless, find the acceleration of the box.

FORCE OF FRICTION

When an object is in contact with a surface there is a forceacting on that object. The component of this force that is parallel to the surface is called the frictional force.

STATIC FRICTION

When the two surfaces are not sliding across one another the friction is called static friction.

Static friction is a ‘lazy’ force: it will only be as large as it needs to.

Static friction has a maximum value. If pull >>, the object will move

Note that the magnitude of the frictional force does not depend on the contact area of the surfaces. It depends on the roughness / smoothness of the two surfaces in contact.