ISAAC NEWTON AND THE FORCE

Dynamics

Kinematics vs Dynamics

Kinematics – the study of how stuff move Velocity, acceleration, displacement, vector analysis Started with Galileo around the year 1600 Describes how things move and how to predict their

movements, but does not explain the movements

Dynamics – the study of why stuff move Started with Isaac Newton and his three laws (1687) Explains why things move the way they do by using

forces and relating the force to acceleration

Newton and Galileo

Galileo is considered by many to be the first real physicist (perhaps the first real scientist) because he was the first to experimentally test and describe the movement of objects (kinematics)

Newton took this to another level, not only describing but also explaining why objects move the way they do (dynamics)

The result is the Newton’s Laws (textbook chapter 5)

The discoveries of Galileo and Newton form the basis of the topic of Classical Mechanics

Force

A force is defined as a push or pullForces occur when objects touch each otherForces cause objects to accelerateForces are vectors and therefore we have to

use vector analysisThere are 4 fundamental forces:

Gravitational (attractive force between all objects) Electromagnetic (force resulting from electric charge) Strong Nuclear (attractive force inside atoms) Weak Force (a different form of electromagnetic force)

Newton’s First Law

An object with no net force acting on it remains at rest or moves in a straight line with constant velocity

Object at rest will stay at rest and moving objects will continue to move unless acted upon by a force

Also known as the law of inertia

Inertia - resistance of any physical object to a change in its state of motion or rest

Newton’s Second Law

The acceleration of a body is directly proportional to the net force on it and inversely proportional to its mass

The larger the mass of an object, the more force needed to make it accelerate

We say that the massive object has more inertia than a less massive object

F = ma

F is force, measured in newtons (N)

Newton’s Third Law

When one object exerts a force on a second object, the second object exerts a force on the first that is equal in magnitude but opposite in direction

These forces are called action-reaction force pairs

An action produces an equal and opposite reaction

Only the forces acting ON an object determine its acceleration

Examples of Newton’s Second Law

1. What force is required to accelerated a 1200. kg car at 2.40 m/s2?

2. How much force must be applied to a car travelling with velocity of 25.0 m/s if we want to stop it in 10.0 s? The mass of the car is 1100. kg

Practice Problems

Page 163 #1, 3Page 168 #7, 8

Mass and Weight

Mass refers to the quantity of matter in an object

Weight is the gravitational force of an objectThe weight is dependent on the mass and the

gravitational force exerted on the object

Astronauts in space are weightless (and even this isn’t exactly true), but they are not massless

Weight

The force of gravity is F = mgOn Earth, g ≈ -9.81 m/s2

Therefore, W = mg

Weight is a vector, so pay attention to the direction

Weight is usually referred to as the gravitational force, Fg

Calculating You Weight

The bathrooms scales you may or may not own does not directly measure your mass

It measures your weight as a force, then estimates your mass from that measurement

The bathroom scale tells Mr. Lee that his mass is 84.6 kg. What is the weight measured by the scale?

Practice Problems

Page 137, #1~4 Please refer to page 132 and 133 for a table

of values used in questions 2 and 3

Free Body Diagrams

A simple diagram used to depict an object and all forces acting on it

Friction

Friction is a force that inhibit relative motion between objects that are in contact with each other

Friction slows things down, they are why objects tend to stop if we stop pushing/pulling them

Ff = μ*Fn

Normal Force (FN)

The force exerted by a surface on a object that prevents the object from penetrating the surface

The force that prevents you from falling through the floor (awesome) or walking through a closed door (not so awesome)

ALWAYS normal (perpendicular) to the surface

Often (but not always) equal to the weight

Coefficient of Friction (μ)

This is a “stickiness” value for two objectsDependent on the surface-to-surface contact

of the two objectsThe coefficient of friction has no unitTwo types: μs for Static Friction and μk for

Kinetic FrictionGenerally, μk < μs for any two materials

Static Friction

Static friction is friction that keep stationary objects stationary

It only applies when we are trying to move an object from rest

Its direction will be where it can prevent the object from accelerating

Kinetic Friction

Kinetic friction only applies to moving objects

A force that tries to make moving objects stop

ALWAYS points in the opposite direction of the object’s movement (opposite the velocity vector)

Smaller in magnitude than static friction

Net Force

Newton’s second law: Fnet = maFnet is the sum of all forces acting on the

objectRemember that forces are vectors, and to add

them we must use vector addition

Example Problems

A cart with a mass of 50.0 kg is being pushed along a rough sidewalk with an applied horizontal force of 200. N and has a constant velocity of 3.00 m/s

a. What other horizontal force is acting on the cart and what is the magnitude and direction of that force?

b. What value of applied horizontal force would be required to accelerate the carriage from rest to 6.00 m/s in 3.00 s?

Another Example Problem

An 5.00 kg block of metal is being pulled across a wooden desk at a uniform velocity. If the block is being pulled with a horizontal force of 50.0 N, what is the coefficient of kinetic friction?

Practice Problems