0014 Force, Mass and Motion: 1. distinguish between mass and weight of an object.

Mass vs Weight

• Quantity of matter in an object

• The measurement of inertia

• Brick = 1kg

• The gravitational force exerted on an object by the nearest, most massive body (Earth)

• Brick = 2.2 pounds

The Newton (metric unit)

• In the metric system, the unit of weight, or any other force, is the newton, which is equal to a little less than a quarter pound.

• Newton = force needed to accelerate 1 kg 1 m/s2

• 1 kg brick weighs about 10 N

• Or a baseball = 1 N

0014 Force, Mass and Motion: 2. identify characteristics of forces that act on objects (e.g. frictional, gravitational)

0014 Force, Mass and Motion: 3. determine the relationship between the velocity and acceleration of an object.

Acceleration

• Acceleration is the amount of change in velocity divided by the time it takes the change to occur.

• Acceleration (m/s2) =

[final velocity – initial velocity (m/s)] / time (s)

• A = (vf - vi) / t

A car traveling at a rate of 10 m/s accelerates to 90 m/s in 12 seconds. Calculate its acceleration.

• A = (vf - vi) / t

• A = 90 m/s – 10 m/s / 12 s

= 80 m/s / 12 s

= 6.67 m/s/s

or 6.67 m/s2

3 devices in your car make it accelerate:

• Accelerator pedal

• Brake pedal

• Steering wheel

• Whenever an object changes speed or direction it accelerates.

Figure 2-8

Galileo’s falling-ball apparatus with a table of measurements and a graph of distance versus time.

Galileo found the following:

• a ball rolling down a ramp moves with constant acceleration

• a ball attains a greater acceleration from steeper inclines

• regardless of weight, when air resistance is negligible, all objects fall with the same acceleration

Free-Fall Velocity

• The velocity of a falling object is proportional to the length of time it has been falling.

• Velocity (m/s) = constant g (m/s2) x time (s)

• V = g x t

• Galileo found g = 9.8 m/s2

Acceleration due to Gravity

• Suppose a falling rock is equipped with a speedometer:

• In each succeeding second of fall, the rock’s speed increases by the same amount: 10 m/s

• Time of Fall (s) Instantaneous Speed (m/s)• 1                                                            10• 2                                                            20• 3                                                            30• 4                                                            40 5 50

Gravity• Suppose a falling rock is equipped with

an odometer:

• The readings would indicate that the distance fallen increases with time according to the relationship d = ½ gt2

• Time of Fall (s) Distance of Fall (m) – 1 5– 2 20– 3 45– 4 80

Free Fall and Air Resistance• In free-fall, force of air

resistance counters force of gravity.

• As skydiver falls, air resistance increases ‘til it approaches the magnitude of the force of gravity. Once the force of air resistance is as large as the force of gravity, skydiver is said to have reached a terminal velocity.

• Skydiving

0014 Force, Mass and Motion: 4. solve quantitative problems involving force, mass, and motion of objects.

0014 Force, Mass and Motion: 5. demonstrate knowledge of Newton’s 3 laws of motion and their application to everyday situations.

Isaac Newton and the Universal Laws of Motion

• English scientist (1642-1727)

• Synthesized the work of Galileo and others

• 3 laws describe all motion

First Law: Inertia(matter resists change)

• A moving object will continue moving in a straight line at a constant speed, and a stationary object will remain at rest, unless acted upon by an unbalanced force.

• animation

Second Law: F = m x a

• The acceleration produced by a force on an object is proportional to the magnitude of the force, and inversely proportional to the mass of the object.

• tutorial

Third Law: action / reaction

• For every action there is an equal and opposite reaction.

• See some examples

calculate the force needed to produce a given acceleration on a given mass

(F = ma)

• A 20 kg mass has an acceleration of 3 m/s2. Calculate the force acting on the mass.

• F = (20 kg) (3 m/s2)

• F = 60 kg m/s2 = 60 N

What force is needed to accelerate a 75 kg sprinter from rest to a speed of 10 meters per second in half a second?

• First find acceleration.

Accel = final vel – initial vel (m/s) / time (s)

= 10 m/s – 0 m/s / .5 s = 20 m/s/s

• Force (N) = mass (kg) x accel (m/s2)

F = 75 kg x 20 m/s2

F = 1500 N

Newton’s Law of Universal Gravitation

• Between any two objects in the universe there is an attractive force proportional to the masses of the objects and inversely proportional to the square of the distance between them.

• F = (G x m1 x m2) / d2

• The more massive 2 objects are, the greater the force between them.

• The farther apart 2 objects are, the less the force between them.

Figure 2-13

An apple falling, a ball being thrown, a space shuttle orbiting the Earth, and the orbiting Moon, all display the influence of the force of gravity.

0014 Force, Mass and Motion: 6. apply knowledge of the concepts of work and power to the analysis of everyday activities.

Work is done when a force is exerted over a distance.

Work

• is equal to the force that is exerted times the distance over which it is exerted.

• W = F x d

• The unit of work combines the unit of force (N) with the unit of distance (m)

• Newton-meter (N-m) aka Joule.

You carry a 20 kg suitcase upstairs, a distance of 4m. How much work did you do?

• W = F x d

• F = ma

• = (20 kg) (10m/s2) = 200 N

• W = F x d

• = (200 N) (4m)

• = 800 J

Power

• measures rate at which work is done.

• Power is the amount of work done, divided by the time it takes to do it.

• Power (watts) = work (joules) / time (sec)

• P = W/t

Power

• Since work performed equals energy expended,

• Power (watts) = energy (joules) / time (sec)

• The watt is defined as the expenditure of

1 joule of energy in 1 second.

(75 watt light bulb consumes 75 J/sec)

0014 Force, Mass and Motion: 7. demonstrate knowledge of types and characteristics of simple machines and their effect on work.

“Simple Machine:

device for

multiplying or

changing the

direction of force.