Chapter 2

MOTION

MOTION

• - one of the more common intangible concepts in science

• - is the act or process of changing position

• TWO IMPORTANT ASPECTS OF MOTION• A) change in position• B) Change in time

3 PROPERTIES OF MOTION

• A. SPEED – how fast an object moves• - distance per unit time

• Speed = change in distance change in time

2nd PROPERTY of MOTION

• B. VELOCITY - describes the SPEED and DIRECTION of a moving object

• Vectors are important because of the concept of direction.

3rd PROPERTY of MOTION

• C. ACCELERATION – “change in velocity” and time; rate at which motion was changed.

• Acceleration = change in velocity change in time

Motion can be changed by:

• 1. changing speed

• 2. changing direction of travel

• 3. changing both the speed and direction (this is the same as changing the velocity of an object = ACCELERATION)

HOW TO CHANGE MOTION

• FORCE can be used to change the motion of an object.

• FORCE – is the push or pull that can change motion

• Net Force – is the sum of all the forces acting on an object. (Again, vectors are important)

Important Concept

• Moving objects that are not pushed or pulled come to a rest in a short period of time.

• Objects keep moving only if a force keeps acting on it.

• Aristotle thought that a heavier rock would fall faster than an lighter rock. He thought that the speed of a falling object is proportional to its weight. (He was wrong!!!)

• Galileo corrected Aristotle.• The velocity of a falling object in FREE FALL

(without air resistance; only gravity involved) is NOT proportional to its weight.

• However, Galileo observed that the velocity of a falling object increases as time increased.

• Galileo observed that the velocity of a falling object depended on the time and the distance that it fell.

• “Distance is proportional to time2.”

• An object should fall 4x as far in 2 seconds (22), 9x as far in 3 seconds (32) and 16x as far in 4 seconds (42).

IMPORTANT CONCLUSIONS!!!• The velocity of a falling object increases at a constant

rate. “Distance is proportional to time2.”

• Acceleration due to gravity is constant for all objects on Earth (9.8 m/s2). [no air resistance]

• All objects in free fall experience constant acceleration.

• Objects falling from the same distance will hit the ground at the same time. [Time is not dependent on the mass of the object.]

2 Type of Motion Discussed so far:

• 1. Horizontal Motion

• 2. Vertical Motion

• 3. Motion due to an object being thrown vertically or horizontally

INERTIA

• Is the behavior of matter to remain in unchanging motion, whether at rest or in motion

VERTICAL PROJECTILES

• Ex. Bullet goes up, slows down and stops, then accelerates back to earth.

• Bullet slows down and stops because gravity is decreasing the velocity of the bullet.

HORIZONTAL PROJECTILES

• If air resistance is ignored, then there are no forces involved in horizontal projectiles.

• The reason an arrow shot horizontally falls to the ground is because the vertical forces (gravity) come into play.

Newtons’ First Law of Motion

• “Every object retains its state of rest or straight-line motion unless acted upon by an unbalanced force.”

• An object resists any change in its state of motion unless acted upon by an unbalanced external force.

Newton’s Second Law of Motion

• “The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to the mass of an object.”

• The acceleration of an object depends on the net force and the mass of an object.

• Equation: Force = mass x acceleration

Unit of Force

• Newton (N)

• 1 Newton =1 N = 1 kg-m/s2

Mass vs. Weight

• Mass = is the amount of matter in an object• = is the property that determines how much

an object resists a change in motion (inertia)

• The greater the mass of an object, the greater the inertia. this is the reason that big rigs takes a longer distance and time to come to a standstill when the brakes are applied.

Weight• = is the force of gravity acting on a mass.

• Because weight is a FORCE, its unit is Newtons.

• Equation: w = mg or weight = mass x acceleration due to gravity

or Downward force = mass x acceleration due to gravity

Acceleration due to Gravity

• Near the Earth’s surface, g (acceleration due to gravity) has an approximate value of :

g = 9.8 m/s2

• Pounds is also a unit of force, aside from Newtons. So pounds is also a unit of weight.

• Kilogram is a unit of mass.

• Conversion: 1 lb = 4.5 Newtons

Newton’s Third Law of Motion

• “ Whenever two objects interact, the force exerted on one object is equal in strength and opposite in direction to the force exerted on the other object.”

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

• Example: 2 Sumo wrestlers doing belly bump

MOMENTUM

• Momentum (p) = mass x velocity = m x v

• It takes a longer time to stops an object’s movement when it has momentum.

• Momentum involves both inertia and velocity.

Law of Conservation of Momentum

• “The total momentum of a group of interacting objects remains the same in the absence of external forces.”

IMPULSE• Is the product of the force and the time of application

of the force. Impulse = Force x time = FT • Brings about a change in momentum (A greater impulse

means greater change in momentum.)

• Equations: Impulse = Force x time = FT

• Change in momentum =(applied force)(time of contact) D p = FT

• The longer the time of contact, the weaker the force.

FORCES and CIRCULAR MOTION

• Centripetal force – is the force that pulls an object out of its straight-line path and into a circular path

• Centrifugal force – is the outward tug or the outward force

• The acceleration of an object moving in uniform circular motion is equal to the square of its speed around the circle (v2) and inversely proportional to the radius of the circle (1/r)

• Equation: ac = v2/r

GRAVITY

• Gravity is the force, a pull that exists between ALL objects in the universe.

• Example: The Earth attracts you and you attract the Earth.

Newton’s Law of Gravitation• “ Every object in the universe is attracted to every other

object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distances between them.”

• Equation: Force = G x mass1 x mass2

distance2

- where G is a universal constant with the value

6.67 x 10-11 N.m2/kg2 and not the acceleration due to gravity.

• Equation: Force = G x mass1 x mass2

distance2

• This equation gives the magnitude of the attractive or gravitational force that each object exerts on the other.