physics unit3

CHAPTER 3

RELATIVE VELOCITY

FRAMES OF REFERENCE

• Velocity measurements differ in different frames of reference.

• Observers using different frames of reference would generally not agree on some features of motion.

RELATIVE VELOCITY

• Write down all information that was given in the problem and that you want to know in the forms of velocities with some kind of subscript.

EXAMPLE PG 108 SAMPLE 3F

A boat heading north crosses a wide river with a velocity of 10 km/hr relative to the water. The river has a uniform velocity of 5.0 km/hr due east. Determine the boat’s velocity with respect to an observer on the shore.

YOU TRY!

(pg 109 #1)

A passenger at the rear of a train traveling at 15 m/s relative to Earth throws a baseball with a speed of 15 m/s in the direction opposite the motion of the train. What is the velocity of the baseball relative to Earth as it leaves the throwers hand?

CHAPTER 4

FORCES & LAWS OF MOTION

4.1: CHANGES IN MOTION

• Force: a push or pull exerted on an object.

• Forces cause a change in velocity.• Force is a VECTOR QUANTITY.• SI unit of force is the Newton (N).• Forces can act through contact or at a

distance

CONTACT VS FIELD FORCES

• Contact Forces – result from physical contact between two objects

• Usually easy to identify• A physical push or a pull

• Field Forces – do not involve physical contact between two objects

• Example: Electric Force, GRAVITY!!!

FBD: FREE BODY DIAGRAMS

• Helps analyze a situation

• Isolate an object and the forces acting on that object.

• FBDs are used to show all the external forces acting on an object.

HOW TO FBD

1. Draw your object as single point. (forces are assumed to act on a single point at the center of an object)

2. Draw and label vector arrows representing all external forces acting on the object.

3. Make sure you are only drawing the forces acting on the object and NOT the forces that the object acts on other things.

4. When finished, an FBD can be used to find the net external force on an object.

THE MASTERMIND

• In the 1630s, GALILEO – yes not Newton – realized that a block sliding on a perfectly smooth surface would slide forever in the absence of an applied force.

NEWTON

• In 1687, Newton further developed the concepts that were initially developed by Galileo which has now come to be known as Newton’s 1st Law of Motion

NEWTON’S FIRST LAW

An object at rest remains at rest, and an object in

motion stays in motion, unless acted upon by an

outside force.

LAW OF INERTIA

• Newton’s 1st Law came to be known as the “Law of Inertia”

• Inertia = tendency of an object to not accelerate

• The more mass something has, the harder it is to …

• Slow it down …if it is already moving• Get it to start moving … if it is at rest

WEIGHT VS. MASS

• Mass = the amount of matter something has.

• Weight = the magnitude of the force of gravity acting on an object.

**You need mass to calculate weight.

MASS ≠ WEIGHT

THE FORCE OF GRAVITY

• The force of gravity as we talk about it in everyday life is called WEIGHT.

• We will call the force of gravity Fg.

• We can calculate the force of gravity using the following formula.

Fg= mgFg= Force of Gravity, measured in Newtons

m= mass, measured in Kilograms

g= acceleration due to gravity, -9.8 m/s2

EXAMPLE

A bag of sugar has a mass of 2.26 kg. What is its weight?

YOU TRY!

If a the mass of Mickey Mouse is 24 kg. Calculate Mickey’s weight on Earth.

THE NORMAL FORCE

• The Normal Force= a force exerted by one object on another in a direction perpendicular to the surface of contact.

FN

Fg

EXAMPLE

A block of mass 26 kg is sitting stationary on a 1 m high table. Calculate the block’s weight and normal force.