Getting an A in Science  Taking ownership or control of the information learned in class and taking responsibility for your work This means: This means: Practicing (reviewing, restating, preparing)Practicing (reviewing, restating, preparing) Applying (creating, thinking, using)Applying (creating, thinking, using) Planning (studying, completing projects and homework, being prepared-pencils, books)Planning (studying, completing projects and homework, being prepared-pencils, books)

Intro to Physics  What is physics?

Motion & Forces Chapter 2

Motion  What does motion mean?  How can we tell if something is in motion?  Is this marble in motion?  How do you know?  How can we provide evidence?

Motion  Motion: a change in position, measured by distance and time.  Motion: a change in position, measured by distance and time.

Frame of Reference Determining if an object changes position requires a frame of reference. Determining if an object changes position requires a frame of reference. Frame of reference is the object or point from which movement is determined Frame of reference is the object or point from which movement is determined What is the frame of reference for this video? How do I know the skater is in motion?

Frame of Reference  Movement is relative to an object that appears stationary  Earth is the most common frame of reference

Motion of the Marble  We can tell if the marble is in motion by measuring its change in position along the ruler (our frame of reference).  Right now, the marble is NOT in motion. It is not changing position. How can we get it to change position?

How to make things move  2IQIZY 2IQIZY 2IQIZY

Force  A force is a push or a pull  Forces can affect motion in several ways.  Forces can make objects: Start movingStart moving Move fasterMove faster Move slowerMove slower Stop movingStop moving Change directionChange direction Change shapeChange shape

What forces are at work…  During a rocket launch? Upwards force (thrust) from engines Upwards force (thrust) from engines Downwards force from gravity Downwards force from gravity Downwards force (drag) from air resistance Downwards force (drag) from air resistance

What forces are at work…  pushing a truck? Forwards force from pushing Forwards force from pushing Backwards force from friction Backwards force from friction Downwards force from gravity Downwards force from gravity Upwards force from ground. Upwards force from ground.

What forces are at work… Kicking a football? Upwards force from kick Downwards force from gravity Backwards force from air friction F kick F grav F air

Friction  Friction force that opposes motion between 2 surfaces force that opposes motion between 2 surfaces It depends on the: types of surfacestypes of surfaces force between the surfacesforce between the surfaces

Friction  Friction is greater... between rough surfaces between rough surfaces when there’s a greater force between the surfaces (e.g. more weight) when there’s a greater force between the surfaces (e.g. more weight)

How strong is the force?  Forces are measured in Newtons ww.youtu be.com/ watch?v =PCxP2 4qj2UQ

Wrong kind of force…

Measuring Forces that cause motion  Units are Newtons (N)  Forces are measured using a spring scale or force meter. =AFu1advEnhw

Forces occur in pairs  The force pairs can be balanced or unbalanced

Balanced Forces  Balanced forces cause NO change in motion  Balanced forces are equal in size and opposite in direction

Balance forces = no motion

Unbalanced Forces  Unbalanced forces always cause a change in motion, speed, or direction  Which way will the boy in the middle be pulled?

Unbalanced Forces Causes Motion Adding Forces Subtracting Forces

Unbalanced Forces Causes motion Adding Forces 5 N Object 5 N Notice that all the forces are pointed in the same direction. Hence they add together.

Subtracting Forces Two forces can subtract to produce a net force in the direction of the larger force. –Negative motion occurs (Slow Down) 10 N 5 N = 5 N

Unbalanced Forces Causes Motion Subtracting Forces 5 N Object 5 N Notice that all the forces are unequal and pointed in the opposite direction. Hence they are unbalanced and in opposition to each other – or one partially cancels the other.

Which game will result in a winning team? How do you know?

Balanced Forces Balanced Push i.e. Pushing a Car No motion Balanced Pull i.e. Tug-of-war No motion 10N

Un-Balanced Forces Additive Un-Balanced Same Direction i.e. sled Faster motion Subtractive Un-Balanced Opposite Direction i.e. football tackle Slower motion 10N

Why do I care? Understanding an object’s motion can help you understand how to interact with the world around you.

Review of balanced and unbalanced forces bRxclk

Stop here

Universal Law of Gravitation  Mass and gravity have a directly proportional relationship.  The distance between the centers of masses of the two objects and gravity have an inversely proportional relationship.

Gravity  Gravity force of attraction between any two objects in the universe force of attraction between any two objects in the universe increases as... increases as... mass increasesmass increases distance decreasesdistance decreases

Gravity  Who experiences more gravity - the astronaut or the politician? less distance more mass  Which exerts more gravity - the Earth or the moon?

Gravity  Would you weigh more on Earth or Jupiter? greater gravity greater weight greater mass Jupiter because... Jupiter because...

Gravity pulls down on you… The ground pushes back up… THIS KEEPS YOU WHERE YOU ARE! If these football players push on each other equally as hard, will either one move? Gravity pulls down Ground pushes up Balanced Forces (Balanced Forces = No Motion)

Newton’s First Law  Newton’s First Law of Motion An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net or unbalanced force. An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net or unbalanced force.

Two Key Points

Newton’s First Law  Newton’s First Law of Motion “Law of Inertia” “Law of Inertia”  Inertia tendency of an object to resist any change in its motion tendency of an object to resist any change in its motion increases as mass increases increases as mass increases

Can you explain how this trick works, using newton’s first law of motion?

Newton’s Second Law  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 its mass. The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. F = ma

Newton’s Second Law F = ma F:force (N) m:mass (kg) a:accel (m/s 2 ) 1 N = 1 kg ·m/s 2 m F a F m

Calculations  What force would be required to accelerate a 40 kg mass by 4 m/s 2 ? GIVEN: F = ? m = 40 kg a = 4 m/s 2 WORK : F = ma F = (40 kg)(4 m/s 2 ) F = 160 N m F a

Calculations  A 4.0 kg shotput is thrown with 30 N of force. What is its acceleration? GIVEN: m = 4.0 kg F = 30 N a = ? WORK : a = F ÷ m a = (30 N) ÷ (4.0 kg) a = 7.5 m/s 2 m F a

ConcepTest  Is the following statement true or false? An astronaut has less mass on the moon since the moon exerts a weaker gravitational force. An astronaut has less mass on the moon since the moon exerts a weaker gravitational force. False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon. False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.

Newton’s Third Law  Newton’s Third Law of Motion When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first. When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.

Newton’s Third Law  Action-Reaction Pairs  The hammer exerts a force on the nail to the right.  The nail exerts an equal but opposite force on the hammer to the left.

Newton’s Third Law  Action-Reaction Pairs  The rocket exerts a downward force on the exhaust gases.  The gases exert an equal but opposite upward force on the rocket. FGFG FRFR

Newton’s Third Law  Action-Reaction Pairs  Both objects accelerate.  The amount of acceleration depends on the mass of the object and the force applied.  Force = mass x acceleration F=ma  Small mass  more acceleration  Large mass  less acceleration