Learning Objectives

• Describe motion with reference frames; how unbalanced forces change it.

• Apply Newton's Laws: inertia (1st), F=ma (2nd), action-reaction (3rd).

• Explain gravity is attractive and depends on objects' masses.

• Describe kinetic energy: energy of motion, depends on mass/speed.

• Motion occurs when an object's position changes.

• We compare its position to a reference frame.

• A reference frame is a fixed spot, like a tree.

• Always state your reference frame to describe motion clearly.

Concept 1: Reference Frames and Motion

• How we see motion depends on our "reference frame." (Viewpoint)

• An object can seem still, yet be moving to another observer!

• Example: On a train, you're still to your seat, moving past the platform.

• Always say your reference frame when describing motion.

Relative Motion

• Imagine a super push or a mighty pull – that’s a force!

• ​Forces have strength (how strong!) and a specific direction they go.

• We measure force strength in Newtons (N) – cool unit, right?

• Forces make things move: speed up, slow down, or even turn!

Force: The Push and Pull Power!

​​Contact Forces

• Applied even if objects don't touch.

• Example: An apple falling to Earth.

• Gravity pulls objects due to their mass.

​​Noncontact Forces

Types of Forces: Contact vs. Noncontact

• Applied only when objects touch.

• Example: Pushing a box across a floor.

• Friction opposes motion when surfaces rub.

​​Balanced Forces

• Forces are unequal or not directly opposite.

• Causes a change in the object's motion (acceleration).

• Net force is greater than zero, changing speed/direction.

​​Unbalanced Forces

Concept 5: Balanced and Unbalanced Forces, and Net Force

• Forces are equal and act in opposite directions.

• No change in the object's motion occurs.

• Net force is zero; object stays still or moves constantly.

• Speed is how fast something moves—its distance traveled per unit of time.

• The formula is simple: Speed = Distance / Time.

• The SI unit for speed is m/s. Others are km/h or mph.

• Greater speed means more distance in same time, or same distance in less time!

Calculating Speed

​​Snapshot Speed: Instantaneous!

• ​Total distance divided by total time.

• ​​Formula: Total Distance / Total Time.

• ​Overall speed for the entire journey.

​​The Big Picture: Average Speed!

Speed Showdown: Instantaneous vs. Average!

• ​Speed at a specific moment in time.

• ​​Like a car's speedometer reading.

• ​Shows quick, moment-to-moment changes.

• Velocity is speed plus the object's direction.

• ​It's a vector quantity: has magnitude (speed) and direction.

• Change speed or direction? Velocity changes too!

• Same speed, different ways? Different velocities!

Velocity: Speed and Direction Together!

• Acceleration is when an object's velocity changes over time.

• Speeding up is positive acceleration; slowing down is negative.

• Changing direction, even at constant speed, is acceleration.

• Any change in velocity (speed or direction) means acceleration.

What is Acceleration?

• Moving things have "kinetic energy" – energy of motion!

• When objects collide, they push or pull with forces.

• These forces can change an object's motion or speed.

• Collisions transfer kinetic energy, making other things move!

Crash Course: Forces and Energy in Collisions!

• An object at rest will stay at rest.

• An object in motion stays in motion with constant velocity.

• This happens unless acted on by a nonzero net force.

• Objects naturally resist changes in their state of motion.

Newton's First Law: The Law of Inertia

• Inertia is an object's resistance to changing its motion.

• Newton's first law is often called the law of inertia.

• An object's inertia depends directly on its mass.

• More mass means more inertia, harder to start, stop, or turn!

Inertia and Mass

• It links net force, an object's mass, and its acceleration.

• Formula: Net Force (F) = Mass (m) × Acceleration (a).

• More net force = more acceleration. More mass = less acceleration.

• Acceleration is in the same direction as the net force.

Newton's Second Law of Motion

• Every action has an equal, opposite reaction.

• If object A pushes B, B pushes A back equally.

• These forces are pairs, acting on different objects.

• Example: Push a wall; the wall pushes you back.

Newton's Third Law: Action & Reaction

• Friction is a force stopping motion between rubbing surfaces.

• Its direction is always opposite to how something moves.

• Rougher surfaces and more pressure mean more friction.

• Heavy box on rough floor? Harder to slide due to friction!

Friction: The Motion Resister!

• Gravity is a noncontact, attractive force between any two objects having mass.

• This force is universal. It acts between all objects in the universe.

• It's strong when objects have large mass, like planets, moons, or stars.

• Earth's gravity pulls objects down. The Sun's gravity holds planets in orbit.

Concept 16: Gravitational Interactions

• Gravitational force depends on two things: mass and distance.

• More mass means a stronger gravitational pull between objects.

• More distance means a weaker gravitational pull between objects.

• So, gravity is strongest for massive objects that are close.

Concept 17: Factors Affecting Gravitational Force

​​All About Mass

• Weight is gravity's pulling force on an object.

• Measured in Newtons (N), weight depends on gravity.

• An astronaut weighs much less on the Moon!

​​Understanding Weight

Concept 18: Unpacking Mass and Weight

• Mass is the amount of "stuff" in an object.

• Measured in kilograms (kg), mass is always constant.

• An astronaut's mass is the same on Earth or Moon.

• Kinetic energy is energy of motion.

• It depends on an object's mass and its speed.

• More mass means more kinetic energy at the same speed.

• More speed means much more kinetic energy (KE = 1/2 mv²).

Kinetic Energy

​Correction

• Constant motion needs no net force. Net force changes motion.

• Without air resistance, all objects fall at the same rate.

• Inertia isn't a force; it's resistance to motion change.

• Action-reaction forces act on different objects, so don't cancel.
  

​Misconception

Common Misconceptions

• Motion means a force always acts in its direction.

• Heavier objects always fall faster than lighter objects.

• Inertia is a force keeping objects moving or at rest.

• Action-reaction forces cancel, so no motion can happen.
  

Summary

• Motion is position change, described by a reference frame.

• Forces (pushes/pulls) cause changes in motion, called acceleration.

• Net force changes motion; balanced forces mean no motion change.

• Newton's Laws: Inertia (1st), F=ma (2nd), Action-Reaction (3rd).