Newtons 1st Law

Chapter 4: Pages 130-135

Holt Physics

“Law of Inertia”

Sir Isaac Newton

• 1687- An object at rest tends to stay at rest, and an object in motion tends to stay in motion unless acted on by an outside force.

• Inertia – the tendency of an object to maintain its current state of motion.

Newton’s 1st Law

• If the car were to abruptly stop and the seat belts were not being worn, then the passengers in motion would continue in motion. The passengers would likely be propelled from the car and be hurled into the air becomes projectiles and continue in projectile-like motion.

Newton’s 1st Law

• In this example, the ladder continues in motion because no outside force changes its original motion, until the ladder hits the ground.

Determining Net external force

• External force – a force acting on an object resulting from interaction between object and environment

• Examples: friction, gravity, every

single push or pull

Net External Force

• Net external force – total force (sum of all forces) acting on an object

Referred to as the Resultant force

• An object may have many forces acting on it, but still be in equilibrium

Equilibrium

• Equilibrium – the state of a body in which there is no change in its motion

Either at rest or in motion

Net external force on an object = 0 Newtons

Inertia

• Inertia is proportional to mass

• Mass is a measurement of inertia

• More mass = More inertia

Equations

• Σ (Greek letter sigma) means “sum of all”

• ΣFx = sum of all forces in x-direction

• ΣFy = sum of all forces in y-direction

Equations

• Fx = Ftotal (cosθ) x-component of force

• Fy = Ftotal (sinθ) y-component of force

Equations

• Ftotal2 = Fx

2 + Fy2 Resultant force (F-total)

(net external force)

Homework

• 1. Read Chapter 4-3

• 2. Page 133 #1-3

• 3. Page 135 #1-3