(Chapter 4)

Newton’s Laws of Motion

Fill in the blanks on your guided notes during the PowerPoint

Newton’s Laws of Motion

• To quote Bill Nye: “Consider the Following”:1. Lifting a backpack:• When you lift a backpack, you exert a force on

it that causes it to move (pull it up)• The backpack initially was at rest (not in

motion)• The force you exerted caused the backpack’s

velocity to change.

Newton’s Laws of Motion

2. Pushing on a table:

• Push down on a table and the table does not move.

• The force you applied did not cause the velocity of the table to change.

Newton’s Laws of Motion

• Sir Isaac Newton (1642-1727) came up with three laws to explain how motion and forces are related:

First Law of Motion

First Law of Motion aka The Law of Inertia:

• An object at rest will remain at rest and an object in motion will remain in motion with a constant speed & direction (velocity) unless acted upon by an external unbalanced force.

(Gravity and Friction are unbalanced forces)

First Law of Motion• If an object is not pushed or pulled upon, it’s speed &

direction (velocity) will naturally remain constant….which means:

–Once moving at a steady speed in a straight line…it will continue to move at a steady speed in a straight line.

–Once standing still…it will stay still.

• (Note: a motionless object is maintaining a constant velocity of 0 m/s)

First Law of Motion

Example: • A skateboard is at rest and doesn’t move

unless you give it a push and increases its velocity while you push it.

• After the skateboard leaves your hand, it slows down and stops because friction acts on it as it rolls.

First Law of Motion

• Inertia: the tendency of an object to resist a change in motion.

– Objects want to remain in uniform motion in a straight line so they resist changes in speed or direction

First Law of Motion

(Inertia Continued)– Depends on mass: Increase Mass, Increase Inertia

• Heavier objects are harder to move because they have more inertia or more resistance to change in motion.

• Pushing a heavier person on a swing is harder than pushing a lighter person.

First Law of Motion

Example: A bowling ball and a volleyball are rolling towards you. You would have to exert a greater force on the bowling ball to make it stop. The bowling ball has more inertia than the volleyball because it has more mass.

First Law of Motion

Example: You are pushing a cart with a box on top and come to a sudden stop, causing the box to slide off the cart. The inertia of the boxes causes them to keep moving even after the cart stops.

First Law of MotionExample:

A car traveling at 85 mph crashes head-on with something solid, crumples, slows and finally stops within 0.1s. Anyone in the car not wearing a seatbelt continues to move forward at the same speed the car was traveling (85 mph), slamming into the windshield, dashboard etc. Note: 85mph is about the speed falling from a three-story building.

Bonus Lesson: Always wear your seatbelt!

Second Law of Motion

Second Law of Motion:

The acceleration of an object is dependent upon the force acting upon the object and the mass of the object.

Second Law of Motion

Newton’s Second Law can be written as the equation:

F = ma or Force = mass x acceleration

F = force (N) m= mass (kg)

a = acceleration (m/s/s or m/s2)

1N = 1kg.m/s2

Second Law of Motion

This equation demonstrates the following two important relationships between force, mass and acceleration:

Second Law of Motion

1. The greater the force on an object, the greater the acceleration of an object.

(Increase Force, Increase Acceleration)

Example:In baseball, if I hit a ball as hard as I can, the ball accelerates more than if I was trying to bunt the ball.

Second Law of Motion

2. The greater the mass of an object, the smaller the acceleration.

(Increase Mass, Decrease Acceleration)

Example: In baseball, if I hit a 50 gram ball and a 25 gram ball with the same force, the 50-gram ball would go slower than the 25 gram ball because it has a greater mass.

Second Law of Motion

Acceleration is directly proportional to the force:

*you push twice as hard and it accelerates twice as much.

(Increase Force, Increase Acceleration)

Second Law of Motion

Acceleration is inversely proportional to the mass:

*If it gets twice the mass, it accelerates half as much.

(Increase Mass, Decrease Acceleration)

Third Law of Motion

Third Law of Motion:

*For every action there is an equal and opposite reaction*

Third Law of Motion

• When one object exerts a force on a second object, the second object exerts the same amount of force back on the first object but in the opposite direction.

• If a student were to jump off of a desk onto the floor, their feet

would sting. This is because the student’s feet would exert a force on the ground but the ground exerts a force back on the student’s feet, causing their feet to sting.

• You jump on a trampoline and exert a downward force while the

trampoline exerts an equal force upward, sending you high in the air.

Momentum

Momentum: a property of a moving object that equals its mass x velocity.

Formula: P = mv

m = mass (kg) V= velocity (m/s)

P=Momentum (Kg.m/s)

Law of Conservation of Momentum

Law of Conservation of Momentum: total momentum is conserved unless an outside force acts on the objects.

Friction is an outside force.

Law of Conservation of Momentum

• The momentum of an object doesn’t change unless its mass, velocity or both change.

• If a group of objects exerts forces only on each other, their total momentum doesn’t change.

• Momentum can be transferred from one object to another, but none is lost.

Gravity

Gravity: the attractive force between any two objects that depends on the mass of the objects and the distance between them.

Gravity

• The earth is close enough and has a large enough mass that you can feel its gravitational attraction.

• The sun has much more mass than the earth, but it is too far away to notice a gravitational attraction to you.

Gravity

The Law of Universal Gravitation: The gravitational force between two masses decreases rapidly as the distance between the masses increases.

i.e Increase Distance, Decrease Gravitational Attraction

Gravity

Gravitational Force Equation:

F=mg

Gravitational Force (N) =

mass (kg) x gravitational acceleration (m/s2)

g=9.8 m/s2

Gravity

Weight: the gravitational force exerted on an object

W=mg

Weight (N) =

mass (kg) x gravitational acceleration (m/s2)

Gravity

Weight and Mass are NOT the same thing! Weight is a force and mass is a measure of the amount of matter in an object. You weigh less on the moon because gravity is 1.6 m/s2. Your mass is the same.