Chapter 2.3 & 3Chapter 2.3 & 3

Force, Mass and AccelerationForce, Mass and Acceleration

Newton’s LawsNewton’s Laws

Sir Isaac NewtonSir Isaac Newton

An English Mathematician and Physicist An English Mathematician and Physicist that formulated the three laws of motion, that formulated the three laws of motion, law of universal gravitation and invented law of universal gravitation and invented calculus before the age of 30calculus before the age of 30

Newton’s discoveries helped to answer Newton’s discoveries helped to answer many questions such as: what causes many questions such as: what causes tides, how do the planets move and why tides, how do the planets move and why do objects of different masses fall at the do objects of different masses fall at the same rate?same rate?

What is Force?What is Force?

Force is a push, pull or any action Force is a push, pull or any action that has the ability to change motion that has the ability to change motion (This doesn’t mean that a force will (This doesn’t mean that a force will always make something move– can always make something move– can you think of a situation such as this?)you think of a situation such as this?)

There are two units that are There are two units that are commonly used to measure force, commonly used to measure force, Newtons and Pounds. Scientists Newtons and Pounds. Scientists prefer Newtons(N)prefer Newtons(N)

Force vs. MassForce vs. Mass

The Newton relates to force and The Newton relates to force and motion. A force of 1 Newton causes motion. A force of 1 Newton causes a 1 kg mass to have an acceleration a 1 kg mass to have an acceleration of 1 m/sof 1 m/s22

We call it a Newton because that is We call it a Newton because that is somewhat easier to say than “1 kg somewhat easier to say than “1 kg times 1 m/stimes 1 m/s22””

Newton’s First LawNewton’s First Law

Known as the Law of InertiaKnown as the Law of Inertia ““An object at rest will stay at rest until An object at rest will stay at rest until

acted upon by an unbalanced force, an acted upon by an unbalanced force, an object in motion will continue with object in motion will continue with constant speed and direction until acted constant speed and direction until acted upon by an unbalanced force”upon by an unbalanced force”

Basically – An object at rest remains at Basically – An object at rest remains at rest and an object in motion remains in rest and an object in motion remains in motion unless acted on by an unbalanced motion unless acted on by an unbalanced force.force.

InertiaInertia

Inertia is dependant on mass, Inertia is dependant on mass, therefore an object with more mass therefore an object with more mass would have a greater inertiawould have a greater inertia

Example: Which is harder to move, a Example: Which is harder to move, a bowling ball or a beach ball? The ball bowling ball or a beach ball? The ball with the greater mass is harder to with the greater mass is harder to move because it has greater inertiamove because it has greater inertia

Newton’s 2Newton’s 2ndnd Law Law

Relates the applied force to an Relates the applied force to an object’s mass and its accelerationobject’s mass and its acceleration

It says that force causes It says that force causes acceleration, mass resists acceleration, mass resists acceleration and the acceleration is a acceleration and the acceleration is a ratio of force over massratio of force over mass

Common sense: If something has Common sense: If something has more mass, it would take more force more mass, it would take more force to cause the object to accelerateto cause the object to accelerate

Using Newton’s 2Using Newton’s 2ndnd Law Law

F = maF = ma

F

M A

An object’s Force equals its mass times its acceleration

Balanced vs. Unbalanced ForcesBalanced vs. Unbalanced Forces

The motion of an object depends on The motion of an object depends on the total of all forces acting on itthe total of all forces acting on it

If the forces are balanced, then the If the forces are balanced, then the object is in equilibrium and there is object is in equilibrium and there is no chance of movement occurring no chance of movement occurring because the net force is zerobecause the net force is zero

If there is a net force, then the forces If there is a net force, then the forces are unbalanced, and movement are unbalanced, and movement occursoccurs

Weight vs. MassWeight vs. Mass

Mass is the amount of matter in an object, Mass is the amount of matter in an object, usually measured in kilogramsusually measured in kilograms

Weight is the force of gravity acting on a Weight is the force of gravity acting on a certain mass, measured in Newtons or certain mass, measured in Newtons or poundspounds

A certain mass of something has a A certain mass of something has a “weight”, which is the force acting on that “weight”, which is the force acting on that mass. It varies depending on the gravity.mass. It varies depending on the gravity.

Gravity, Mass and WeightGravity, Mass and Weight Imagine that you travel to the moon. Imagine that you travel to the moon.

Which of these will be the same as on Which of these will be the same as on earth: Gravity, your mass or your weight?earth: Gravity, your mass or your weight?

An object will have the same mass on the An object will have the same mass on the moon as on earth – it still has the same moon as on earth – it still has the same amount of matteramount of matter

An object weighs less on the moon but has An object weighs less on the moon but has the same mass. It weighs less because the same mass. It weighs less because there is less gravity.there is less gravity.

You would weigh more on Jupiter than You would weigh more on Jupiter than Earth because Jupiter has more gravity Earth because Jupiter has more gravity than Earththan Earth

GravityGravity

The acceleration due to gravity on The acceleration due to gravity on Earth is 9.8 m/sEarth is 9.8 m/s22

On earth, every kilogram of mass On earth, every kilogram of mass weighs 9.8 Newtonsweighs 9.8 Newtons

You can figure out the weight by You can figure out the weight by multiplying the mass times the multiplying the mass times the acceleration of gravityacceleration of gravity

F= m X a F= m X a

Gravity, Cont.Gravity, Cont.

On Earth, every object will fall at the same On Earth, every object will fall at the same rate (not counting air friction)rate (not counting air friction)

The Acceleration of gravity is 9.8 m/sThe Acceleration of gravity is 9.8 m/s22 meaning that every second, a falling meaning that every second, a falling object accelerates 9.8 m/sobject accelerates 9.8 m/s

In other words, every second something is In other words, every second something is falling it is moving 9.8 m/s fasterfalling it is moving 9.8 m/s faster

If you drop a bowling ball and a match box If you drop a bowling ball and a match box car at the same time, they will hit the car at the same time, they will hit the ground at the same timeground at the same time

FrictionFriction

Friction is a force that resists other Friction is a force that resists other kinds of movementkinds of movement

Friction always works against the Friction always works against the movement of the object– it comes movement of the object– it comes from two objects moving against from two objects moving against each othereach other

We use the term “friction” to We use the term “friction” to describe any force caused by motion describe any force caused by motion and that acts to slow motion downand that acts to slow motion down

Types of FrictionTypes of Friction

Air friction- Air moving around Air friction- Air moving around objects creates an opposing forceobjects creates an opposing force

Sliding friction- Caused when two Sliding friction- Caused when two surfaces rub against each othersurfaces rub against each other

Viscous friction- Objects that move in Viscous friction- Objects that move in water or other fluids create this type.water or other fluids create this type.

Rolling friction- Caused by one object Rolling friction- Caused by one object rolling over anotherrolling over another

Newton’s Third LawNewton’s Third Law

““For every action, there is an equal and For every action, there is an equal and opposite reaction”opposite reaction”

In other words, for every object, there is a In other words, for every object, there is a PAIR of forces acting on itPAIR of forces acting on it

If you press down on the table, you are If you press down on the table, you are exerting a force on it. Is the table exerting exerting a force on it. Is the table exerting a force on your hand?a force on your hand?

Your force on the table is action force, the Your force on the table is action force, the table’s force on you is reaction forcetable’s force on you is reaction force