Motion and Forces

Sir Isaac Newton(1643 – 1727)

One of the world’s greatest scientists

Developed the 3 Laws of Motion

Newton’s Laws of Motion

•1st Law – Law of Inertia

•2nd Law – Force = Mass x Acceleration

•3rd Law – For every Action, there is a Reaction

Everything to do with motion, forces, gravity, speed, or acceleration is governed

by these laws.

Newton’s First Law of Motion“Law of Inertia”

• An object will stay at rest

unless acted on by an

unbalanced force

• An object in motion will

stay in motion unless acted

on by an unbalanced force

What happens when a car hits a stationary object?

What happens to you riding in that car?

Inertia

Bike Race Crash

Inertia

• Inertia – the tendency of an object to resist change in motion (whether at rest or moving)

• There are 3 key points about the law of inertia

• Table Cloth Inertia

• Egg Drop Inertia

What does this mean?

• An object will “keep doing what it was doing”

unless acted on by an unbalanced force

• If the object was sitting still, it will remain

stationary (still)

• If it was moving at a constant velocity, it will

keep moving

• It takes force to change the motion of an

object

1st point: Mass and Inertia

The amount of inertia an object has depends on its mass

The greater the mass the greater the inertiaMass is measured in

grams or kilograms.Eureka video

• Objects with a greater mass have greater inertia

• Example: It is more difficult to change the motion of bowling ball than the motion of a golf ball

Inertia at work

Point 2: Objects at rest remain at rest (or will keep doing what they were doing) unless acted on by an unbalanced force.

• Since the train is so heavy, it is difficult to stop it once it is moving

• Since the bicycle has such a small mass, it is very easy to stop.

Point 3: Objects in motion remain in motion with a constant speed and direction(unless acted upon by an unbalanced force).

• Since the train is so heavy, it is difficult to change its speed.

• Since the bicycle is so light weight, it is very easy to change its speed.

Friction and Gravity

• Newton’s First Law of Motion states that an object in motion will stay in motion unless acted on by an outside force.

• Two forces that act on objects are friction and gravity.

Friction Friction acts in a direction

opposite to the object’s direction of motionWithout friction, the

object would continue to move at a constant speed forever

The strength of the force of friction depends on two factors: 1. the types of surfaces

2. how hard the surfaces push together.

Starting a fire

Racing on ice

Types of Friction1. Sliding Friction – solid surfaces

slide over each other.

2. Rolling Friction – when an object rolls over a surface.• Force of friction is less in

rolling friction than with sliding friction.

3. Fluid Friction – when an object moves through a fluid.• Force of friction is less in

fluid friction than with sliding friction.

Gravity• Gravity – is the force that pulls objects toward

each other.

Air Resistance

Objects falling through air experience a type of friction called air resistance

As surface area increases, air resistance increases.

Eventually, air resistance equals gravity

The greatest velocity an object reaches is called terminal velocity

In a vacuum, all objects fall at the same rate

Force, Mass, and Acceleration

Weight vs. Mass

• Weight – a measure of the force of gravity on an object. (weight is a force)

• Your weight on the moon is 1/6 of your weight on Earth – the moon is 1/6 the size of Earth

• Mass – a measure of the amount of matter in that object.

Weight = Mass x Acceleration due to gravity

• Units of Measure:

• Weight – measured in Newtons

• Mass – measured in Kg

• Acceleration – measured in m/s2

Newton’s Second Law of Motion•When a net force acts on an object the object will accelerate in the direction of the net force.

Unit of measurement for force: A “Newton” - is the force needed to accelerate one kg of mass at 1 meter/sec 2 (1 N = 1kg x 1m/s2)

Newton’s 2nd Law

Inversely Proportional

• Mass and acceleration are Inversely proportional:• As the mass increases,

the acceleration decreases

Directly Proportional

• Directly proportional: • As the force increases,

the acceleration increases

Push the shopping cart

Shopping Cart Example:

Graphing Newton’s 2nd Law

• The graphs show the relationship between force, mass and acceleration

(F = ma)

• For the same force:• more mass causes

less acceleration• less mass causes

more acceleration

Graphing Newton’s 2nd Law

• For the same mass:• more force

causes more acceleration

• less force causes less acceleration

Graphing Newton’s 2nd Law

• For the same acceleration:• more mass needs

more force• less mass needs

less force

What does F = ma say?

F = ma basically means that the force of an object comes from its mass and its acceleration.

Something very small (low mass) that’s changing speed very quickly (high

acceleration), like a bullet, can still have a great force.

Something very small changing speed very slowly will have a very weak force.

Something very massive (high mass)

that’s changing speed very slowly (low

acceleration), like a glacier, can still

have great force.

A Little Practice

• How much force will you need to accelerate a 200kg wagon at 5m/s2?

1000 N

Newton’s Second Law Summary

• How much force will you need to accelerate a 10,000kg train at 5m/s2

50,000 N

Let’s try out Newton’s work for ourselves!

•Find this lab page:

Wacky Washers!

• Make sure leave each station set up and ready for the next group!

• Read the instructions and answer the questions in complete sentences!

Mouth to Mouth

• We are not using money, but an index card ;)

Newton’s Race

• Follow the directions on this one!! Fill out the data table and then answer the questions.

Newton’ s 3rd Law of Motion

• For every action there is an equal and opposite reaction

• All forces occur in pairs, and these two forces are equal in strength and opposite in direction.

•Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force.

•This is why you are not moving. There is a balanced force acting on you– gravity pulling down, your seat pushing up.

What does this mean?

Equal but Opposite

• Ice Skaters (push off of each other with equal force in opposite directions):

The speed at which the two skaters move depend on their masses. (the greater the mass the slower the speed)

What happens if you are standing on a skateboard and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.

Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

Think about it

Check Your Understanding• While driving down the road,

an unfortunate bug strikes the windshield of a bus.

• The bug hits the windshield and the windshield hits the bug.

• Which of the two forces is greater: • the force on the bug or

• the force on the bus?

Answer•Trick Question!• Each force is the same size.

• For every action, there is an equal ... (equal!).

• The fact that the bug splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction.

Check Your Understanding: Sports

• The racer’s foot is pushing on the starting block, and the starting block is pushing back

• When the runner pushes off, the motion is the result of action-reaction force pairs.

• The runner accelerates slowly at first and then faster and faster as the mass is carried along

More explanation:• The force on the foot equals the force on the

block. (Newton’s 3rd Law)

• Yet, acceleration depends on both force and mass. (Newton’s 2nd Law)

• Remember: acceleration and mass are inversely proportional. (Newton’s 2nd Law)

Check Your Understanding: Cars• According to Newton's First Law of Motion, an object at rest tends

to stay at rest and an object in motion tends to stay in motion, unless acted upon by an unbalanced force.

• A car will tend to either keep standing still or keep moving in a straight line at the same speed unless some unbalanced force makes it do otherwise.

• Examples of unbalanced forces are air resistance or the friction between tires and the road

Check Your Understanding: Cars

• Newton’s 2nd Law is F = ma

• If F = ma, then a = F/m. The acceleration of a car equals the sum of the external forces applied to the car, divided by the car's mass.

• For a car of any given mass, the more unopposed force is applied, the faster the car accelerates

• If the driver applies the brakes and produces enough friction to stop the car, then F = 0. In this case, acceleration also equals zero, because zero divided by anything equals zero.

Check Your Understanding: Cars• Newton’s 3rd Law states: for action there is an equal and opposite

reaction

• The force exerted by the tires on the road is matched by an equal and opposite force exerted by the road on the tires. If that force is enough to overcome the car's inertia, the car accelerates forward.

Review

• Newton’s First Law (Inertia):• Objects in motion tend to stay in motion and objects at rest

tend to stay at rest unless acted upon by an unbalanced force.

• Newton’s Second Law (F=ma):• Force equals mass times acceleration

(F = ma).

• Newton’s Third Law:• For every action there is an equal and opposite reaction.

Car Balloon Project

Let’s look at the project

Final due date is: March 2-3.

If Time – Match Rockets!