Motion and Forces

Newton’s Laws of Motion

•1st Law – Law of Inertia

•2nd Law – Law of Force and Acceleration

•3rd Law – Law of Action – Reaction

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

by these laws.

Motion

Many of the objects we encounter in everyday life are in motion or have parts that are in motion, even down to the atomic level. Motion is the rule of all objects, not the exception. The physical laws that govern the motion of these objects are universal, i.e. all the objects move according to the same rules.

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

Car crashes and seatbelts:

Physics of car crashes

Why wear Seatbelts?

Inertia

1. Inertia is the resistance of any physical object to any change in its state of motion (whether at rest or moving)

• Egg Drop Inertia

Explaining Inertia a bit more:

2. An object will “keep doing what it was doing”

unless acted on by an unbalanced force

3. If the object was sitting still, it will remain

stationary (still)

4. If it was moving at a constant velocity, it will

keep moving

5. It takes this (unbalanced) force to change the

motion of an object

6. Equilibrium is reached when the net force on an object is zero. An object is in equilibrium if it is at rest or moving at a constant velocity (velocity is speed in a certain direction).

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.

• 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

Why?

• The bowling ball is much heavier (it has more mass) than the golf ball. So it can resist a change in its motion more than the golf ball.

• It takes more force to move the bowling ball and also to stop it

What about changing it’s direction?• Again, the bowling ball has more mass and it took

more force to get it going. However, now that it has accelerated, changing the speed or direction is difficult!

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 these factors: 1. the types of surfaces

2. how hard the surfaces push together.

Starting a fire

Racing on ice

For example -

• Type of surface – ice versus concrete

• How hard the surfaces are pushed together

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. The larger object (more mass) has more gravity.

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 (the air resistance is eliminated)

Vacuum Video

Newton’s Laws –Inertia Lab

The lab is at the end of your notes

Lab Behavior expectations:

• Use Principled and on-task behavior, do not disrupt someone else’s learning opportunity

• Communicate with your group and work on the ATL skill of communication

• Be knowledgeable and work diligently to record data

• When asked to clean up the lab, put supplies back where you retrieved them from.

Lab Purpose:

• The object of this activity is to observe the properties of Newton’s First Law by attempting to remove the notecards from a tower of wooden blocks without causing enough of a disturbance to the tower’s equilibrium to cause it to tumble. This is similar to another famous inertia demonstration; the tablecloth trick.