Newton’s 1st Law

Forces and Motion

Think about it…

• What causes an object that is not moving to move?

• What causes an object to keep moving?

Newton’s Laws

• If this book started to move, you’d look for a cause, or a force.

• A force is a push or a pull. Anything that makes an object move, or stop moving.

• Aristotle thought this was reasonable. He thought that a force had to maintain motion.

Aristotle 384 BC – 322 BCGreek Philospher

Example:• A cannon ball in a cannon is at rest. The

gases ignite and expand and force the ball out.

• Turn and Talk (elbow partner):• What keeps the ball moving after it leaves

the cannon? • Are the gasses still working on the ball?

Another example:

• When you throw a baseball. • Turn and Talk (table group):• What keeps the ball moving?

Is your hand still working on the ball?

No, inertia is what keeps objects moving• Inertia - Another word for “laziness” or the

resistance of an object to change states of motion.

• If an object is moving, it “wants” to stay moving.

• If an object is at rest, it “wants” to stay at rest.

Inertia is a property of mass

• Imagine trying to catch a thrown bowling ball. • Now imagine trying to catch a volley ball.

• The bowling ball has more mass, and therefore has more inertia.

• It takes more force to overcome the inertia of the bowling ball.

Inertia Demonstrations

• Table Cloth• Bowl of Cheerios• Pen vs Pencil• Piece of Paper under a cup

Newton’s 1st LawSometimes called the Law of Inertia

• An object at rest will stay at rest, and an object in motion will stay in motion unless acted upon by an outside force.

Sir Isaac Newton 1642-1727English physicist, mathematician,

astronomer, natural philosopher, alchemist, and theologian

Balanced Forces

• So…According to Newton, an object will want to stay at rest or keep moving at the same speed in the same direction.

• Balanced force – an object at rest, or moving at a constant velocity (same speed in the same direction), or equal forces in opposite directions.

Unbalanced ForcesAny force that changes the direction, speed or

motion can act as an unbalanced force on an object:

• A kick• A pull• A twist

Outside Forces

Unbalanced force – causes a change in motion of an object.

• It can change the speed (make it go slower or faster)• It can change the direction it is moving.• It can make a resting object move.• It can make a moving object stop.

So….

• In a vacuum, like space, a baseball thrown would go on forever.

• But what happens on Earth?

• What are the outside forces that stop the baseball?

AND

What are these unbalanced forces that act on an object in motion?

Other Outside Forces(that cause an unbalanced force)

• Friction – a force that opposes motion

• Gravity - a force of attraction between two objects.

Friction

The resistance of air on the baseball slows the ball, creating an unbalanced force.

http://www.physics.umd.edu/lecdem/services/demos/demosf5/f5-32a.htm

Friction

From Glencoe Physical Science

• Imagine if you were trying to move this box on a sheet of ice, instead.

• It would be easier because there would be less friction.

Gravity• The pull of the earth on the baseball keeps the

ball from exiting the atmosphere.

• It takes a lot of force to overcomegravity of large objects like Earth.

Newtons’s 1st Law and You

Don’t let this be you. Wear seat belts.

Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.

Outside Forces

Unbalanced force – causes a change in motion of an object.

• It can change the speed (make it go slower or faster)• It can change the direction it is moving.• It can make a resting object move.• It can make a moving object stop.

Think about it…

• What causes an object that is not moving to move?

• What causes an object to keep moving?• What new learnings help answer these

questions?

Recap of 1st Law

• Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless.

These pumpkins will not move unless acted on by an unbalanced force.

Recap of 1st Law

• Once airborne, unless acted on by an unbalanced force (gravity and air friction), it would never stop!

Recap of 1st Law

• Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.