What keeps the Earth spinning?

Now that we have a better understanding of motion (speed, velocity, acceleration, etc.) we need to investigate the ideas behind what actually causes motion and what prevents it

Objectives for today• Recognize that force is needed to

change an object’s motion• Explain Newton’s 1st law• Describe how inertia and mass are

related• Based on this knowledge, be able

to answer the question at the top of the slide!

Suppose you are playing mini golf

What action must you take to make the ball move toward the hole?

You would have to hit the ball with the golf club to get it rolling• The club applies a force to the ball• This force is what changes the ball from being at rest to being in motion

What is force?

A force is a push or pull, or any action that has the ability to change motionWhat will the golf ball do if you do nothing to it?• It will just continue to sit there motionless!• The ball will stay at rest until you apply a force to set it in motionOnce the ball is moving…..• It will continue to move in a straight line at a constant speed, unless

another force changes its motionForces can be used to increase/decrease speed of an object, or to change the direction in which an object is moving

Force is required to change motion

There can be no change in motion without the presence of a force• Anytime there is a change in motion, a force must exist, even if you cannot

immediately recognize the force

Example:When a rolling ball hits a wall and bounces, its motion changes rapidly.• What is that change in motion caused by? Caused by the wall exerting a force that changes the direction of the ball’s motion

Forces, mass, and inertiaLet’s keep playing golf. Once the ball is moving, how can you stop it?• Only way to stop it is to apply a force in a direction opposite its motionIn general, objects want to keep doing what they are already doing. This is known as Newton’s 1st law

Newton’s 1st law:• An object at rest will stay at rest and an object in

motion will continue in motion with the same speed and direction UNLESS acted on by a force

InertiaAre all object’s super easy or really hard to move?• Some object’s resist changes in motion better than others• Inertia is the property of an object that resists changes in its

motion

Bowling ball has more inertia than golf ball

The greater an object’s inertia, the greater the force needed to change its motion• Because inertia is an important idea,

Newton’s 1st law is sometimes called the “law of inertia”

Units of ForceAlthough in America we like to express forces in units like “pounds” (lbs), scientists prefer to use the unit “Newtons” (N) because it is much more universal1 N = amount of force needed to cause a mass of one kilogram to speed up by one meter per second each second.

The Net ForceMultiple Forces:• When you hit a golf ball, the force from the club is not the only force that

acts. What other forces are present that affect the ball’s motion?Forces act together:• The motion of objects changes in response to the total force acting on the

object, including gravity and any other force that is presentNet Force:• Adding up forces can be different from simply adding numbers because

the directions of forces matter• For this reason, the term net force is used to describe the total of all forces

acting on the object

So what keep the Earth spinning?

NOTHING!!!!!!!!

The earth has been spinning for over 4.5 billion years and won’t speed up or slow down unless another force is applied to it • Great example of Newton’s 1st

Law