Unit 1 Lessons (Forces, Equilibrium, Inertia)

Unit 1 Lessons

by Mr. Castillo

Multiple Choice

What do the arrows or vectors represent?

• forces acting on the object

• all the acceleration acting on the object

• all the ways an object can move

Multiple Choice

What is the upward force called ?

• Force of Gravity

• Force of Fricton

• Normal Force

• Force Applied

Multiple Choice

What is the downward force?

• Force of gravity

• Force applied

• Force of Friction

• Normal Force

Multiple Choice

Can an object still be moving if all forces are balanced?

• yes, if velocity is constant

• no, is in a state of equilibrium

Balanced and Unbalanced Forces

A Force

A force is a push or a pull that causes a change in an objects motion. Forces are measured in units called Newtons.

Forces work in pairs

There are always two or more forces that affect an objects motion. The size of the forces determines the motion an object will have.

Balanced Forces

When two forces acting on an object are equal and in opposite directions then the forces are balanced and the object will not move. The forces "cancel" each other out.

Unbalanced Forces

When two forces acting on an object are in opposite directions but are not equal then the forces are unbalanced and the object will move. The forces don't "cancel" each other out so there is movement.

Motion of an object

When forces are unbalanced they cause a change in an objects motion. Unbalanced forces can cause objects to start or stop moving, speed up or slow down, or change directions.

Multiple Choice

What affect do forces have on an object?

• They can change the motion of an object

• They can make objects bigger

• They can make objects change from one form into another

• They can make objects smaller.

Multiple Choice

What type of force causes objects to not move?

• Balanced forces

• Unbalanced forces

Multiple Choice

What type of force causes objects to move?

• Balanced forces

• Unbalanced forces

Multiple Choice

When the forces acting upon an object are balanced then the object

• will not move

• will move

Multiple Choice

When the forces acting upon an object are unbalanced then the object

• will not move

• will move

Forces in action

The motion of a baseball that gets hit by a batter changes because of forces.

How forces affect the motion of a baseball

When a batter hits a ball the ball rises into the air and at point A it starts to SLOW DOWN because of the force of gravity.

How forces affect the motion of a baseball

At point B the ball CHANGES DIRECTIONS. It was moving upward but at point B it has changed directions and is moving downward.

How forces affect the motion of a baseball

At point C gravity pulls the ball towards the center of the Earth and it SPEEDS UP.

How forces affect the motion of a baseball

At point D the ball hits the ground and once it stops moving it is experiencing balanced forces.

Multiple Choice

What is the balls motion at point A?

• Slows down

• Speeds up

• Changes directions

• Hits the ground

Multiple Choice

What is the balls motion at point B?

• Slows down

• Speeds up

• Changes directions

• Hits the ground

Multiple Choice

What is the balls motion at point C?

• Slows down

• Speeds up

• Changes directions

• Hits the ground

Multiple Choice

What is the balls motion at point D?

• Slows down

• Speeds up

• Changes directions

• Hits the ground

Inertia

INERTIA: KEEP IT MOVING

FORCE & MOTION

Phenomena

• At Newton’s Skate Park, Lauren is pushing her twin cousins, Jonathan and Cody, on her skateboard.

• She has to push hard to get the skateboard started, but once it begins moving, it takes much less effort to keep it rolling over the smooth, flat pavement.

• In fact, when Lauren tries to stop the rolling skateboard, it takes just as much effort to stop it as it did to start it rolling in the first place.

Open Ended

Why is it harder to start and stop the skateboard than it is to keep it rolling?

Inertia

• Inertia is the tendency of an object to resist a change in its motion. All objects have inertia, whether they are stationary or moving.

• Inertia explains Newton’s first law of motion, which states that an object at rest will remain at rest and an object in motion will stay in motion unless it is acted on by an unbalanced force.

• That’s why Newton’s first law of motion is sometimes called the law of inertia.

Open Ended

You probably don’t realize it, but you experience inertia all the time, and you don’t have to ride a skateboard. For example, think about what happens when you are riding in a car that stops suddenly. Your body moves forward on the seat and strains against the seat belt. Why does this happen?

Inertia and Mass

• The inertia of an object depends on its mass.

• Objects with greater mass also have greater inertia.

• It would be easier for Lauren to push just one of her cousins on her skateboard than both of them. With just one twin, there would be only about half as much mass on the skateboard, so there would be less inertia to overcome

Multiple Choice

Which rolling item would be harder to stop?

Multiple Choice

Which object would be harder to move?

Open Ended

What if Lauren didn’t stop the skateboard in this way? If it remained on a smooth, flat surface, would it just keep rolling forever?

**Think about what other forces are acting on the skateboard**

Let's Talk About It !

The inertia of the moving skateboard would keep it rolling forever if no other unbalanced force opposed its motion. However, The force of friction opposes the motion of the rolling skateboard and would eventually bring it to a stop without any help from Lauren.

Friction opposes the motion of all moving objects, so—like the skateboard—all moving objects eventually stop moving even if no other forces oppose their motion.

Open Ended

Later that day, Jonathan rode his skateboard and did some jumps. You can see him in the picture below.

When he’s in the air, there is no rolling friction between his wheels and the ground, but another unbalanced force is acting on the skateboard and changing its motion.

What force is acting on the skateboard when it is in the air above the ground? And how will this force change the skateboard’s motion?

Let's Talk About It!

The force of gravity is acting on the skateboard. It will pull the skateboard back down to the ground. Once it’s on the ground, friction will slow its motion.

Summary

• Inertia is the tendency of an object to resist a change in its motion. Because of inertia, a resting object will remain at rest, and a moving object will keep moving.

• Objects with greater mass have greater inertia.

• To change the motion of an object, inertia must be overcome by an unbalanced force acting on the object.

Multiple Choice

True of False? An object's inertia is effected by its mass.

• False

• True

Multiple Choice

the tendency of an object to resist a change in its motion

• acceleration

• inertia

• mass

• frame of reference