NEWTON’S FIRST LAW

I could sure go for some Fig Me’s and a glass of milk!!!

Demonstration 1: An “Apple” Sitting on a Table

Draw an FBD for this apple

What is the net force on this apple?

FN – Normal Force

Fg or w - Weight

Fnet = FN – w = 0-N

What will happen to this apple during this demonstration?

The apple will sit forever --- No net force (equilibrium) no change in motion!!

Demonstration 2:Pool Hustle

Draw an FBD for the quarter after the pool ball has been knocked from under it.

What is the net force on the quarter?

Fg or w - weight

Fnet = w

What will happen to the quarter after the pool ball is hit?

What happened to the quarter?

It fell “straight” down after the ball was knocked out from under it.

It only changed its motion in the direction of its net force.

What will happen to the dishes when the tablecloth is pulled?

Demonstration 3:Dishes on a Table Cloth

What happened?

The tablecloth moved, but the dishes didn’t (much)!!!!

Why?

The tablecloth had the net force so its motion changed, but the dishes had no net force so their motion didn’t

change!!!!!

We saw three demonstrations that involved an object at rest with no net force acting on

it.

In each demonstration we saw that the object remained at rest.

This is the first part of Newton's First Law:

If an object or system is at rest and has no net force acting on it (it is in equilibrium), then the object or system will remain at

rest.

Newton’s First Law of Motion

Newton’s First Law of MotionIn our lab we repeated a thought experiment first

done by Galileo.

In this experiment we determined that if an object is in motion and it has no net force acting on it,

then it will continue moving at the same speed in the same direction forever.

We could also say it will have a constant velocity or that it will keep its motion.

This is the second part of Newton's First Law:

If an object or system that is in motion has no net force acting on it then it will continue with that

motion forever. It will travel at a constant velocity.

Newton’s First Law of Motion

An object or system at rest will remain at rest and an object or system in

motion will remain in motion with a constant velocity as long the object or system has no net force acting on it

(it is in equilibrium).

Applying Newton's First LawA car is driving along a straight horizontal road. The car's mass is 1200-kg. The engine is pushing the car forward with 250-N of force and the friction between the car and the road is 50-N. There is no air resistance.

When the car reaches 65-mph, it encounters a steady wind that pushes it back with 200-N of force.

a) Draw a FBD for this car. {Hint: WAIT!!!! It is NORMAL to forget one of the forces acting on the car.}

b) What is the net force on the car?

c) What should be happening to the car's speed?

w = mg = (1200)(9.8) = 11,760-N

FN

200-N

250-N50-N

Since the normal force equals the weight and the horizontal forces cancel out. The net force on the car is 0-N

= w = 11,760-N

Since the car is in equilibrium, Newton’s First Law says it will not change its motion, so it will continue moving at 65-mph until the wind stops blowing.