Objective

• SWBAT use Newton’s first and second laws to identify and explain changes in the velocity of objects.

The man who follows the crowd will usually get no further than the crowd. The man who walks alone is likely to find himself in places no one has ever been.—Albert Einstein.

Balanced Forces No acceleration

Newton’s First Law

Check For Understanding• Which statement below correctly summarizes

Newton’s first law?A. When a force acts on an object, the object accelerates

in the direction of the force.B. When the net force acting on an object is zero, the

object stays at rest, or if the object is already moving, it continues to move in a straight line with a constant speed.

C. When unbalanced forces act on an object, the object will not move.

D. When balanced forces act on an object, the object will accelerate.

Supertankers illustrate the principal of inertia; their engines are cut off

a mile or more before the tanker enters port and it still coasts right

on in.

You are driving in a car with no seat belt on. You see a family of ducks and slam on your brakes to avoid hitting them. Since you are not wearing your seat belt, you fly out of the car (remain in motion) until you hit the ground (an unbalanced force).

AHH!

Ouch

In motion… At rest…

Unbalanced Forces Acceleration

Forces are either balanced, or unbalanced

When forces are equal, no acceleration

occurs.

Forces are either balanced, or unbalanced

But when they are unbalanced, that’s

when objects accelerate

Forces are either balanced, or unbalanced

But when they are unbalanced, that’s

when objects accelerate

Forces are either balanced, or unbalanced

But when they are unbalanced, that’s

when objects accelerate

Forces are either balanced, or unbalanced

But when they are unbalanced, that’s

when objects accelerate

Newton’s Second Law

Check For Understanding

• If an object is accelerating, which statement below is true?

A.The object is being acted on by balanced forces.B.The object is being acted on by unbalanced

forces.C. The object is not being acted on by any forces.D.You do not know whether the forces are

balanced or unbalanced.

If F = ma…

• The more force you put in, the more acceleration an object will have.

If I throw the ball really hard

(F = 10 N), then the ball will

go really fast, (A = 20 m/s2)

If I throw the ball softly

(F = 1 N), then the ball will

go slowly, (A = 2 m/s2)

Newton’s Second Law

The more mass an object has, the more force you have to put in to move it.

It would be hard to move an elephant (more force), because he is so heavy (more mass).

It would be easy to move a baby duck (less force), because he is so light (less mass).

Newton’s Second Law

• If you exert an equal force on two objects, the lighter object (the one with less mass) will accelerate faster.

F = 10 N

Check For Understanding

• What property of a stalled car determines how much effort is required to move it?

A.HeightB.MassC.VolumeD.Length

Check For Understanding• Elrick pushes a textbook and a file cabinet. The

textbook has a mass of 10 kg. The file cabinet has a mass of 300 kg. He pushes them both with a force of 50 N. Which box will accelerate at a faster rate?

A.The textbookB.The file cabinetC. They will accelerate at the same speedD.Neither will accelerate – they will move with

constant velocity.

Check For Understanding• Tanesha is mad at Maegan and Jahina, but she is

REALLY mad at Maegan. She pushes Maegan with a force of 40 N, and she pushes Jahina with a force of just 5 N. If Jahina and Maegan have the same mass, who will accelerate faster after being pushed?

A.JahinaB.MaeganC. They will accelerate at the same speedD.Neither will accelerate – they will move with

constant velocity.