How does an object move when a force acts on it?

Connects FORCE, MASS, ACCELERATION

Newton’s Second Law of Motion

(Textbook pages 400-402)

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Newton's 2nd Law of Motion

Recalling Newton's first law of motion, how will an object be affected when the forces acting on it are not in equilibrium?

What is the cause of this change?

The object will accelerate.

The net force (the applied force that is not canceled by other forces) on the object causes the acceleration.

Newton's Second Law (cont.)

How do you expect the acceleration to depend on the applied force? The greater the applied force, the

greater the acceleration

What property of an object determines how much acceleration a given force will cause? The inertia of the object (as measured

by its mass)

More precisely, Newton’s 2nd Law in terms of FORCE:

maFNet Where FNet is the “net force” or the combination of all forces on the system

So, what exactly is Fnet? A few examples…

1.

2.

20 lbs20 lbs Fnet = _____

Fnet = _____5 lbs 20 lbs

(like a “tug of war”… the object will accelerate to the _______!)

40 lbs

15 lbs

right

Fnet = _____20 lbs20 lbs3.

So… What is this “object” doing?

According to the 2nd Law (Fnet = ma), if the Fnet is zero, the acceleration must be ______. So the “object” is either at _____ or moving at ________ __________.

Only “__________ forces” produce accelerations.

0 lbs

zerorest

constant velocity

unbalanced

• In the SI (metric) system .... the ____________ • In the English system …. ___________

(One N is equal to 0.225 lb, or 1 lb equals 4.45 N.)

So, what’s a Newton?

1 N is the amount of force required to accelerate a 1 kg object at ___ m/s2….

1 N = (1 kg)(_____)

Newton (N)

Pound (lb)

1

1 m/s2

2nd law in terms of ACCELERATION:

A. More Force = More Acceleration

What type of relationship is this?

The larger the net force acting on an object, the greater its

acceleration!2X the force -------- 2X the acceleration

3X the force ----------- 3X the acceleration

(1/2) half the force ------------- 1/2 the acceleration

Practice Problem:

a. You push on a crate with 100 N of force. If the friction force is 100 N, does the crate accelerate?

b. Does that mean that the crate is not moving?

Practice continued…

d. A friend helps push with an additional 150 N. By how much does acceleration increase?

c. Now push with 150 N; friction is still 100 N. Does the crate accelerate?

B. More Mass = Less AccelerationMass: quantity of matter in an object (kg)

What type of relationship is this?

2X the mass -------- _____ the acceleration

3X the mass ----------- _____ the acceleration

(1/2) half the mass --------- _____ the acceleration

Mass & Acceleration

For a given force, the greater the mass, the smaller the acceleration

1/2

1/3

2x

?????

?????

?????

?????

IMAGINE THAT THERE IS NO FRICTION

Newton's Second Law (cont.)

Given everything you now know…

using ‘a’ for acceleration, ‘F’ for net force, and ‘m’ for mass,

…how would you write Newton's 2nd law as an equation?

Newton's 2nd Law - Defined “The acceleration of an object is directly

proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object.”

Inversely Proportional? Acceleration decreases as mass increases

(for same amount of force)

a = Fm

Newton’s Second Law

Solving for FORCE, MASS, ACCELERATION

a

F

m

NEWTON’S SECOND LAW