Newton’s Second Law

How the apple fell from the tree…

The second Law

F= m x a

History Isaac Newton was one of the world’s great

scientists. He combined his ideas and the ideas of earlier scientists, such as Galileo, into a unified picture of how the universe works.

Isaac Newton explained the workings of the universe through mathematics. He formulated laws of motion and gravitation. These laws are math formulas that explain how objects move when a force acts on them. Principia, his most famous book, explained three basic laws that govern the way objects move. These three laws are known as Newton’s Laws.

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What is a force?

A force is a push or a pull on any object with a mass.

The second law States that the sum of all the forces pushing or pulling the object is directly proportional to how fast the object is speeding up.

History Newton’s First law of Motion

I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

Newton's Second Law of Motion:II. The relationship between an object's mass m, its

acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.

Newton's Third Law of Motion:III. For every action there is an equal and opposite

reaction.

What is a Force? If an object is

speeding up at a constant rate, then a force is being exerted on the object.

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What is the net force?This is the most powerful of Newton's three Laws, because it allows us to calculate how objects move: It allows us to relate what moves to why it moves.

It also answers how and why speeds change when forces are applied

How do we measure Force? Forces are measured using the units of

mass and acceleration combined. The SI standard unit of force is the Newton, N. In America, the standard unit of measure of weight is pounds, or lbs.

Weight Weight is a force acting

down on a mass Your weight is the force

that the earth exerts on you to keep you on the ground. The Earth exerts gravity that pulls you down.

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Example If an object has a mass of 10 kg and you

push on the object so that it speeds up uniformly at 10 meters per second every second the Force exerted on the object would be:

(10 kg) x (10 m/s2) = 100 Newtons

10 kg

If a rope attached to an object pulls with a force of 24 newtons and the mass of the object is 6 kg, how fast is the object speeding up?

6 Kg

24 N = (6kg) x acceleration

24 / 6 = acceleration = 4 m/s2

Atwood’s MachineAtwood’s machine

is a simple machine allows us to calculate gravity and mass.

Concept of Atwood’s MachineGreater weights will

cause the machine to accelerate in the direction of that weight.

What forces are acting on each

individual mass?

Tension in rope

Gravity is the only force “pulling” on the lager mass.

For the smaller mass, gravity pulls down and the rope pulls up.

Weight of smaller mass

Weight of larger mass

Concept of Atwood’s Machine

The system accelerates in the direction with the heaviest weight. Is this in concord with the laws of gravity and balance?

2 kg

1 kg

What about friction?Friction is defined as the resistant force between two objects’

nonsmooth surfaces

It always acts against an object.

What about friction? Imagine a wooden brick tied to a weight on

a horizontal table. If the weight is hung over the edge of the table, the wooden brick will accelerate in the direction the string is being pulled (horizontally).

Block

Table

String

What about friction?In this scenereo, gravity is not the only

force acting on the block-weight system. Friction is also exerting a force opposite that of gravity.

Block

Table

Weight exerted by gravity

String

Friction

What about friction? That means that friction is acting against

the pull of the string and the direction of the motion.

What can you conclude about the relationship between gravity and the net acceleration by adding in friction?

Practice Problems Try some problems on your own:

Practice Problems An apple that has a mass of

2 kg falls with a net acceleration of 2m/s2 from the largest tree in the world. Air resistance also acts on the apple. What is the total force acting on the apple? How does this compare to the apple’s weight on the ground? What if there was no air resistance?

Practice answers We can set up our equation using what we

know from Newton’s second law

Force = mass x acceleration

So:

Force = (2kg) x (2m/s2)

Force = 4N

Practice answers The weight of the object on the ground is:

(mass) x (gravity)This is equal to:

(2kg) x (9.81 m/s2)

We can conclude that the apple’s weight is greater than the force acting on the apple in

the air

Practice answers If there was no air resistance, the apple

would fall with an acceleration of gravity, 9.81 m/s2, and the force acting on the apple in the air would be equal to that of the apple’s weight on the ground.

Practice Problems Two monster dogs pull on an 80 kg rope.

Monster dog #1, Pepe, pulls to the left with a force of 12 N. Jumbo, monster dog # 2, pulls to the right with a force of 6 N. How fast is the rope speeding up and in what direction?

Practice answers

First we must find the direction of each force.

One dog pulls to the left, the other dog pulls to the right.

Pepe Jumbo

Practice answers Both dogs pull in opposite directs so the

total force is the result of one force subtracted from the other

Practice answers We can then set up our equation:

Force = 12N - 6N = mass x acceleration

_+

12N 6N

Practice answers We know the mass of the rope is 80 kg, so:

12N - 6N = (80kg) x a

6N = (80kg) x a

a=0.075 m/s2_+

12N 6N

80 kg

Questions and Discussion

References http://csep10.phys.utk.edu/astr161/lect/hist

ory/newton3laws.html

http://en.wikipedia.org/wiki/Image:Atwoodmachine.gif

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