Newton’s Newton’s Second LawSecond Law

TEKS 8.6A demonstrate and calculate how TEKS 8.6A demonstrate and calculate how unbalanced forces change the speed or unbalanced forces change the speed or direction of an object’s motiondirection of an object’s motion

TEKS 8.6C investigate and describe TEKS 8.6C investigate and describe applications of Newton’s law of force and applications of Newton’s law of force and accelerationacceleration

Net Force, Mass Net Force, Mass and Acceleration and Acceleration

Isaac Isaac NewtonNewton discovered one of the discovered one of the most important laws most important laws

of nature; the of nature; the relationship between relationship between

net forcenet force, , massmass (inertia) and (inertia) and accelerationacceleration..

Newton’s second law Newton’s second law states that the net force states that the net force acting on a mass causes acting on a mass causes the the massmass to accelerateto accelerate in in the the directiondirection of the net of the net

force.force.

A shorthand way of A shorthand way of writing this law is:writing this law is:

f = maf = ma

f = force in newtons (N)f = force in newtons (N)m= mass in kilograms (kg)m= mass in kilograms (kg)a = acceleration in meters per a = acceleration in meters per

second per second (m/ssecond per second (m/s22))

Objects with Objects with moremore mass are more mass are more

difficult to difficult to accelerateaccelerate. .

If the If the samesame force is applied force is applied to two objects to two objects with different with different masses, the masses, the one with the one with the smaller masssmaller mass will accelerate will accelerate

moremore..

More mass More mass means less means less

acceleration, acceleration, unless a unless a larger larger forceforce is applied. is applied.

small mass

leads to

large mass

leads to

large acceleration

Same Forces

small acceleration

small mass

can lead to

large mass

same acceleration

small force

large force

Different Forces

Falling Objects and Falling Objects and Newton’s 2nd LawNewton’s 2nd Law

Galileo Galileo GalileiGalilei, the , the

Italian Italian physicist and physicist and astronomer, astronomer,

studied studied falling falling

objects. objects.

He found that He found that when two when two objects of objects of different different

massesmasses are are dropped, they dropped, they

fell at the fell at the same same raterate. .

This was never fully This was never fully understood until Isaac understood until Isaac Newton announced his Newton announced his second law of motion. second law of motion.

In the case of free In the case of free falling objects, the falling objects, the forceforce is equal to the is equal to the weightweight of the object, of the object, which is determined which is determined

by the acceleration of by the acceleration of gravity (gravity (9.8 m/s9.8 m/s22). ).

Try dropping a book Try dropping a book and a ball of paper. and a ball of paper. See what happens. See what happens.

Now try dropping a Now try dropping a piece of paper and a piece of paper and a

book. book.

Most likely you will see Most likely you will see that the paper falls more that the paper falls more

slowlyslowly. .

Remember the book Remember the book has more has more weightweight than than the paper, so it has the paper, so it has more more forceforce to resist to resist

friction from the air. friction from the air.

Try dropping the book Try dropping the book with the paper against the with the paper against the

lower surface of the lower surface of the book. book.

What happens? What happens?

Now try dropping the Now try dropping the book with the paper book with the paper on top of the book. on top of the book.

How will the How will the accelerations of accelerations of the book and the book and the paper the paper compare?compare?

Will they Will they separate and separate and

fall fall differently?differently?

How much force will be How much force will be required to move the required to move the

object in each picture? object in each picture?

2 m/s2 m=50kg

f =

m=2000 kg .05 m/s2

f=

100N100N

100N100N

What direction are the What direction are the forces applied in each forces applied in each

picture? picture?

2 m/s2 m=50kg

f =

m=2000 kg .05 m/s2

f=

leftleft

rightright

Will the rock and the van Will the rock and the van accelerate at the same accelerate at the same

rate? rate?

2 m/s2 m=50kg

f =

m=2000 kg .05 m/s2

f=

No, the rock will No, the rock will accelerate faster.accelerate faster.