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NSTA Web Seminar:

Force and Motion: Stop Faking It!

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Bill RobertsonDecember 5, 2006

NSTA Web SeminarForce and Motion: Stop Faking It!

What’s going to happen when he drops the paper clip and the box of paper clips?

They will land at the same time

The heavier object—the box

—will hit first

There’s no way to tell what will

happen

Objects don’t always hit the ground at the same time.

A geologic hammer in Apollo 15’s astronaut David Scott's right hand and a falcon feather in his left hand reached the surface of the moon at the same time.

Okay, so if you ignore air resistance, all objects land at the same time. Why?

The Earth’s gravity exerts the same

force on all objects, so they end up with the

same acceleration.

The Earth’s gravity exerts different

forces on different objects, but they end up with the

same acceleration.

The mass of an object has

nothing to do with how it is affected by

gravity.

change in velocity

time for the change

If you ignore air resistance, all objects dropped from the same height land at the same time. It turns out they have the same acceleration. Near the surface of the Earth, that acceleration is represented by the letter g.

g = 9.8 meters per second per second = 32 feet per second per second

Remember that acceleration =

Apply Newton’s second law to a falling object

F = ma

Force of gravity = (mass of object)(accleration of object)

Fgrav = mobjectg

If we go to a larger mass, what should happen to the size of the girl in this

drawing?

If we go to a larger mass, what should happen to

the size of the girl in this drawing?

She should get smaller

She should get larger

She should stay the same size

As the mass gets larger, meaning the girl gets larger, what happens to the acceleration? Remember that

we’re talking about objects falling under the influence of gravity.

The acceleration gets larger

The acceleration gets smaller

The acceleration

stays the same

If the mass gets larger and the acceleration stays the same, what must happen to the gravitational

force in order to keep things in balance?

The gravitational force must get

larger

The gravitational force must get

smaller

The gravitational force must

remain the same.

If the mass gets larger and the acceleration stays the same, what must happen

to the gravitational force in order to keep things in

balance?

Objects with different masses experience different

gravitational forces. This happens in such a way that all

objects have the same acceleration.

Objects with different masses experience different gravitational

forces. This happens in such a way that all objects have the same

acceleration.

Why?

“When I pull the wagon, the ball rolls to the back of the wagon. And when I’m pulling it along, and I suddenly stop, the ball rolls to the front of the wagon. Why is that?”

‘That, nobody knows,’ he said. ‘The general principle is that things which are moving tend to keep on moving, and things which are standing still tend to stand still, unless you push them hard. This tendency is called inertia, but nobody knows why it’s true.’ “Now, that’s a deep understanding.”

-- R.P. Feynmann

A definition of weight

Weight is defined as the force the Earth’s gravity exerts on an object.

What’s the difference between mass and weight?

Mass and weight differ by a conversion

factor, such as 2.2 pounds per

kilogram.

When you head to the moon, your weight changes but your mass

remains the same.

Mass is a measure of inertia, and is the quantity that goes on the right side of F=ma. Weight is a

force, and it goes on the left side of

F=ma.

F = maweight mass

For an object near the surface of the Earth:

F = ma

The acceleration is g, so

Weight = mg

Away from the surface of the Earth, we need a different expression for gravitational force. A

general expression that applies to the gravitational force between any two objects is:

G = a very tiny number

m1= mass of one object

m2= mass of other object

r = distance between centers of spherical objects.

1 22grav

mmF G

r

This is not an expression of F=ma!

1 22grav

mmF G

r

Let’s write F=ma for an object near the Earth’s surface

Fgravity = ma

2

Earth objectobject object

Earth

m mG m a

r

Earth objectm mG

2 objectEarth

mr

objecta

2Earth

objectEarth

mG ar

The acceleration of the object does not depend

on the mass of the object

When is an object weightless?

When is an object weightless?

When it’s in orbit around the

Earth

When it’s in free-fall toward

the Earth

Objects are never

weightless

Weight is the force that the Earth exerts on an object.

This force is never equal to zero.

1 22grav

mmF G

r

When in free-fall, you feel weightless

Orbiting is free-fall

National Science Teachers AssociationGerry Wheeler, Executive Director

Frank Owens, Associate Executive Director Conferences and Programs

Al Byers, Assistant Executive Director e-Learning

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