Section 3

Motion and Forces

1. A marble is placed at the top of a smooth ramp. What happens to the marble? What force causes this?

2. A marble is rolling around in the back of a small toy wagon as the wagon is pulled along the sidewalk. When the wagon is stopped suddenly by a rock under one of the wheels, the marble rolls towards the front of the wagon. Why does the marble keep going when the wagon stops? (Hint: Consider what it takes to change the velocity of the wagon and the marble.)

3. If you dropped a flat uncrumpled sheet of notebook paper and a similar piece of notebook paper that was crushed into a ball from the same height, which would reach the floor first? Why are the forces on these two pieces of paper different?

The concept of force explains many occurrences in our everyday lives. From your own experience, state what will happen in the following situations:

Bellringer/10.3 Notes

What is FORCE?

Are these people

exerting FORCE?

Which people are exerting a balanced force?

WHY?

BALANCED

UNBALANCED

A. Net Force – Combination of ALL forces acting on an object.

1. Net force greater than zero equals acceleration of that object.

Why is there a net force?

I. Unbalanced Forces

Are there unbalanced forces in the picture below?

Will he be able to move the box?

Unbalanced forces are 2 & 4

II. Balanced Forces

The men are the same size and have the same strength.

What will happen?

A. Balanced forces DO NOT change motion

Man’s mass pushing down

Force (Earth) pushing up on the

man

Are there balanced forces in the picture below?

Balanced forces are 1 & 3

III. FrictionA. Friction is an unbalanced force

1.Friction is caused by tiny bumps and ridges on a surface bumping into the tiny bumps and ridges on another surface.

What FORCE helps these cars going around the corner?

The force of friction is greater than the force of the car’s motion.

What FORCE prevents this car from rolling down the hill?

Friction opposes the force of gravity

Why do they “sweep” the ice in front of the stone?

IV. Static & Kinetic Friction

A. Static Friction – two objects not moving.

1. Takes a greater force to oppose static friction.

B. Kinetic Friction – friction between moving surfaces.

1. Takes less force than static friction.

2. Two kinds of kinetic friction:

a. Sliding friction

b. Rolling friction

3. Air resistance – friction between an object and the air passing over it.

When the force of gravity = the force of air, no acceleration

takes place.

4. Air resistance increases as speed increases.

a. More air per second must pass by due to the increase of speed.

b. Size & shape also affects air resistance.