1. Oregon State University PH 106, Lecture #6
Newton’s Laws (a quick summary/review)
Every object has inertia (mass)—the property that causes it to
maintain a constant velocity unless acted upon by a net force.
2. When an object is acted upon by a net force, it accelerates in the direction of that force, at a rate proportional to the strength of that
force and inversely proportional to the object’s mass. (F = ma)
3. A force always involves two objects. It acts on each object with the same strength but in opposite directions.
4/14/17
Oregon State University PH 106, Lecture #6

2. Oregon State University PH 106, Lecture #6
Common Forces
• Tension force (pull)
• Normal force (push)
• Friction force (drag)
• Gravitational force (weight)
4/14/17
Oregon State University PH 106, Lecture #6

3. Oregon State University PH 106, Lecture #6
Tension Force
One object pulls on another object—
via direct “contact.”
4/14/17
Oregon State University PH 106, Lecture #6

4. Oregon State University PH 106, Lecture #6
Normal Force
A surface pushes on another object—
in a direction perpendicular to the surface.
4/14/17
Oregon State University PH 106, Lecture #6

5. Oregon State University PH 106, Lecture #6
Friction Force
A surface pushes on another object—
in a direction parallel to the surface.
This can happen even when one object
is moving with respect to the other.
4/14/17
Oregon State University PH 106, Lecture #6

6. Gravitational Force (“weight”)
Gravity here at the earth’s surface accelerates all masses equally (Galileo’s famous demonstration). That is, it exerts a force (called weight) on each object that is proportional to the object’s mass.
But each force must involve two objects. What’s the other object?
The farther away you are from the other object, the weaker the gravi-tational force on you (and on it). Weight is inversely proportional to the square of the distance between the centers of the two masses.
4/14/17
Oregon State University PH 106, Lecture #6

7. Common Combinations of Forces
See if you can identify all forces acting on Object A in these situations.
-- A book lying at rest on my hand.
-- A book moving up (or down) at constant speed on my hand.
-- A book sliding across the table, slowing.
-- Sir Edward Horse, pulling a cart, moving steadily faster.
-- A ball falling freely straight down.
-- A ball tossed across a room.
4/14/17
Oregon State University PH 106, Lecture #6

8. Oregon State University PH 106, Lecture #6
Projectiles
The motion, the forces on, and the resulting acceleration of, any object along one axis (say, east/west) can be analyzed entirely separately than along another axis (say, up/down). Look, for example, at a projectile —any object moving under the influence of only gravity, such as:
-- A ball dropped straight down.
-- A bullet (after it’s left the gun).
-- The moon, the earth, all planets, etc...
4/14/17
Oregon State University PH 106, Lecture #6

9. Oregon State University PH 106, Lecture #6
Consider a ball thrown horizontally across the room, from west to east.
After it is released, what forces act on it eastward or westward?
After it is released, what forces act on it upward or downward?
If you figure out how an object would move in response to the forces in each direction separately, you can describe how it actually moves.
(The same goes for a drop of water—as in the lab next week.)
4/14/17
Oregon State University PH 106, Lecture #6