Newton’s First Law

Free Body Diagrams

Newton’s First Law States:

• An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

F=maIn fact, it is the natural tendency of objects to resist changes

in their state of motion. This tendency to resist changes in their state of motion is described as inertia.

Mass as a Measure of the Amount of Inertia

  All objects resist changes in their state of motion. All objects have this tendency - they have inertia. But do some objects have more of a tendency to resist changes than others? Absolutely yes! The tendency of an object to resist changes in its state of motion varies with mass. Mass is that quantity which is solely dependent upon the inertia of an object. The more inertia which an object has, the more mass it has. A more massive object has a greater tendency to resist changes in its state of motion.Suppose that there are two seemingly identical bricks at rest on the physics lecture table. Yet one brick consists of mortar and the other brick consists of Styrofoam. Without lifting the bricks, how could you tell which brick was the Styrofoam brick? You could give the bricks an identical push in an effort to change their state of motion. The brick which offers the least resistance is the brick with the least inertia - and therefore the brick with the least mass (i.e., the Styrofoam brick).                       A common physics demonstration relies on this principle that the more massive the object, the more that object tends to resist changes in its state of motion. The demonstration goes as follows: several massive books are placed upon a teacher's head. A wooden board is placed on top of the books and a hammer is used to drive a nail into the board. Due to the large mass of the books, the force of the hammer is sufficiently resisted (inertia). This is demonstrated by the fact that the hammer blow is not felt by the teacher. (Of course, this story may explain many of the observations which you previously have made concerning your "weird physics teacher.") A common variation of this demonstration involves braking a brick over the teacher's hand using the swift blow of a hammer. The massive bricks resist the force and the hand is not hurt. (CAUTION: do not try these demonstrations at home.) 

Forces Don't Keep Objects Moving

• Newton's first law of motion declares that a force is not needed to keep an object in motion. Slide a book across a table and watch it slide to a rest position. The book in motion on the table top does not come to a rest position because of the absence of a force; rather it is the presence of a force - that force being the force of friction - which brings the book to a rest position. In the absence of a force of friction, the book would continue in motion with the same speed and direction - forever! (Or at least to the end of the table top.) A force is not required to keep a moving book in motion. In actuality, it is a force which brings the book to rest.

Free Body Diagrams

Free-body diagrams are used to show the relative magnitude and direction of all forces acting on an object.

This diagram shows four forces acting upon an object. There aren’t always four forces, For example, there could be one, two, or three forces.

Show Animation

Problem 1A book is at rest on a table top. Diagram the

forces acting on the book.

Problem 2An egg is free-falling from a nest in a tree.

Neglect air resistance. Draw a free-body diagram showing the forces involved.

Gravity is the only force acting on the egg as it falls.

Problem 3A flying squirrel is gliding (no wing flaps) from

a tree to the ground at constant velocity. Consider air resistance. A free body diagram for this situation looks like…

Gravity pulls down on the squirrel while air resistance keeps the squirrel in the air for a while.

Problem 4A rightward force is applied to a book in order to

move it across a desk. Consider frictional forces. Neglect air resistance. Construct a free-body diagram. Let’s see what this one looks like.

Note the applied force arrow pointing to the right. Notice how friction force points in the opposite direction. Finally, there is still gravity and normal forces involved.

Problem 5A skydiver is descending with a constant

velocity. Consider air resistance. Draw a free-body diagram.

Gravity pulls down on the skydiver, while air resistance pushes up as she falls.

Problem 6A man drags a sled across loosely packed snow

with a rightward acceleration. Draw a free-body diagram.

The rightward force arrow points to the right. Friction slows his progress and pulls in the opposite direction. Since there is not information that we are in a blizzard, normal forces still apply as does gravitational force since we are on planet Earth.

Problem 7A football is moving upwards toward its peak

after having been booted by the punter. Draw a free-body diagram.

The force of gravity is the only force described. It is not a windy day (no air resistance).

Problem 8A car runs out of gas and is coasting down a hill.

Even though the car is coasting down the hill, there is still the dragging friction of the road (left pointing arrow) as well as gravity and normal forces.

Net ForcesNow let’s take a look at what happens when

unbalanced forces do not become completely balanced (or cancelled) by other individual forces.

An unbalanced forces exists when the vertical and horizontal forces do not cancel each other out.

Example 1

Notice the upward force of 1200 Newtons (N) is more than gravity (800 N). The net force is 400 N up.

Example 2Notice that while the normal force and gravitation

forces are balanced (each are 50 N) the force of friction results in unbalanced force on the horizontal axis. The net force is 20 N left.

Another way to look at balances and unbalanced forces

Another way to look at balances and unbalanced forces