Review: Newton’s 1st LawReview: Newton’s 1st Law

An object in motion stays in motion in a straight line, unless acted upon by unbalanced force. A push or pull will cause object to speed up, slow down, or change direction.

Review: Forces are BalancedReview: Forces are Balanced

Object at Rest

V = zero m/s

Objects in Motion

V  ≠ zero m/s

Stay at Rest Stay in Motion (same speed and direction

a = 0 m/s2 a = 0 m/s2

Basically, objects just keep on doing whatever they are doing unless they are acted upon by an unbalanced force.

Free-body diagramsFree-body diagrams

Free-body diagrams are pictures that show the size and direction of all forces acting on an object.

Steps to drawing a free body diagram

1.Pick one object to analyze

2.Draw a box to represent the object

3.Draw an arrow to represent each force acting on the object

4.Make sure the arrow shows the direction and relative size of the force

Force Symbol Definition Direction

Friction Ff The contact force that acts to oppose sliding motion between surfaces

Parallel to surface & opposite direction of

sliding

Normal FN The contact force exerted by a surface on an object

Perpendicular to & away from the surface

Spring Fsp A restoring force, that is, the push or pull a spring exerts on an object

Opposite the displacement of the

object at end of spring

Tension FT The pull exerted by a string, rope, or cable when attached to a body &

pulled taut

Away from object & parallel to spring, rope

or cable at point of attachment

Thrust Fthrust A general term for the forces that move objects such as rockets,

planes, cars & people

In same direction as acceleration of object

Weight Fg A long-range force due to gravitational attraction between two objects, generally Earth & an object

Straight down toward center of Earth

Problem 1Problem 1

A book is at rest on a table top. Diagram the forces acting on the book.

Problem 1Problem 1

In this diagram, there are normal and gravitational forces on the book.

Problem 1Problem 1

The forces are balanced (they cancel each other out)

Problem 2Problem 2

An egg is free-falling from a nest in a tree. Neglect air resistance. Draw a free-body diagram showing the forces involved.

Problem 2Problem 2

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

Problem 2Problem 2

The forces are unbalanced, so the egg will accelerate downward.

Problem 3Problem 3 A 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 squirrel looks like…

Problem 3Problem 3

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

Problem 4Problem 4

A rightward force is applied to a book at rest, in order to move it across a desk. Consider frictional forces. Neglect air resistance. Construct a free-body diagram for the book.

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

Problem 5Problem 5

A skydiver is falling with a constant velocity. Consider air resistance. Draw a free-body diagram for the skydiver.

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

Problem 6Problem 6

A man drags a sled across loosely packed snow with a rightward acceleration. Draw a free-body diagram of the forces acting on the sled.

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 7Problem 7

A football is moving upwards toward its peak after having been booted by the punter. Neglect air resistance. Draw a free-body diagram of the football in mid-air.

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

Problem 8Problem 8

A car runs out of gas and coasts to a stop on flat ground. Draw a free body diagram of the forces acting on the car.

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.