Introduction to Forces & FBDs

Forces and Free Body

Diagrams

Multiple Choice

The unit for Force is

• meters

• Newton

• meters per second

• meters per second squared

What is a Force?

●

A force is a push or a pull on an object resulting from the interaction
of the object with something else

●

2 types of forces - contact forces and at-a-distance forces

Contact Forces

●Frictional Force

●Tension Force

●Normal Force

●Air Resistance Force

●Applied Force

●Spring Force

At-a-Distance Forces

●Gravitational Force

●Electrical Force

●Magnetic Force

Categorize

Options (8)

friction

gravitational force

tension

normal force

magnetic force

applied force

air resistance

electrical force

Organize these options into the right categories

Contact force

NonContact Force

Unit of Forces

● Force is measured in Newtons
● Newtons are equal to 1 kg*m/s2

● Forces are a vector, meaning they have a magnitude AND

direction

● Forces can be represented in pictures by drawing vectors

○ These drawings are called free-body diagrams

Gravitational Force

●

Attractive force between two objects

●

On earth, gravity is always directed
downward

●

Gravitational force is also called
weight

●

Fg = m*g
○

m = mass

○

g = acceleration of gravity

●

g on earth is always -10 m/s2

Multiple Choice

Which of the following statements about gravitational force is FALSE

• It is measured in Newtons

• It always points down towards the Earth

• It is also known as mass

• It is also known as weight

Normal Force

●The force that is exerted on an object
in contact with a surface

●When two objects are in contact with eachother, they each exert a Normal Force on the other

●Normal force is always in the direction
perpendicular to the surface

Multiple Choice

The picture shows an individual leaning against a wall. Which of the following correctly describes the normal force acting on the person? Select TWO answers.

• The person is pushing the wall to the left

• The wall is pushing the person to the right

• The ground is pushing the person upwards

• The Earth is pulling the person downward

Applied Force

●

This is simply any force
applied to an object

●

Examples:
○

Pulling a desk across
the room

○

Pushing a ball down a
ramp

Open Ended

Give two examples of an applied force not mentioned in the previous slide.

Tension

●

Force exerted by a rope, string,
cable, chain, etc.

●

Tension only occurs when rope,
chain, etc. is pulled taut and has
forces acting on both ends

●

When objects are hanging directly
downward, tension will equal
gravitational force and be directed
upward

Dropdown

The picture shows a birdfeeder hanging from a tree in my backyard. The direction of the tension force on the birdfeeder is . . . . ​

Select answer

Frictional Force

●The force a surface exerts on an object
that resists the object’s motion

●Frictional force will most often be
opposite direction of the object’s
movement

●The closer an object is to a surface, or
the more firmly pressed down it is to the
surface, the higher the frictional force is

●Friction can be found using:
Ffrict = µ • Fnorm

Drag and Drop

Drag all of the TRUE statements to the empty boxes shown here: ​

Placeholder

​

Placeholder

​ ​

Placeholder

Drag these tiles and drop them in the correct blank above

friction acts parallel to a surface

friction is measured in Newtons

friction opposes motion

friction always points left or right

friction is measured in meters

​

​

Dropdown

If an object has no air resistance force acting on it, and only has gravitational force acting on it, we say that the object is in FREE FALL​

Select answer

Multiple Choice

What type of force is the BLUE arrow indicating?

• Applied Force

• Gravity

• Friction

• Normal Force

Checkpoint!
Practice

A flying squirrel is gliding
(no wing flaps) from a tree to
the ground at constant
velocity. Consider air
resistance. Diagram the
forces acting on the squirrel.

Checkpoint!
Practice Continued

An egg is free-falling from a
nest in a tree. Neglect air
resistance. Diagram the forces
acting on the egg as it is falling.

Checkpoint!
Practice Continued

A gymnast holding onto a bar,
is suspended motionless in
mid-air. The bar is supported by
two ropes that attach to the
ceiling. Diagram the forces acting
on the combination of gymnast
and bar.

Checkpoint!
Practice Continued

A rightward force is applied to a
book in order to move it across a
desk with a rightward acceleration.
Consider frictional forces. Neglect
air resistance. Diagram the forces
acting on the book.

What do Net Forces mean for motion?

● A Net Force simply means that the forces on the object

are unbalanced

● If an object has a Net Force, it is accelerating
● Net forces, just like regular forces, are a VECTOR and

have a DIRECTION

Multiple Choice

The net force acting on the box shown in the picture is

• 600 N up

• 13 N right

• 637 N

• 13 N left