Forces Mastery Lesson 8.9A

​FORCES

​8.6(A) demonstrate and calculate how unbalanced forces change the speed or direction of an object’s motion

​TEK

Multiple Choice

Force

• A push or a pull

• No motion; forces are equal to each other.

• Motion; Forces are not equal

• Newtons (N)

Multiple Choice

Unit of Force

• Newtons (N)
• The resistance to motion of one object moving relative to another; two forces opposing each other.
• The force on an object due to gravity.
• Energy that is stored

Multiple Choice

Friction

• The resistance to motion of one object moving relative to another; two forces opposing each other.
• The force on an object due to gravity.
• Energy that is stored
• Energy of motion

Multiple Choice

Balanced force

• No motion; forces are equal to each other.
• Motion; Forces are not equal
• Newtons (N)
• The resistance to motion of one object moving relative to another; two forces opposing each other.

Multiple Choice

Unbalanced Force

• Motion; Forces are not equal
• Newtons (N)
• The resistance to motion of one object moving relative to another; two forces opposing each other.
• The force on an object due to gravity.

Match

Match the following vocabulary words to the definitions.

• Unbalanced Force

• Balanced Force

• Friction

• Newton’s 3rd Law

• Newton’s 1st Law

Forces acting on an object that combine and form a net force that is not zero

Forces acting on an object that combine and form a net force of zero

A force that resists the motion of two surfaces that are touching  

For every action there is an equal and opposite reaction.

An object will not change its motion unless acted upon by an external force.

• Forces acting on an object that combine and form a net force that is not zero

• Forces acting on an object that combine and form a net force of zero

• A force that resists the motion of two surfaces that are touching  

• For every action there is an equal and opposite reaction.

• An object will not change its motion unless acted upon by an external force.

Unbalanced Forces

Unbalanced forces exist on an object when the net
force acting on that object is greater than zero. In the
above example, gravity pulls down more strongly than
the air resistance pushes back. The two forces can be
represented by arrows, as shown to the right. The size
of the arrows represent the relative strength of the two
forces. The larger down arrow represents gravity, and
the smaller up arrow represents air resistance. By
subtracting the down arrow length from the up arrow
length, the net force arrow shows the smaller
downward acting force

balanced Forces

Balanced forces exist when all of the forces acting
on an object cancel each other out, resulting in a
net force of zero. If the object pulled down by
gravity is resting on the ground, which is strong
enough to push up with equal force, the two
forces cancel each other out, and no motion
results In the diagram to the right, the down arrow
(gravity) and the up arrow (ground support) are of
equal length but opposite direction. If the net force
is zero, the forces are balanced, and no
acceleration (change of motion) occurs.

balanced Forces

A good example of balanced and unbalanced forces is a
tug-of-war between two teams of students. If the opposing
teams are of equal strength, neither team can move the
other, because their net force is zero and the forces are
balanced. This is shown in the diagram to the right.

The same result occurs when a force pushes on an
unmovable object, or when friction prevents the motion of an
object because not enough force was applied to overcome
that friction. In both of these cases, the two forces are equal
but in opposite directions—they cancel each other out.

Unbalanced Forces

The same result occurs when a force pushes on an
unmovable object, or when friction prevents the motion
of an object because not enough force was applied to
overcome that friction. In both of these cases, the two
forces are equal but in opposite directions—they cancel
each other out.

If Team B becomes tired, and pulls less hard than Team
A, Team B eventually lose because they were moved
(accelerated) to the left, the direction of the greater
force.

Unbalanced Forces

Calculations of net force, where the two forces act in opposite directions, simply
require that you subtract the weaker force from the stronger force. The result of
this subtraction is the net force, acting in the direction of the stronger force.
Study the examples below to see how this process works.

Multiple Choice

The diagram shows a homemade car being pushed with the force of 25 N.

The force causes the car to move at a constant speed of 3 m/s. What will happen if the force is changed to 35 N?

• The car will move at a constant speed of 13 m/s.

• The speed of the car will not change.

• The speed of the car will increase.

• The speed of the car will decrease to 1 m/s.

Multiple Choice

The diagram shows a boat moving north in a river at 3 m/s while the current in the river moves south at 1 m/s.

How will the boat be affected if it enters a part of the river where the current is moving south at 2 m/s?

• The boat will move slower.

• The boat will move faster

• The boat will stop

• The boat will move to the west.

Multiple Choice

What is the net force on this box?

• 30 N left
• 25 N right
• 5 N left
• 55 N right

Multiple Choice

To calculate net force you must

• Add the forces

• Subtract the forces