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Forces
Ch 7
6th grade
7.1 Vocabulary
• Force
• Net force
How are Forces Described?
• A force is described by its strength and by
the direction in which it acts.
• The direction and strength of a force can
be represented by an arrow.
• The strength of a force is measured in the
SI unit called the newton (N).
How do forces Affect Motion?
• The net force determines if and how an
object will accelerate.
• A nonzero net force causes a change in the
object’s motion.
• When forces are moving in the same
direction, add all of the forces together to
find the net force.
• When two forces are moving in opposite
directions, subtract the smaller force from
the larger force. The net force is the same
direction as the larger force.
7.2 Vocabulary
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Friction
Gravity
Mass
Weight
What factors Affect Friction?
• Types of surface: smooth surfaces
produce less friction than rough surfaces.
• Hardness that surface is pushed together:
friction increases when surfaces push
harder against each other
• Without friction, a moving object will not
stop until it strikes another object.
• Sliding friction: when two solid surfaces slide
over each other that slows objects down.
• Static friction: acts between objects that
aren’t moving
• Fluid friction: occurs when a solid object
moves through a fluid (such as water or air)
• Rolling friction: when an object rolls across a
surface
What factors Affect Gravity?
• Law of Universal Gravitation: the force of
gravity acts between all objects in the
universe that have mass.
– You are attracted not only to Earth, but also to
all planetary bodies and all objects around
you.
– You do not notice the attraction because
these forces are extremely small compared to
the force of Earth’s attraction.
– Gravitational force between some objects is
stronger than the forces of others. You only
observe the effects of the strongest gravitational
forces.
– The more mass, the greater the gravitational
force.
– Gravitational force also depends on the distance
between them.
– Your mass is the same on Earth as it would be
on any other planet, but weight varies with the
strength of the gravitational force.
7.3 Vocabulary
• Inertia
Newton’s First Law of Motion
• An object at rest with remain at rest unless
acted upon by a nonzero net force.
• An object moving at a constant velocity will
continue moving at a constant velocity
unless acted upon by a nonzero net force.
Newton’s First Law
• All object, moving or not, resist changes in
motion. So this law is sometimes called
the Law of Inertia.
• The greater the mass of an object, the
greater its inertia, and the greater the force
required to change its motion.
• Inertia
Newton’s Second Law of Motion
• An object’s acceleration depends on its
mass and on the new force acting on it.
• Acceleration = Net Force
Mass
OR:
Net Force = Mass (Acceleration)
• Acceleration is measured in meters per
second per second (m/s2).
• Mass is measured in kilograms (kg).
• So, force is measured in kilograms times
meters per second per second (kg· m/s2),
or more simply the newton (N).
• One newton is the force required to give a
1-kg mass an acceleration of 1 m/s2.
Newton’s Third Law of Motion
• If one object exerts a force on another
object, then the second object exerts a
force of equal strength in the opposite
direction on the first object.
• For every action, there is an equal but
opposite reaction.
Action-Reaction Pairs
• Pairs of action and reaction forces are all
around you. As you walk, your feet push
on the ground as an action force, but the
ground pushes with an equal and opposite
reaction force.
Detecting Motion
• When gravity pulls an item downward,
Newton’s third law states that the item is
also pulling upward on the Earth. Since
the Earth has such a large mass, its
acceleration is so small, that we can’t see
it.
Cancellation?
• Equal and opposite forces cancel each other
out, but only when applied to same objects.
• Action and reaction forces do not cancel out
because they act on different objects.
• If you pushed on a friend’s hand with your
hand with the same amount of force, neither
of you would move. But if each of you push
on the other’s shoulder, there would be action
and reaction forces that do not cancel out.
• 7.3 Review