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Forces
What is a force?
• Forces are what makes things move, like a
push or a pull.
• When you kick a football your foot applies
a force to the ball.
• Forces can
– start motion
– stop motion
– change directions
• The engine of a train uses a pulling force to
make the carriages move along the track.
Forces are either balanced or
unbalanced
• The more energy used to create force, the
stronger the force will be.
• Forces cause Energy to be transformed or
transferred. Forces cause motion to occur.
Balanced Forces
• All forces acting on an object are equal
• When forces are balanced, there is no
change in motion. (If the object is not
moving, there will be no movement. If the
object is moving, its speed will not change;
no slowing down or speeding up.
– It will remain constant.
Unbalanced Forces
• An unbalanced force changes the motion of
an object. An unbalanced force will have
acceleration.
• All forces acting on an object are not equal
• One or more force is stronger than others
• Motion occurs
Net Force
• The sum of all forces acting on an object
– A net force of 0
• No motion
– A net force of more than 0
• Motion occurs
• AND has direction
Mass is a measure of how much matter is in an
object. Unit of mass is grams (g)
Weight is the force of gravity on an objects mass.
Weight is measured in Newtons (N).
A spring scale is used to measure a force. It
measures a force in Newtons.
Kinds of
Forces
Friction
• Force that slows down motion or opposes
motion
– The force of friction is affected by the type of
surfaces involved
– The greater the load or mass the more force needed
to overcome friction
– The force of friction does NOT depend on the
amount of surface area
Three types of Friction
• Sliding- when two solid surfaces slide past each
other. Example pushing a box across the floor
• Rolling- involving wheels or round objects.
Reduces sliding friction. Example: wheels on a
car
• Fluid- involving liquids or gases. Reduces both
sliding and rolling friction. Example: Air
resistance or use of a lubricant such as grease
• Air resistance creates friction in most situations
Attractive Forces
• A force between two mass
– Types of Attractive Forces
• Gravity
• Magnetic
• Electrical
Gravityis an attractive force
.
• More mass = more gravity
• Gravitational force is the force of attraction
between two different masses.
• The larger the mass, the Larger the effect of
the gravitational force has on a body.
• Acceleration because of gravity is 9.8 m/s/s
• All objects accelerate at the same rate
Law of Universal Gravitation states: That all objects
in the universe attract each other by the force of
gravity and the size of the forces depends on the
mass of the objects and the distance between them.
Magnetic force- an attractive force
between electrically charged particles
within certain metals
Electrical force is an
attractive force between
electrical charges
Applied force
• refers to a force that is applied to an
object such as when a person moves a piece
of furniture across the room or pushes a
button on the remote control. A force is
applied.
Normal Force
• A book resting on a table has the force of gravity pulling it toward the
Earth, but the book is not moving or accelerating, so there must be
opposing forces acting on the book. This force is caused by the table
and is known as the normal force. . Normal Force is the force of an
object pushing back.
• You can "see" the normal force in some situations. If you place a ruler
so that it is supported by both ends and place a small heavy object in
the center, the ruler will bend. Of course it wants to straighten out so
it exerts an upward force on the object. This upward force is the
normal force. You can feel the force yourself if you push down in the
center of the ruler. The harder you push, the more the ruler bends and
the harder it pushes back
Tension Force -the force created by a stretched wire,
rope or cable
Spring Force- the force created by the compression of
a spring
Buoyancy
• An upward pushing force in a fluid. A fluid
is a liquid or a gas.
Newton’s Laws
• Mechanical forces obey three laws
called Newton’s Laws of Motion.
• Motion is a change of position in a certain
amount of time.
– To stop a motion, we must have an opposing
force
• Describe motion and changes in motion
First Law of Motion
• Law of inertia
– Inertia: a property of all objects that is related to its
mass - moving objects tend to stay moving and
motionless objects remain motionless
– An object at rest [not moving] will stay at rest until a
force acts on it.
– An object in motion will stay in motion until a force
acts on it.
– Objects with more mass have more inertia
• Bigger objects are harder to start and stop
…If there
are no forces
acting on an
object which is
not moving, it
will stay at rest.
or …..
if it was already
moving at a
steady state in a
straight line, it
will continue to
move in a straight
line as before.
So to start something moving ...
…or speed it up, slow it down or stop it ...
…or change its direction... A
force must be applied.
The Second Law
Law of acceleration
• A force applied to an object will change its speed
in the same direction as the force.
• Objects accelerate in the direction of the force
• The more force applied, then more acceleration
• There is a RELATIONSHIP between mass of
object and the force needed to move an object.
• The more mass an object has, the more force is
needed to accelerate the object. Which can be
expressed:
F = ma
A big force will give a bigger acceleration
than...
…a
small
force
applied
to the
same
mass.
• Acceleration is the change of speed of an
object. It means the object is speeding up or
slowing down.
• Speed is the rate of motion of an object. The
speed of an object is equal to the distance the
object travels divided by the time it takes to
travel that distance. Speed = distance divided by
time
The third law
Third Law often called the Law
of action-reaction
• For each force there is another force acting in
the opposite direction with the same strength.
• (For every action there is an equal but opposite
reaction)
– Forces occur in pairs
– The forces are equal and opposite
– One force is an action force
– The other force is a reaction force
• So if you push against the wall, the
wall pushes you back with the same
force, otherwise the wall would fall
over!
• This also explains how a rocket works.
The force of the gases rushing out of
the end of the rocket produces a force
in the opposite direction pushing the
rocket forward.