Transcript Forces and Motion

Forces and Motion
6th grade science
Mrs. Reese
Sir Isaac Newton
(1642-1727)
• While many scientists and philosophers like Aristotle
and Galileo tried to observe and discuss how objects
move on Earth, Sir Isaac Newton was the first scientist
to formulate a set of laws or rules to summarize how
all objects move on Earth and in the universe. It is
said that Newton came up with these ideas when an
apple fell on his head when he was sleeping under a
tree!
What is a force?
• Force is a push or pull one body exerts on another.
Another way to describe a force is any action that has
the ability to change the motion of an object by giving the
object energy.
• Forces are a big part of physics. Physicists devote a lot
of time to the study of forces that are found everywhere
in the universe. The forces could be big, such as the pull
of a star on a planet. The forces could also be very
small, such as the pull of a nucleus on an electron.
Forces are acting everywhere in the universe at all
times.
Examples of Forces
• Contact Force When two objects are in
contact, each object
exerts a force on the
other. Pushing and
pulling.
• Gravity – attraction
force
Force that pulls
everything near the
Earth’s surface toward
the Earth’s center.
• Magnetic Force attraction between
magnets. N +S
• Electrical Force attraction between
charges: protons(+) and
electrons (-)
Do we use forces in our body?
• Yes, during breathing, muscles contract
(use force) to move air in and out of the
lungs. The heart also contracts (uses
force) to pump blood through the blood
vessels, carrying oxygen to the muscles.
Forces in Nature
• Erosion is the process by which fragments
of soil and rock are broken off from the
ground (sediments) and are carried away.
Eventually these fragments are deposited
in a new place.
• Water has the ability to cause erosion if it
carries enough force and speed. The
more force and speed water has, the
greater the impact on changing the shape
of the land.
Soil Erosion Due To Water
Forces in Nature
Balanced Forces
• Balanced forces are forces that are equal in
size and opposite in direction.
• Example: tug-of-war between two people who
are pulling with equal strength. The net force
is zero!
There is no change in motion!
Unbalanced Forces
• Unbalanced forces are forces that change the
velocity (speed and direction) of an object.
• Example: tug-of-war between two people of
unequal strength.
• Suppose one student is pulling with a force of 30N
and the other with 45N, the difference in the NET
force is 15N in the direction of the greater force.
30N
Hey! Pick on
Somebody your own
size!
45N
Newton’s First Law: Law of Inertia
• Inertia is defined as an object’s tendency to resist
any change in its motion.
• If an object is moving, it will keep moving at the
same speed and direction unless and unbalanced
force acts on it.
• Example: When you are riding in car, both you and
the car are in motion. If the car stops suddenly,
inertia will make you keep going forward unless you
are wearing your seatbelt!.
MORAL: WEAR YOUR SEATBELT!!!
Newton’s First Law: Law of Inertia
• The opposite is also true. If an object is at
rest, then the object will remain at rest
unless an unbalanced force acts on it.
• Example: Imagine if you have a toy car on
the floor and you want it to move. Will it
move unless you apply a force to it? NO!!!
Newton’s First Law
Does mass affect inertia?
• The more mass (amount of matter in object) an
object has, the greater its inertia.
• Example: Which has more inertia, a golf ball or
bowling ball? Ask yourself, who resists a change
in motion the most?
Force of Friction
• Friction is the force that opposes motion
between two surfaces that are touching each
other.
• Friction depends on the type of surfaces that are
being pressed together and the force at which
they are pressed together.
• Example: Friction between your shoes and the
floor is what allows us to walk! If there was no
friction, you would be constantly slipping and
falling!
Types of Friction
• You can have rolling, sliding, and static friction.
• Static friction is friction between two solid objects that
are not moving relative to each other. The static friction
force must be overcome by an applied force before an
object can move.
• Rolling friction is the resistive force that slows down the
motion of a rolling ball or wheel. This type of friction is
typically a combination of several friction forces at the
point of contact between the wheel and the ground or
other surface.
• Sliding friction is the friction between moving surfaces.
What type of friction is this?
NOT MOVING

Newton’s Second Law
• Newton’s Second Law: Newton’s second law
describes how force, mass, and acceleration are
connected.
• Newton’s second law states that a force acting on
an object causes the object to accelerate in the
direction of the force.
Newton’s Second Law
• The larger the force that is applied on the object,
the greater its acceleration.
• An object with more mass requires more force to
accelerate.
• Objects with more mass accelerate slower than
object’s with less mass if an equal force is
applied to both objects.
• Example: semi-truck vs. car
What is the Formula?
F=mxa
F = Force in Newtons (N) or kg (m/s2)
m = mass in kilograms (kg)
a = acceleration in meters per second
squared (m/s2)
Math Example
• With what force will a car hit a tree if the
car has a mass of 3000kg and it is
accelerating at a rate of 2m/s2?
Math Example (Use 5 Steps)
• A 10 kg bowling ball would require what
force to accelerate it down an alleyway at
a rate of 3 m/s2?
F= ?
m = 10 kg F = m x a F = 10 kg x 3 m/s2
a = 3 m/s2
Step 5 solve by hand! 10 x 3 = 30
F = 30 kg m/s2 or F = 30N
Newton’s Third Law
• Describes action-reaction pairs: When an object
exerts a force on a second object, the second
one exerts a force on the first that is equal in
size and opposite in direction.
• In other words, for every action force, there is an
equal and opposite reaction force. The forces
are balanced.
• It is important to remember that the actionreaction forces always act on different objects.
Example 1
Example 2
• Consider the propulsion of a fish through the
water. A fish uses its fins to push water
backwards. But a push on the water will only
serve to accelerate the water. In turn, the
water reacts by pushing the fish forwards,
propelling the fish through the water.
Example 3
• Consider the motion of your automobile to
school. An automobile is equipped with
wheels which spin backwards. As the
wheels spin backwards, they push the road
backwards. In turn, the road reacts by
pushing the wheels forward.