Friction and Gravity
(Text pages 340 to 348)
Key Concepts:
What factors determine the strength of the
friction force between two surfaces?
What factors affect the gravitational force
between two objects?
Why do objects accelerate during free fall?
Gravity is pulling this rock
down, however, friction is
holding it in place!
Friction
Friction is the force that two surfaces exert on each other when they rub
against each other.
When an object rubs against another the force of friction depends on two
factors:
•How hard the surfaces are pushed together
•The type of surfaces involved
•Smooth surfaces have very little friction (like snow and ice)
•Rough or bumpy surfaces have more friction (like a road or rocky
surface)
Friction acts in a direction opposite to the direction of the object’s motion
Push
Friction
Types of Friction
1. Static Friction
•The friction that acts on objects that are not moving
•Extra force is needed to start the motion
•To get the object moving you must overcome static
function
2. Sliding Friction
•Occurs when two objects are sliding over each other
(static friction has been overcome)
Types of Friction
3. Rolling Friction
• Occurs when an object rolls over a surface
4. Fluid Friction
• Occurs when a solid object moves through
a fluid
• Remember, Fluids can be liquids or gases
Gravity
Gravity is a force that pulls objects towards each other
Gravity acts everywhere in the universe
•Makes an apple fall to the ground
•Keeps the moon in orbit around earth
•Creates the tides on earth
•Keeps all the planets in orbit around the sun
Newton stated the Law of Universal Gravitation:
Every object in the universe attracts every other object with a force
that is proportional to the product of their masses and inversely
proportional to the square of the distance between the two objects.
Gravity
So what does this mean?
All objects have gravity and pull other objects towards them
So that means you have a gravitational pull on others around you
and they on you!
But, you don’t all stick together…why?
Well, its due to the two factors that affect the strength of the
gravitational force…
1. Mass of the objects
2. Distance between the objects
Your mass is so little compared to the Earth’s that the gravitational
pull you have on everyone is overtaken by the Earth’s gravitational
pull.
Also, the force of friction is greater than your gravitational pull!
Gravity
Gravity between objects depends on
1. The mass of each object
• The more mass the object has, the greater its gravitational
force
• Example: The sun is so massive, it’s gravity is the greatest in
the Solar System so all the planets orbit the sun, not each
other!
2. The distance between them
• The further apart two objects are, the lesser the
gravitational force between them
• The closer they are, the greater the force
Weight and Mass
Weight ≠ Mass!!!
Mass is a measure of the amount of matter in an object
Mass never changes
Weight is a measure of the gravitational force that is exerted on an object
Weight varies with the strength of gravity
•Remember gravity depends on the mass of an object so, since the
Moon’s mass is less than Earth’s your weight will be less on the Moon.
Gravity and Motion
•On Earth, gravity acts as a downward force.
•It affects ALL objects on and outside the Earth
•So the book you hold coming to class is affected by gravity.
•As you hold it, you balance the force of gravity and the book stays put,
•But, if you let go, the forces become unbalanced and the book drops due
to gravity
When the only force acting on an object is gravity we say it is in “free fall”
Any object experiencing “free fall” is accelerating
In “free fall” the force of gravity is an unbalanced force, which causes an
object to accelerate.
Objects accelerate at the rate of 9.8 m/s2 when in “free fall”
Gravity and Motion
Amazing, but True!!! All objects experience the same “free fall” acceleration
regardless of mass
But……when I drop a piece of paper and a ball at the same time, the ball hits
the ground first not both at the same time!
Since the paper has more surface area the friction of the air pushes up on
the paper more than the ball.
This is called air resistance
Gravity and Motion
•Air resistance increases with velocity
•As the falling object speeds up, the force of air resistance becomes
greater and greater
•A falling object will fall fast enough that the upward force of air
resistance = the downward force of gravity
•Balanced Forces....so no more acceleration
•The object falls but it is now at its terminal velocity
•Terminal velocity is reached when the force
of the air resistance = the weight of the
object (force of gravity)