 # Forces.

## Presentation on theme: "Forces."— Presentation transcript:

Forces

Change in motion? UNBANLANCED FORCE
What causes an object to start moving, stop moving, or change direction? UNBANLANCED FORCE

What is a force? The strength of a force is measured in the Newton (N)
A push or pull A force is described by strength and direction The strength of a force is measured in the Newton (N) Arrows are used to represent force:

Combining forces Most of the time, there are more than 1 force acting on an object at a time Net force- the combination of all forces acting on an 15 N 15 N 15 N 15 N

Unbalanced vs. Balanced Forces
Unbalanced Forces Result in a net force Can cause a change in the object’s motion Balanced Forces Equal forces acting in opposite direction Do not cause a change in the object’s motion They cancel each other out

Draw Force Diagrams in Notebook
A book is at rest on a tabletop. Diagram the forces acting on the book A gymnast is holding on the rings suspended from the ceiling waiting to being. A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance

Examples Continued 5. A student rests a backpack upon his shoulder. The pack is suspended motionless by one strap from one shoulder. 6. A skydiver is descending with a constant velocity. Consider air resistance 7. A force is applied to the right to drag a sled across loosely packed snow with a rightward acceleration 8. A car is coasting to the right and slowing down

Force = mass x acceleration
Formula F=m•a Force = mass x acceleration Units: Newtons (N) N=(kg)•(m/s2)

Friction The force that one surface exerts on another when the two surfaces rub against each other Force that oppose motion- always in opposite direction of motion

SURFACE AREA DOES NOT AFFECT THE AMOUNT OF FRICTION
Friction produces heat! – how have you seen this? The amount of friction depends on: How hard the surfaces push together (force) Types of surfaces involved SURFACE AREA DOES NOT AFFECT THE AMOUNT OF FRICTION

Static Friction Acts on an object that are not moving
Strongest- most difficult to overcome Examples: Desk on the floor Parked car on the pavement

Sliding Friction Occurs when one solid surface slides over another solid surface Not as strong as static friction Examples: Bicycle brakes Pushing a box along the floor

Rolling Friction Occurs when an object rolls over a surface
Not as strong as sliding friction Examples: Shopping cart Car traveling on the road

Fluid Friction Occurs as an object moves through a fluid Examples:
Surf boarding in the ocean Car as it moves through the air

Gravity Is the force that pulls objects toward each other
The law of Universal Gravitation- the force of gravity acts between all objects in the universe! That means: any two masses exert an attractive force on each other

Factors affecting gravity
Mass- a measure of the amount of matter in an object The more mass an object has, the greater its gravitational force on other objects is Distance The farther apart objects are, the lesser the gravitational force between them The sun has a gravitational force on you..why don’t you feel it? The pencil in front of you has a gravitational force on you…why don’t you feel it?

Gravitational Acceleration
The gravitational attraction of Earth causes all falling objects to have an acceleration of 9.8 m/s2 Which direction is Earth’s gravitational force?

Weight What is it? W=m x g  g=a
The gravitational force exerted on an object W=m x g  g=a Calculate the weight of a truck with a mass of 600 kg. G: m = 600 kg F: W=m x g g = 9.8 m/s/s Find: w = ? W.S.: w = 600kg x 9.8 m/s/s Answer: 5,880 N

Mass vs. Weight Mass is _________ Weight is a FORCE
the more mass an object has, the stronger the gravitational force between the object and Earth. So, the more mass an object has, the more it weighs Weight can be different if the gravitational force is different

Weightlessness When an object is influences only by the force of gravity, it is said to be in free fall.

Air resistance Air resistance acts in the opposite direct of an object’s motion The force it opposes is GRAVITY The amount of air resistance depends on: Speed Size Shape Terminal velocity- when the force of air resistance = weight of object no more acceleration

What happens if you drop 2 objects?
What would happen if there was no air resistance? REMEMBER: ALL OBJECTS IN FREE FALL ACCELERATE AT THE SAME RATE, REGARDLESS OF MASS!!

Projectile Motion A projectile is an object that has been projected through the air A projectile has horizontal and vertical motion Neglecting air resistance, all falling objects accelerate at the same rate No matter how fast and object is moving horizontally, the force of gravity acts the same as if it was falling straight down