Force and Friction

Friction

Forces and its components

⚫Force is a vector

Needs magnitude and direction.

⚫If at an angle can be broken down into

components
Fx – horizontal component (Fx = ?)
Fy – vertical component (Fy = ?)

⚫Net force: The sum of all forces.

Weight and Mass

⚫Weight is also called the gravitational

force which means that weight is a force.

⚫Weight depends on gravity, so in different

planets weight is different.

⚫Weight is measured in Newtons, mass in

kilograms and gravity in m/s2.

⚫Fg = mg

​Weight = mass x acceleration due to gravity

Two main causes of Resistance

Friction

⚫Two solid objects

sliding against
each other.

Drag

⚫The backwards

drag of a solid
object moving
forward through a
fluid.

Cause of Friction

⚫The hills and valleys of 2 objects acting

upon each other.

Cause of Friction

⚫Friction varies with the weight, normal

force, pull, and force of friction on an
object.

⚫Objects are considered to be in equilibrium

if the weight and normal force are equal in
size and opposite in directions.

2 Modes of Friction

Static

Initial force used to
start moving an
object.

Fs = µs • FN

Kinetic

Force used to keep
an object in motion

F

k = µk • FN

Static > Kinetic

​Force of = coefficient of x Normal Force
static friction static friction

​​Force of = coefficient of x Normal Force
kinetic friction kinetic friction

Ways of Reducing Friction

Wet Friction

Adding lubricants to
reduce the frictional
forces between parts.
-Examples-

Rolling Friction

Adding rolling parts to
an object to reduce
frictional forces
between parts.
-Examples-

wheels, ball
bearings, etc.

Oil, grease,
water, etc.

How lubricants work

⚫Separates surfaces
⚫Places friction on lubricant

How rolling friction works

⚫Surfaces don’t scrape together as much
⚫Frictional Force is always less than sliding

friction.

Drag

⚫Produced by a fluid moving over a solid

object.
Fluids can be a liquid or a gas
Acts just like a lubricant in its movement
Causes turbulence on the solid object
Increases in a linear pattern to a point

Ways of Reducing Drag

⚫Streamlining

Creating a more aerodynamic solid object to

reduce turbulence

Multiple Choice

A student moves a box of books by attaching a rope to the box and pulling with a force of 200.0N. The box of books has a mass of 20.0 kg, and the coefficient of kinetic friction between the bottom of the box and the sidewalk is 0.50. Find the acceleration of the box.

• 900 m/s²

• 5 m/s²

• 15 m/s²

• 10 m/s²

Multiple Choice

A student moves a box of books by attaching a rope to the box and pulling with a force of 215N. The box has a mass of 35.0 kg, and the µk between the box and the floor is 0.27. Find the acceleration on the box

• 2032 m/s²

• 3.44 m/s²

• 8.84 m/s²

• 6.14 m/s²