Find 17 creatures in this paragraph

Kneel in the kayak grasping the boat, but don't wrench the bullion or scowl at the chart. Behind the taped and sealed planter is a benevolent collier. The foxglove is in the bath.

Everyday Forces

Dawson High School Physics

Force of Gravity and Weight

• The force of gravity is represented by Fg

– Fg is a vector quantity.

– Fg always points down toward the center of the Earth.

• The scalar portion (magnitude) of the force of gravity is what we call weight.

Weight and Mass

• Mass is a measure of the amount of matter an object has. – It also tells us how much “inertia” an object has.

• Weight is a measure of the force of gravity. – In the absence of gravity, a substance has no weight,

but it still has mass.

• Mass is proportional to weight.– Weight = m g

Normal Force

• The normal force is the force that a support surface provides.

• This force is always perpendicular to a surface.– Ex.- A .50 kg book rests on a table.

Fg=-mg= - 4.9 N

Fn

Normal Force

• The normal force is not always equal to an object’s weight.– Example- A student presses down on with

a force of 3 N on a 4.9 N book.

Fg=-mg= - 4.9 N

Fn

Fs = -3 N

Frictional Forces

• Frictional forces oppose the applied force.– They act in the opposite direction of the

motion.

Static Friction

• Static friction (Fs) is the frictional force that acts on a static (nonmoving) object.

• When an object is not moving, the frictional force will equal the applied force but be in the opposite direction.– Fs= - Fapplied

• There is a maximum amount of static friction, Fs,max . Once the applied force exceeds Fs,max the object breaks free and begins moving.

Kinetic Friction

• Kinetic friction is the frictional force on a moving object.

• The force of kinetic friction is less than the maximum static friction.

• The net force on a moving object is equal to Fapplied - Fk

Coefficient of Friction

• The force of friction is proportional to the normal force.

• The coefficient of friction is the ratio of the force of friction to the normal force for particular surfaces.– Coefficient of kinetic friction mk = Fk/Fn

– Coefficient of static friction ms = Fs,max/Fn

Coefficient of Friction Example

• A 24 kg crate, initially at rest, requires a 75 N force to set it in motion. Once moving, a force of 53 N is needed to keep it moving with constant velocity.

• Find the coefficient of static friction and the coefficient of kinetic friction.

Coefficient of Friction Example

32.0

)m/s kg)(9.81 (24

N 75

0

2max,

max,

s

ss

n

n

n

ss

mg

F

mgF

mgF

F

F

Coefficient of Friction Example

23.0

)m/s kg)(9.81 (24

N 35

0

2

k

kk

n

n

n

kk

mg

F

mgF

mgF

F

F

Air Resistance

• The force of air resistance (FR) is a form of friction.

• FR depends on velocity.• Terminal speed: At some speed, the air

resistance will be equal in magnitude to the force of gravity. (FR = -Fg). Once an object reaches that speed, it will no longer accelerate. It will continue to fall at a constant speed.