find 17 creatures in this paragraph kneel in the kayak grasping the boat, but don't wrench the...

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.