Physics: Force

Physics• Physics is the science of understanding

forces• A force is a push or pull. It has both

direction and magnitude (strong/weak)• General forces include gravity and

friction• Elastic forces include compression,

tension, and torque

• Gravity is a force that pulls towards the center of the Earth

Gravity & Weight

• Weight is a force that is a measure of gravity’s pull on an object

Gravity & Weight

Turkey

Scale

To calculate the weight force of gravity’s pull on an object, multiply mass by gravity:

1. Convert mass in grams to kilograms (move the decimal to the left three places)

2. Multiply by gravity which on Earth is 9.8

Calculating Weight Force

• The unit for all forces is the Newton (N)• Named after Isaac Newton, the famous

physicist

Unit

If mass = 215 gforce = .215 x 9.8

= 2.107 = 2.1 N

Examples:

If mass = 1.2 gforce = .0012 x 9.8 = 0.01176 = 0.01 N

• Gravity increases with mass

Understanding Gravity

•Gravity decreases with distance

Large MassMore Gravity

Small MassLess Gravity

•Think of gravity as massive objects creating warps in the fabric of space

Understanding Gravity

Understanding Gravity – Newton vs Einstein

• Two forces are always opposing motion: –Gravity–Friction

Gravity & Friction Oppose Motion

• Gravity is a force that slows upward and trajectory (on a path) motion by pulling objects towards the Earth

Gravity Opposes Motion

Gravity slows upward motion

Gravity slows forward motion

• Gravity pulls all objects to the Earth at the same rate no matter if they are dropped or shot from a gun

Gravity Opposes Motion

• Friction is a force that opposes motion between touching surfaces

Motion Force

Friction Force

Friction Opposes Motion

It is hard to walk without friction

• The space shuttle and meteors (shooting stars) have air friction between their surface and our atmosphere

Friction Opposes Motion

• Friction creates heat and wears out objects

Hip Replacement

Friction Opposes Motion

• Lubrication, such as oil and grease, help reduce friction

Friction Opposes Motion

• There are two basic types of friction: static and kinetic.

• Static friction is the stronger force of the two.

Friction Opposes Motion

• Static friction occurs before movement takes place - it resists movement.

Motion ForceFriction

Force

Friction Opposes Motion

• Kinetic friction occurs during movement - it tries to slow or stop movement.

Motion Force

Friction Force

Friction Opposes Motion

Example QuestionWhat two forces oppose the motion of this bike?

Motion ForceFriction

Force

GravityForce

Elastic ForcesCompression

• Forces come together from opposite directions (crushing a can)

Don’t try this at home!

• Forces applied in opposite directions (tug of war)

Elastic Forces (continued) Tension

Elastic Forces (continued)T o rque

• A twisting force (the ‘O’ in torque)