Forces, Inertia, Weight and Gravity Heavy stuff.

Newton’s First Law An object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted on by UNBALANCED forces. –Also called the Law of INERTIA (laziness!) Think about the baseball thrown in space. What will it do AFTER leaving your hand?

What is a Force? A force is ANY push or pull on an object Without force, an object cannot accelerate. Think of some forces acting on you right this moment.

NET FORCE How many forces can act on an object at once? The SUM of all these forces is known as the NET FORCE. If the net force is NOT zero, then we say the forces are UNBALANCED Unbalanced forces are what cause acceleration!

Real world scenario: Take your textbook and push it across your desk. –(keep the speed constant) Are you applying a force the whole time? –Then why isnt the book accelerating? There must be a force BALANCING your push on the book.

Friction! Friction is a force that OPPOSES (works against) motion. It happens between any two solid objects that are touching It works against motion, but without it motion for us would be nearly impossible! –(Why?)

Gravity Gravity is the attraction between ANY two objects. –This is felt as a force acting on BOTH objects The strength of a gravitation force depends on: –The mass of the two objects –The distance between them (squared) On earth, the gravitational force from earth causes ALL objects to accelerate at the same rate –9.8 m/s/s

Weight Weight is the force that gravity exerts on a body –Measured in Newtons (This is the SI unit for FORCE) Mass is the amount of matter in an object –Measured in kilograms –Doesn’t change no matter where you are To calculate, We use the formula W = m * g –On earth, g = 9.8 m/s/s

The Formula W = Weight (N) the force of gravity acting on an object m = mass (kg) The amount of stuff (matter) an object is made of g = gravity (m/s 2 ) The acceleration due to gravity W = m  g

Practice Problems W = ? m = 10 kg g = 9.8 m/s 2 W = 98 N

Practice Problems W = 15 N m = ? g = 9.8 m/s 2 m = 1.53 kg Is this correct?

Practice Problems W = 25 N m = 5.0 kg g = ? g = 10 m/s 2 Is this correct?

On Planet X, the acceleration due to gravity is 15 m/s 2. If Jim’s mass is 90 kg, what is his weight? W = 1350 N

Stephanie steps on scale that says her weight is 500 N. If the acceleration due to gravity is 9.8 m/s 2, what is her mass? m = kg

Jeff’s mass is 75 kg. On another planet, his weight is 900 N. What is the acceleration due to gravity on that planet? A = 12 m/s 2