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/s2)• The acceleration due to

gravity

• W = m g

Practice Problems

• W = ?

• m = 10 kg

• g = 9.8 m/s2

• W = 98 N

Practice Problems

• W = 15 N

• m = ?

• g = 9.8 m/s2

• m = 1.53 kg Is this correct?

Practice Problems

• W = 25 N

• m = 5.0 kg

• g = ?

• g = 10 m/s2 Is this correct?

On Planet X, the acceleration due to gravity is 15 m/s2. 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/s2, what is her mass?

• m = 51.02 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/s2