Force and Motion Unit: Newton’s Laws and Gravity

Ch. 11

Newton’s Laws: Why do objects move?

• Newton’s 1st Law of Motion:– Objects at rest remain at rest unless a force is

applied– Objects in motion remain in motion unless a

force is applied• (wheels on the bus….)

Everyday exapmles of Newton’s First Law

• Seat Belts—they hold us in place if car has sudden force applied

• Table cloth/ dishes• Coin on note card over beaker• Ripping paper towel off roll• Tiolet paper off its roll

What is a force?

• A force is a Push or a pull• The SI unit for force is called a NEWTON. (N)• 1 Newton is the force needed to cause a 1-kg

mass to accelerate at a rate of 1 m per second for each second of motion

How is gravity a force?

Universal Law of Gravity:

All objects in the universe attract (pull on) each other through gravitational force.»Universal Gravitation

Equation:• F=Gm1m2

d2

What this means:• G= a gravitation constant• m1= mass of object 1• m2= mass of object 2• d= distance between

objects• The force of gravity

depends on the sizes of the objects and the distance between them squared.

Gravity

Gravitational force is small between objects that have small masses.

Gravitational force is larger when one or both objects have larger massesGravitational force becomes stronger as

distance is decreasedGravitational force becomes weaker as

distance is increased

Acceleration Due to Gravity

• On earth, this means that all objects are pulled to earth at the same rate.

• A falling object, no matter of its mass, will fall with an acceleration of 9.81 m/s2

Feather vs. Penny demo

• If gravity acts equally on all objects, what then could cause a coin to fall to the floor faster?

• When objects fall through the air, its motion is opposed by the force of FRICTION.

• When the forces of gravity(downward) and air(upward) are equal, an object reaches terminal velocity.

Newton’s 2nd Law of Motion:

– Force varies with Mass and Acceleration– F= m x a• (Bingo was his name-o….)• Is weight the same as Mass?

• Force is measured in Newtons (N)

Newton’s 3rd Law of Motion:

– For every force (action) there is an equal but opposite force (action)• Give some everyday examples of this