12.2 Newton’s First and

Second Laws

Review• Force=push or pull that acts on an object

– Force can cause a resting object to move or an object already in motion to accelerate.

– Unit of Force=Newton (N)

• Friction=force that opposes the motion of touching objects as they move past one another– Four types of friction:

• Static• Sliding• Rolling • Fluid

• Gravity=an attractive force that pulls objects together.– Ex: gravity pulls you and the Earth together

Where did this knowledge come from?

• Modern scientists have gained their knowledge about force and motion from early scientists.

• These scientists are:– Aristotle– Galileo– Newton

• It only took 2000 years to get here!!

So, What did they do?

• Aristotle incorrectly proposed that force is required to keep an object moving at constant speed. – This error held back progress in the study of motion for almost

2000 yrs.

• Galileo Galilei studied how gravity produces constant acceleration. – He concluded that moving objects not subjected to friction or

any other force would continue to move indefinitely.

So, What did they do?

• Newton built on the work of scientists such as Galileo.

• Published his work in Principia.• First he defined mass and force.• Then he introduced his three laws

of motion.

Newton’s First Law of Motion (aka Law of Inertia)

• According to Newton’s first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero.

• An object at rest stays at rest, an object in motion stays in motion, until an outside force acts upon it.– Ex: a ball sitting on the ground will stay there until someone picks it

up or kicks it.– Ex: once the ball is kicked it will continue to roll until stopped by a

person or will eventually slow due to friction between the ball and the grass.

• Inertia is the tendency of an object to resist a change in its motion.– The ball mentioned above has inertia.

Newton’s Second Law• According to Newton’s second law of motion,

the acceleration of an object is equal to the net force acting on it divided by the object’s mass.

• The acceleration of an object is always in the same direction as the net force.

• When a net force acts in the direction opposite to the object’s motion, the force produces a deceleration.

Newton’s Second Law• The acceleration of an object also depends on its mass. • Mass is a measure of the inertia of an object.

• The formula can also be written:– F = ma

Weight and Mass• Mass and weight are related but are not

the same. • Mass is a measure of the amount of

material an object contains.• Weight is a measure of the force of

gravity acting on an object. • Weight is the product of mass and acceleration

due to gravity.

Weight and Mass• W = mg is a different form of Newton’s Second Law, F = ma.• The value of g in the formula is 9.8 m/s2.

Calculating Weight• If an astronaut has a mass

of 112 kilograms, what is his weight on Earth where the acceleration due to gravity is 9.8 m/s2?

Calculating Weight• If an astronaut has a mass of 112

kilograms, what is his weight on Earth where the acceleration due to gravity is 9.8 m/s2?Weight = Mass × Acceleration due to

gravity= 112 kg × 9.8 m/s2

Calculating Weight• If an astronaut has a mass of 112

kilograms, what is his weight on Earth where the acceleration due to gravity is 9.8 m/s2?

Weight = Mass × Acceleration due to gravity

= 112 kg × 9.8 m/s2

= 1100 kg•m/s2 × 1100 N

Weight and Mass on the Moon

On the moon, the acceleration due to gravity is only about one sixth that on Earth.

• The astronaut weighs only about one sixth as much on the moon as on Earth.

• The mass of the astronaut is the same on the moon and on Earth.

Weight and Mass on the Moon

Astronaut on EarthMass = 88.0 kg, Weight = 863 N

Astronaut on MoonMass = 88.0 kg, Weight = 141 N