CHAPTER 3

Newton’s Second Law of Motion

• F = m × a• Force = mass ×

acceleration• The faster you run

into a wall, the more force you exert on that wall

• Units are kg×m/s2 • Units are Newtons (N)

F = m x a

• You run into the wall at 50 N of force. Your mass is 10 kilograms. Calculate your acceleration.

• Carmine has a mass of 100 g, and he is running toward you at 2 m/s2. Calculate the force he will hit you with. (Tricky)

Friction

• Friction is the force that opposes motion between two surfaces touching each other

• There are 3 types:

• 1. static = not moving

• 2. sliding = pushing a heavy box across the floor

• 3. rolling = your car spinning its wheels on the ice

Air Resistance

• Air resistance affects anything moving in earth’s atmosphere

• The amount depends on the speed, size, and shape of the object

• Negating air resistance, which will fall faster: a feather or a bowling ball?

Terminal Velocity

• The force of air resistance increases with speed

• Remember that the acceleration due to gravity is always 9.8 m/s2

• When the object is falling so fast that it cancels the force due to gravity, it is now at terminal velocity and is no longer accelerating

• This is the greatest falling velocity possible

Law of Gravitation

• Any two masses exert a gravitational force on each other

• The size of this force depends on the mass and the distance between the objects

• Greater the mass, greater the force

• Closer together, greater the force

Weight vs. Mass

• Weight = mass × acceleration due to gravity (9.8 m/s2)

• Weight is in Newtons

• Weight is a force

• Mass is in kg

• Mass is the measure of amount of matter an object contains

Calculating weight

• How much does a person with a mass of 70kg weigh on planet Earth?

• How much does a baby with a mass of 500g weigh on planet Earth? (Tricky)

Projectile Motion

• When you throw a ball on Earth, there are two forces working on it

• Vertical force pulling down (gravity)

• Horizontal force pushing out (your throw)

Newton’s Third Law of Motion

• For every action there is an equal and opposite reaction

• Ex: rocket propulsion

Momentum

• momentum = mass × velocity

• p = m × v• Law of Conservation of

Momentum • Momentum can be

transferred from one object to another

• Total momentum of the system remains the same

Calculating momentum

• Compare the momentums of a 50kg dolphin swimming 16.4m/s and a 6,300kg elephant walking 0.11m/s.