Section 6.2Using Newton’s Laws

Physics

Objectives• Describe how the weight and the mass of

an object are related.• Differentiate between the gravitational

force weight and what is experienced as apparent weight.

• Define the friction force and distinguish between static and kinetic friction.

• Describe simple harmonic motion and explain how the acceleration due to gravity influences such motion.

Using Newton’s Second Law

• What did Galileo do to prove that objects fall at the same rate?– Supposedly dropped two balls of different

weights from the top of the Leaning Tower of Pisa.

– Why do objects fall at the same rate?• They have the same acceleration due to gravity:

9.8 m/s²

Newton’s Second Law

• Newton’s Second Law can explain the weight force, Fg, exerted on an object.

• Fg = mg

• Which sign are force and direction?– Both are negative.

Newton’s Second Law

• Would acceleration due to gravity exist on other planets?– Yes– Would it be the same?

• No

• How would the mass and weight of an object differ on another planet?

Practice Problems

• Pg 129

• 12

Weight vs. Apparent Weight

• Weight, Fg, = mg

• Apparent weight = the weight of an object that is sensed as a result of contact forces acting on it.– Give me an example of when you would

apparently weigh more than you actually do.– Give me an example of when you would

apparently weigh less that you actually do.

Friction Force

• Friction is everywhere and has a real effect on everyday actions.

• Friction is the force that opposes motion.– Static Friction Force: The force of friction

between two objects when there is no relative motion between the two surfaces.

– Kinetic Friction Force: The force exerted on one surface by the other when the surfaces are in relative motion.

Friction Forces

• Kinetic Friction = Ff = μkFN

– Friction Force = coefficient of kinetic friction times the normal force.

• Static Friction: 0 < Fs < μsFN

– Static Friction is greater than or equal to zero, but less than or equal to coefficient of static friction times the normal force.

Practice Problems

• Pg 133

• 14-16

Air Drag and Terminal Velocity

• Air causes friction on objects that are falling.

• This type of friction depends on the speed of the object – the friction increases as the speed of the object increases.

• Eventually, the friction will equal the force of gravity – this is known as terminal velocity.

Periodic Motion

Simple Harmonic Motion: Motion that returns an object to its equilibrium position as a result of a restoring force that is directly proportional to the object’s displacement.

Harmonic Motion

Two Quantities in Simple Harmonic Motion:

Period -- T -- Time needed to complete one complete cycle of motion.

Amplitude -- maximum distance the object moves from equilibrium.

The Pendulum

• The bob is suspended by a string of length l.

• The string exerts a tension force and gravity exerts the weight force on the bob.

• The period of the pendulum can be found using: T = 2π(square root of l/g).

Practice Problems

• Pg 136

• 17-19