Newton’s Laws

Newton’s First Law of Motion

• “Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state of motion by forces impressed upon it ”

• The tendency of matter to maintain its state of motion is known as INERTIA.

Newton’s First Law of Motion

• Inertia prevents an object at rest from moving spontaneously or an object in uniform linear motion from changing its speed or direction spontaneously.

• Force is simply a push or a pull.

Newton’s First Law of Motion

• Changes in motion do not happen spontaneously.

• Example: 1) Tablecloth under dishes

Newton’s First Law of Motion

Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless.

These pumpkins will not move unless acted on by an unbalanced force.

Forces

• Usually think of a force as a push or pull• Vector quantity• May be a contact force or a field force

• Contact forces result from physical contact between two objects

• Field forces act between disconnected objects• Also called “action at a distance”

Contact Forces…

• Contact forces are forces which exert a push or pull (interact) while touching.

• Examples:• frictional forces, • tensional forces, • air resistance forces, • applied forces (e.g., pushes or pulls).

Action at a Distance Forces…

• Action-at-a-distance forces exert a push or pull (interact) without physical contact.

• Examples:• Gravity (e.g., when you jump off the ground,

there is still a gravitational pull between you and the Earth),

• Electric (e.g., static electricity), • Magnetic (e.g., two magnets).

Contact and Field Forces

How Many FUNDAMENTAL Forces?

• Four• Strong Nuclear Force • Electromagnetic Force• Weak Nuclear Force• Gravitational Force

What do they DO?

• Strong Nuclear Force• Holds nucleus together

• Electromagnetic Force• Interaction of electric & magnetic fields

• Weak Nuclear Force• Causes radioactivity

• Gravitational Force• Attracts mass

How Strong are they?

• On a relative scale, we let the strong force = 1, then…

• Strong Nuclear 1.00

• Electromagnetic 1 x 10-2

• Weak Nuclear 1 x 10-13

• Gravitational 1 x 10-38

External and Internal Forces

• External force• Any force that results from the interaction

between the object and its environment• Internal forces

• Forces that originate within the object itself• They cannot change the object’s velocity

Example

When the ball at the end of the string swings to its lowest point, the string is cut by a sharp razor. Which path will the ball then follow?

Example: Answer (b)

When the ball at the end of the string swings to its lowest point, the string is cut by a sharp razor. Which path will the ball then follow?

Answer: Inertia

Example

When the pellet fired into the spiral tube emerges, which path will it follow? (Neglect gravity).

Example: Answer (b)

When the pellet fired into the spiral tube emerges, which path will it follow? (Neglect gravity).

Answer: Inertia