Forces: Newton’s Laws of MotionChapters 6 & 7

Any push or pull exerted on an object. The object is the system The forces exerted on the system is the

environment.

Force

Contact: acts on an object by touching it. Long-Range: exerted without contact.

Gravity Cause of a force is the agent.

Contact vs. Long-Range Forces

“An obect that is at rest will remain at rest or an object that is moving will continue to move in a straight line with constant speed, if and only if the net force acting on the object is zero” Sir Isaac Newton

Law of Inertia. Inertia is the tendency of an object not to

accelerate. Equilibrium: occurs if the net force on an

object is zero.

Newton’s 1st Law of Motion

The object is at rest, or moving at a constant velocity.

Net force is anything that disturbs a state of equilibrium

Table 6-2 pg. 123

Newton’s 1st Law of Motion

A force is needed to keep an object moving. Inertia is a force. Air does not exert a force When an object is thrown, the force of the

hand that throws it remains with the object.

Force Misconceptions

If an outside force is applied to an object, the object will accelerate.

a= F net/m Net force is the vector sum of two or more

forces on an object. Weightlessness: no contact forces are acting

upon an object.

Newton’s 2nd Law of Motion

Friction is an opposing force acting on an object.

Second force acting in the opposite direction of the applied force on an object.

Static friction: occurs when there is no relative motion.

Kinetic friction: occurs when there is relative motion.

Friction

Frictionlike force exerted when an object moves through a fluid.

Force depends on the speed of the motion. Faster the motion, the greater the force.

Air Drag

Initially, it has very little velocity and only a small drag force

Gravity is stronger than the upward drag force, so there is downward acceleration

As the object’s velocity increases, so does the drag force.

Drag force will equal force of gravity. No net force, no acceleration. Velocity

becomes constant. Terminal Velocity

When dropping an object

Simple harmonic motion: occurs when the force that restores the object to equilibrium is directly proportional to the displacement of the object.

Period: time needed to repeat one complete cycle of motion.

Amplitude: maximum distance that the object moves from equilibrium

Periodic Motion

Pendulum: demonstrates periodic motion Mechanical Resonance: increase in

amplitude, occurs when small forces are applied at regular intervals to a vibrating or oscillating object.

Periodic Motion

“For every action, there is an equal, opposite reaction”

Interactive forces: two forces that are “equal and opposite”

F A on B = -FA on B

Newton’s 3rd Law of Motion

Gravitational Force Magnetic Strong Nuclear Force Weak Nuclear Force

Four Fundamental Forces