How do forces change the motion of objects?

• What are some contact forces and some noncontact forces?

• What is the law of universal gravitation?

• How does friction affect the motion of two objects sliding past each other?

Newton’s Laws of MotionLesson 1

Gravity & Friction

Types of Forces

A push or a pull is called a force. An object or a person can apply a force to another object or person.

A contact force is a force that is applied when two objects touch.

A force that one object can apply to another object without touching it is a noncontact force.

The SI unit for force is the newton (N).

Sir Isaac Newton developed the law of universal gravitation in the late 1600s.

The law of universal gravitation states that all objects are attracted to each other by a gravitational force.

Friction is a force that opposes the movement between two touching surfaces.

There are several types of friction.

•static friction•sliding friction•fluid friction

Static friction prevents surfaces from sliding past each other.

Sliding friction opposes the motion of surfaces sliding past each other.

Fluid friction is friction between a surface and a fluid—any material, such as water or air, that flows.

Fluid friction between a surface and air is air resistance.

Lesson 2 Newton’s 1ST Law

Identifying Forces

The sum of all the forces acting on an object is the net force.

The net force depends on the directions of the forces applied to an object.

A force moving in the reference direction is positive, and a force in the opposite direction is negative.

When the forces applied to an object act in the same direction, the net force is the sum of the individual forces.

When forces act in opposite direction on an object, the net force is still the sum of the forces.

Balanced forces are forces that combine and form a net force of zero.

Forces that combine and form a net force that is not zero are unbalanced forces.

Newton’s First Law of Motion

According to Newton’s first law of motion, if the net force on an object is zero, an object at rest will stay at rest, and a moving object will continue moving in a straight line with constant speed.

When unbalanced forces act on an object, the object’s velocity changes.

The tendency of an object to resist a change in its motion is called inertia.

For an object to start moving, a force greater than static friction must be applied to it.

Objects stop moving because friction or another force acts on them.

EXAMPLE: IF YOUR ON A MOTOR CYCLE AND IT HITS A WALL, THE MOTORCYCLE COMES TO A STOP. BUT YOUR BODY, WHICH WAS IN MOTION WITH THE MOTORCYCLE, RESISTS STOPPING AND CONTINUES IN MOTION.

Newton’s Second Law of Motion

According to Newton’s second law of motion, the acceleration of an object is equal to the net force acting on the object divided by the object’s mass.

EXAMPLE: A LARGE LINEBACKER, TACKLES A SMALLER RUNNING BACK. THE LINEBACKER HAS MORE MASS AND IS ABLE TO EXERT MORE FORCE, THUS PUSHING THE RUNNING BACK BACKWARDS EASILY.

SAYS: THE CHANGE IN MOTION OF AN OBJECT DEPENDS ON THE OBJECTS MASSES, AND THE SIZE OF THE FORCE EXERTED ON THE OBJECT.

Force=mass X acceleration

Newton’s Third Law of Motion

According to Newton’s third law of motion, when one object applies a force on a second object, the second object applies an equal force in the opposite direction on the first object.

According to Newton’s third law of motion, every action force has a reaction force in the opposite direction.

EXAMPLE: IF YOU FIRE A CANNON OR A GUN, THE FORCE OF THE CANNON PUSHES OUT ON THE CANNON BALL AND THE CANNON BALL PUSHES BACK ON THE CANNON, PROPELING IT BACKWARDS.

Momentum

Momentum is a measure of how hard it is to stop a moving object.

Momentum is the product of an object’s mass and velocity.

According to the law of conservation of momentum, the total momentum of a group of objects stays the same unless outside forces such as friction act on the objects.

EXAMPLE: A SEMI TRUCK AND A MOTORCYCLE ARE BOTH TRAVELING DOWN THE HIGHWAY AT 60 mph, THE SEMI TRUCK HAS MORE MOMENTUM BECAUSE IT HAS MORE MASS.

OR: TWO SEMI TRUCKS THE SAME MASS TRAVELING DOWN THE HIGHWAY, BUT ONE TRUCK IS ONLY GOING 50mph AND THE OTHER TRUCK IS GOING 60 mph. THE TRUCK GOING 60 mph HAS MORE MOMENTUM, BECAUSE IT HAS MORE VELOCITY.