Northeast Middle School


 Physics: Forces and Motion

Forces and Motion – What makes objects move the way they do?


8.1 - An object’s inertia causes it to continue moving the way it is moving unless it is acted upon by a force to change its motion.

  • The motion of an object can be described by its position, direction of motion and speed.
  • An unbalanced force acting on an object changes its speed and/or direction of motion.
  • Objects moving in circles must experience force acting toward the center.

C 22. Calculate the average speed of a moving object and illustrate the motion of objects in graphs of distance over time.

C 23. Describe the qualitative relationships among force, mass and changes in motion.

C 24. Describe the forces acting on an object moving in a circular path.


Key Vocabulary words to remember for this section:

Inertia: An object's resistance to a change in motion.  More mass, more inertia: the harder it is to change the motion of the object.

Momentum: product of the mass and velocity of an object.  More mass, more momentum.  More velocity, more momentum.

Speed: distance traveled in a period of time.

Acceleration: change in speed that happens over a period of time.

Force: a push or a pull on an object




The Three Laws of Motion by Sir Isaac Newton explain motion:

Newton's First Law describes motion produced by balanced forces. 


  • The ball will not move unless a force is applied to it, as illustrated in the picture above.  Once in motion, the ball will continue to move until the forces of gravity and friction cause the ball to stop moving.

An object at rest will remain at rest, and a moving object will remain at a constant velocity unless unbalanced forces act on it

The box is not belted onto the truck.  The truck moves forward, but the box stays in place and falls to the ground as the truck drives away.


  •  Newton was first to use the term inertia to describe the tendency of objects to remain in motion or stay at rest. Inertia comes from the Latin word iners, which means "lazy". 


Inertia is a measurement of how hard it is to change the motion of an object.  The more mass an object has, the greater its inertia is.

In this image, the man is not wearing a seatbelt.  The car and the man are moving forward until they hit the brick wall.  The car is stopped by the wall, but there is no outside force to stop the man so he continues to move in the same direction until gravity and friction cause him to stop moving.  



An object in motion stays in motion, and an object at rest stays at rest unless acted on by an outside foce.


Newton's Second Law:

Newton's Second Law describes motion produced by unbalanced forces.


This law is best stated using the equation:

Force = mass X acceleration


  • Acceleration is always in the direction of the unbalanced force.
  • The units of force are "Newtons" 

                                 1 N = 1 kg X 1 m/s/s

Newton's second law states that the greater the mass of an object, the more force is required to move the object.  The more force applied to the object, the faster it will move.



Newton's Third Law


Newton's Third Law explains why forces act in pairs.


  • For every action, there is an equal and opposite reaction.
  • Forces always act in pairs. 
  • Action force, object putting a force on a second object.  Reaction force, second object putting force back on the first object.



Action force: man jumping off the boat.

Reaction force: the boat pushing off the man and moving backwards


Newton's Third Law:

The most common example of Newton's Third Law is the rocket.


The engine "burns" causing the rocket to thrust forward.

Newton's Third law can be seen in animals in the ocean, like the squid, the jelly fish, the nautilus, and the octopus:




Squid takes water into the mantle cavity.

Action force: Squid pushes water out of mantle cavity

Reaction force: water pushing squid in the opposite direction as the water



Jelly fish takes water in through the mouth (between the oral arms)

Action force: Jelly fish pushes water out of the mouth

Reaction force: water pushes jelly fish in the opposite direction








Northeast Middle School - 530 Stevens Street Bristol, CT 06010 Voice: 860-584-7839 Fax: 860-584-7837

email page print page large type small type
powered by finalsite