Newtons Laws

  • Newton's First Law = Inertia

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

    "Every object remains in a state of rest or of uniform motion in a straight line unless acted upon by an outside force." (An outside force is a net force.)

    Inertia is the resistance to changes in motion. Inertia is proportional to mass. Big masses resist changing their motion more than smaller masses.

    Objects at rest remain at rest unless acted on by a net force.

    A lot of inertia! The large train resists changing its motion.

    Very little inertia. The small baby carriage has very little resistance to changes in motion.

    Since the train is so huge, it is difficult to change its speed. In fact, a large net force is required to change its speed or direction.

    Since the baby carriage is so small, it is very easy to change its speed or direction. A small net force is required to change its speed or direction.

     

    Objects in motion remain in motion in a straight line (unless acted upon by an outside force).

    A lot of inertia!

    Very little inertia

    Since the train is so huge, it is difficult to stop it once it is moving. It is difficult to change its speed. In fact, a large net force is required to change its speed.

    Since the soccer ball is so small, it is very easy to stop it once it is moving. A small net force is required to change its speed.

     Second Law

    The acceleration of an object is directly proportional to the net force acting upon it. The constant of proportionality is the mass.

    F=MA  A=F/M

    = Force = A push or pull

    = Mass

    A= Acceleration = speeding up, slowing down or changing direction

     

    Mass = 5 kg

    Let’s assume that the wheels of a 5-kg car apply 10 N of force. What is the net force if friction and drag are negligible?

     

    The net force would equal 10 Newtons.

     

     

    What is the acceleration of the car?

    Force 

    = MA

     

    10

    = 5A

     

    Acceleration =

    2 m/s2

     

     

     

    Mass = 6 kg

    What is the net force if the wheels of the 5-kg car apply 10 Newtons but a 1-kg parachute applies 3 Newtons in the other direction?

     

    The net force would equal 3 Newtons. The total mass = 6 kg.

    What is the acceleration of the car?

    Acceleration 

    = F/M

     

     

    Acceleration 

    = 3/6

     

     

    Acceleration =

    0.5 m/s2

     

     

     

     

    Mass = 10 kg

    A rocket is added to the car and applies an additional force of 10 Newtons. The wheels still apply 10 N. What is the net force if the parachute continues to apply 7 Newtons in the other direction? The total mass of the car, rocket and parachute is 10 kg.

     

    The net force would equal 13 Newtons. The total mass = 10 kg.

    What is the acceleration of the car?

    Acceleration 

    = F/M

     

     

    Acceleration 

    =13/10

     

     

    Acceleration 

    =1.3 m/s2

     

     

    Big masses are hard to accelerate. Big masses require big forces to change speed.

     

    Small masses are easy to accelerate. Small masses require small forces to change speed.

    Objects move in the direction they are pushed or pulled.

    Objects accelerate more quickly when a greater force is used.

     

    Assume that both steam engines below apply the same amount of force.

     

    A heavy train has a difficult time accelerating. Big masses require big forces to change speed.

    Acceleration =

    Force / Mass

    Acceleration =

    100% / 100%

    Acceleration =

    1

     When the same force is applied to a less massive train its acceleration is greater. Small masses require small forces to change speed.

    Acceleration =

    Force / Mass

    Acceleration =

    100% / 10% of the big train

    Acceleration =

    10 times greater than the big train



    Newton's Third Law of Motion

     Back To Forces and Motion

     

    "When object A exerts a force on object B, object B will exert an equal and opposite force back on object A."

     

    Any time you touch an object, it is touching you with the same amount of force.

     

     

     

     

    The forces here are equal and opposite. Neither the dog nor its owner pulls with greater force. They pull with the same force in opposite directions.

     

     Forces always come in equal and opposite pairs. This means that the forces are equal even when a big object crashes into a small object.

    The forces will be equal when the truck crashes into the car.

    Since the car is smaller, the car will have a greater acceleration.

    How can anything move?