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Unit 3 Lesson 7. Force and Laws of Motion. S.8.C.3.1.1,2. The physics classroom website. Force. 1 N= 0.225lbs. A push or pull on an object. Can change the motion of an object Measured in Newtons (N) (kg*m/s 2 ) Friction and gravity are examples. Friction

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Force and laws of motion

Unit 3

Lesson 7

Force and Laws of Motion


The physics classroom website


1 N= 0.225lbs

  • A push or pull on an object.

  • Can change the motion of an object

  • Measured in Newtons (N) (kg*m/s2)

  • Friction and gravity are examples.

  • Friction

  • A force that acts against motion.

  • Between objects that are touching

    • Between the legs of a desk and the floor when you push a desk


  • A force that pulls matter together

  • Between objects that are not touching

  • Isaac Newton

    • “the force of gravity acts between all objects in the universe”

  • Gravity exists between all matter

    • Gravity exists between you and your pencil however it is very weak

  • The more mass an object has, the stronger its gravity

  • Keeps objects in orbit around a star, planet or moon.

  • Makes objects “fall”

Combining forces
Combining Forces

Force Diagrams:

  • We use arrows to show the different forces acting on objects

  • Direction of arrow shows the direction the force is acting

  • Length of the arrow shows the strength (size or magnitude) of the force

  • Net force: the combination of all forces acting on an object



= 10N









0 N

Balanced forces
Balanced Forces

  • When two forces are the same strength (magnitude or size) but acting in opposite directions on a the same object.

  • Cannotchange an objects motion (speed and direction)

  • An object at rest to start with balanced forces will continueat rest

  • An object moving at 30 mph to the left with balanced forces acting on it will continue to move at 30 mph to the left

  • Forces are equal

  • Motion remains the same

    • If not moving, it continues to not move

    • If moving , it continues to move exactly the same

Balanced Forces

Forces on the person are balanced: Gravity pulls down on person, chair pushes up on person

10 N

10 N

Unbalanced forces
Unbalanced Forces

  • Will change an objects motion (speed and direction)

  • A net force results.

  • Net force is calculated by adding forces in the same direction or subtracting forces in opposite directions

  • If an object is starting at restwhen a net force of 40 N  is applied, the object will move to the right

  • If an object is already moving to the right at a constant speed when a net forceof 40 N  is applied, the object will speed up.

  • Net force of 40 N to the Right

  • 40 N

  • New motion depends on the starting motion

Newton s 1 st law of motion
Newton’s 1st Law of Motion

Law of Inertia

  • Object at rest will remain at rest

  • Object in motion will continue in motion in a straight line

  • The tendency of an object to resist a change in motion.

  • The higher the mass of an object , the higher its inertia is.

Unless an unbalanced force acts upon it

Circular motion animation

car making a sharp turn


Video Clip

Bill Nye- Inertia

Newton s 2 nd law of motion
Newton’s 2nd Law of Motion

  • F = MA

  • The larger the net force on an object , the more the objects motion will change (greater acceleration).

  • Objects with larger masses, need more force to change their motion.

Newton’s 2nd Law of Motion

Newton s 3 rd law of motion
Newton’s 3rd Law of Motion

  • When one object exerts a force on another object, the second object exerts an equal but opposite force on the first object

  • They are not balanced forces because the forces act on different objects

100 N

100 N

Newton s 3 rd law
Newton’s 3rd Law

  • Action-Reaction

  • For every action (force), there is an equal but opposite reaction (force)

3 laws Video clip

Newton’s 3 Laws