Newton’s 3rd Law and Free Body Diagrams

Newton’s 3rd Law of Motion- “For every action, there is an equal and opposite reaction”. Forces always exist in “action – reaction” pairs These pairs do not cancel each other out. Newton’s 3rd Law of Motion-

Newton’s 3rd Law of Motion-

Newton’s Third Law Identify action-reaction pairs of forces. Explain how the horse can move.

Forces Push or Pull on an object Forces always occur in pairs Balanced forces have zero net force and result in no movement. Unbalanced forces have a net force and result in movement.

Newton’s Third Law Behavior of Forces Internal Forces – Come in pairs (action-reaction), cancel one another, and can NOT accelerate an object. External (applied) Force – Individual forces which may accelerate an object (SF > 0). All type of forces (Fg, FN, FT, Ff) can behave as internal and external forces.

Forces Forces are vectors measured in Newtons Force = mass × acceleration (shorthand F = m a) 1 Newton (N) = 1 kg m / s2

Weight W = mg Find the weight of a 100 kg object on earth. Weight is the force of an object due to gravity. The weight of an object is the product of its mass and the acceleration due to gravity (9.8 m/s²). W = mg Find the weight of a 100 kg object on earth.

Types of Forces Contact Forces Field Forces Result from physical contact between two objects. Field Forces Do not involve physical contact between two objects. Applied Force – push or pull on an object Normal Force – supporting force (perpendicular to surface) Weight – gravitational force on object Tension – force on a rope Spring Force – force on a spring Frictional Force – contact force that opposes motion

If all forces on an object are balanced how would the object behave? Tension Fair Fgravity Predict what would happen if forces on an object are balanced. No change in motion Gravity

What if the Forces are Unbalanced? Fg = Weight Acceleration Predict what would happen in each case above.

Unbalanced or Balanced?

Unbalanced or Balanced?

What’s a Net Force? Sum of all forces “Left-over” Force when forces are “added” together. Same direction- the forces are added. Opposite directions- the forces are subtracted. Similar to your “net” pay on your paycheck. Its what’s left after subtracting taxes, insurance, etc.

Net Force Examples

Net Force Examples

What’s a Net Force? “Left-over” Force when forces are “added” together. Same direction the forces are added. Opposite directions the forces are subtracted. Similar to your “net” pay on your paycheck. Its what’s left after subtracting taxes, insurance, etc.

What if the Forces are Unbalanced? Predict what would happen in each case above.

Net Force Examples

Your Turn Find the net force for each situation. What will happen to its motion?

YOUR TURN: LABEL ALL THE FORCES #1 Equilibrium #2 Equilibrium #3 Rock is sliding at constant speed on a frictionless surface #4 Rock is sliding to a stop #5 Rock falling. No friction #6 Rock falling at constant velocity (terminal velocity) #7 Rock is tied to a rope and pulled straight upward., accelerating at 4.90 m/s2. No air resistance. #8 Rock is tied to a rope and accelerates straight downward at 4.90 m/s2. No air resistance.