Download Force Pairs

Demonstration Questions
1. How did the direction the ball moved
compare to the direction the teacher moved?
2. The ball moved because the teacher applied
a force to it by pushing it. Why did the
teacher move? What pushed the teacher?
Chapter 4: Newton’s 3rd Law of
Motion
Force Pairs
What is a force?
• Force: a push or a pull upon an object.
• They result from interactions between two
objects.
What is Newton’s 3rd Law of Motion?
• Newton’s 3rd Law of Motion: for every action,
there is an equal and opposite reaction.
What is a force pair?
• Force pair: two things that exert force on one
another.
– Forces are always equal.
– Forces are always opposite directions.
Where can we see Newton’s 3rd Law?
• Fish swimming in the water.
Newton’s 3rd Law and Flying Birds
Newton’s 3rd Law and Driving
• Wheels interacting with the road.
Check for Understanding
• While driving down the road, a firefly strikes the
windshield of a bus and makes a quite obvious mess
in front of the face of the driver. This is a clear case of
Newton’s 3rd Law of Motion. The firefly hit the bus
and the bus hit the firefly. Which of the 2 forces is
greater?
– The force on the firefly.
– The force on the bus.
– Neither force is greater
than the other.
Check for Understanding
• For years, space travel was believed to be
impossible because there was nothing which
rockets could push off of in order to provide the
propulsion necessary to accelerate. This
statement is not true because…
– Space is void of air so the rockets have nothing to
push off of.
– Gravity is absent in space.
– Space is void of air and so there is no air resistance in
space.
– Rockets do accelerate in space because of Newton’s
3rd Law of Motion.
Check for Understanding
• Many people are familiar with the fact that a rifle
recoils when fired. This recoil is the result of
action-reaction force pairs. A gunpowder
explosion creates hot gases which expand
outward allowing the rifle to push forward on the
bullet. Consistent with Newton’s 3rd Law, the
bullet pushes backward upon the rifle. How fast is
the acceleration of the recoiling rifle?
– Greater than the acceleration of the bullet.
– Smaller than the acceleration of the bullet.
– The same size as the acceleration of the bullet.
Check for Understanding
• In the top picture below Mr. Weinandt is pulling upon a rope which
is attached to a wall. In the bottom picture, Mr. Weinandt is pulling
on a rope attached to an elephant. In each case, the force scale
reads 500 N. In which case is Mr. Weinandt pulling with more force?
– Scenario 1: Rope attached to the wall.
– Scenario 2: Rope attached to the elephant.
– He is pulling with the same force in both scenarios.
Identifying the parts of force pairs
• Forces come in pairs that are known as
“action-reaction” forces pairs.
– Action: object A exerts a force on object B.
– Reaction: object B exerts a force on object A.
Identifying “Action-Reaction” Pairs
• Action: baseball pushes the glove to the left.
• Reaction: glove pushes baseball to the right.
Action
Identifying “Action-Reaction” Pairs
• Action: Bowling balls pushes the pins to the
right.
• Reaction: Pins push bowling ball to the left.
Action
Identifying “Action-Reaction” Pairs
• Action: Enclosed air particles push balloon
wall outwards.
• Reaction: Ball wall pushes the air particles
inward.
Check for Understanding
• Consider the interaction depicted below between
foot A, ball B, and foot C. The 3 objects interact
simultaneously. Identify 2 pairs of action-reaction
forces. Use the terms foot A, ball b, and foot C when
writing your statements.
Answers
1. Foot A pushes ball C to the right, ball C
pushes foot A to the left.
2. Foot B pushes ball C to the left, ball C pushes
foot B to the right.
Check for Understanding
• Identify at least 4 pairs of action-reaction
force pairs in the following diagram.
Answers
1. The elephants feet push down on the ground, the
ground pushes up on the elephant’s feet.
2. The elephant pulls the rope to the right, the rope
pulls the elephant to the left.
3. The man pulls the right part of the rope to the left,
the rope pulls the man to the right.
4. The tractor pulls the rope to the left, the rope pulls
the tractor to the right.
5. The tractor’s tires push down on the ground, the
ground pushes up on the tires.
6. The man pulls the left part of the rope to the right,
the rope pulls the man to the left.
Exit Slip
• On a separate sheet of paper to be turned in,
explain an “action-reaction” force pair that
you have experienced in your own life.
Identify the two objects and then identify the
action and reaction.
• You may NOT use one of the examples
discussed in class today!
How does this demonstrate Newton’s
3rd Law?