3.1.6 Actions, Reactions, and Newton's Third Law Flashcards
Actions, Reactions, and Newton’s Third Law
- Newton’s third law states: if object A exerts a force on object B, then object B always exerts a force back on A. The two forces are equal in magnitude but opposite in direction to each other.
- It is sometimes useful to think of Newton’s third law in terms of action and reaction pairs.
- It is important to understand that the opposite forces in Newton’s third law apply to different objects.
- Newton’s third law is used when comparing forces on objects.
Taken together, Newton’s three laws form the basis of modern dynamics. Which of the following correctly states these three laws?
In the absence of a net force, a body at rest remains at rest and a body in motion continues moving in a straight line with constant speed. Force is equal to mass times acceleration. For every action, there is an equal but opposite reaction.
A hammer strikes a nail, driving it deep into a block of wood. The force on the nail by the hammer is represented by FNH. The force on the hammer by the nail is FHN. Which of the following correctly reflects what occurred when the hammer struck the nail?
FHN = -FNH
Newton’s third law states that for every action, there is a reaction. Which of the following accurately restates this law?
If object A exerts a force on object B, then object B always exerts a force back on A. The two forces are equal in magnitude, but opposite in direction to each other.
Which of the following correctly completes Newton’s third law? If object A exerts a force on object B, then object B always exerts a force back on A. The two forces are _____ in _____ but _____ in _____ to each other.
equal, magnitude, opposite, direction
Which of the following best describes what would happen if a nail were hammered in outer space?
The nail would accelerate in the direction of the force during contact and would continue to move in the same direction with constant speed.
Newton’s third law is sometimes counterintuitive. For instance, it may be difficult to accept that when a car collides with a sport-utility vehicle (SUV), the small car pushes back with equal force on the large SUV. Given that Newton’s third law is correct, which of the following is correct, and what does it suggest?
Note: S = SUV, C = car
Both A and B.
Newton’s third law for any two objects, A and B, can be written as which of the following?
FAB = -FBA
Consider a horse pulling a cart, and all the forces involved:
FCH = the force on the cart by the horse
FHC = the force on the horse by the cart
FHG = the force on the horse by the ground
FGH = the force on the ground by the horse
FCG = the force on the cart by the ground
For the horse to accelerate and pull the cart, which of the following needs to be true?
FHG > FCH
FCH > FCG
