Question

a.) "for every action, there is an equal and opposite reaction." You plan to stan on a skateboard and throw an orange backward. someone tells you that since the orange is moving backward, the reaction is the skateboard moving forwards. how would you correct their statement? or is it already correct?

b) "for every action, there is an equal and opposite reaction." a book sits on a desk. The Earth pulls the book down. What is the reaction to this action, according to newton's 3rd law?

c.) for a problem like the Atwood machine, where two objects are connected by a rope, what is the physical significance of the rope not stretching?

d.) an object experiences a net force that is directed up and to the let 25 degrees. Since the mass is constant, Newtown's 2nd Law reduces F=ma which implies that the acceleration of the object is also up and to the left at 25 degrees (since the mass must be postive value). Must the motion of the object also be up and to the left at 25 degrees? (are there any cases where the velocity of the object would not also be up and to the left at 25 degrees?)

Answer 1

a. The statement is correct, since boy apply force on the orange , the orange applies force on boy + skateboard system, so they both move forward.

B. Earth pulls the book down, the reaction is that the book pulls the earth up by an equal force

C. If the rope is stretchable then the acceleration on both sides will be different, solid mechanics has to be included to solve, there will even be vibration considerations due to stretching of the rope.

D. The force and acceleration will be in the same direction, but that's not true for velocity and displacement, the direction of velocity depends on the initial conditions, if the velocity has a component perpendicular to the applied force the the body will follow a curved path. Example is a circular motion by a body, where the acceleration is towards center but the velocity is tangential.

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