Open Question
The foot of a 55 kg sprinter is on the ground for 0.25 s while her body accelerates from rest to 2.0 m/s. (b) What is the magnitude of the friction force?
6. Intro to Forces (Dynamics)
Newton's Third Law & Action-Reaction Pairs
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Which of the options is NOT an action-reaction pair in the following situation? A book slides across the floor, slowing down due to friction.
A
Friction on the book from the floor & friction on the floor from the book
B
Weight of the book from the Earth & gravitational force on the Earth from the book
C
Weight of the book & normal force on the book
D
Normal force on the book from the floor & normal force on the floor from the book
Verified step by step guidance1
Identify the concept of action-reaction pairs: According to Newton's Third Law, for every action, there is an equal and opposite reaction. This means that forces always come in pairs, acting on two different objects.
Analyze the first option: 'Friction on the book from the floor & friction on the floor from the book'. These forces are indeed an action-reaction pair because the frictional force exerted by the floor on the book is matched by an equal and opposite frictional force exerted by the book on the floor.
Examine the second option: 'Weight of the book from the Earth & gravitational force on the Earth from the book'. These forces are also an action-reaction pair. The Earth exerts a gravitational force (weight) on the book, and the book exerts an equal and opposite gravitational force on the Earth.
Consider the third option: 'Weight of the book & normal force on the book'. These forces are not an action-reaction pair. The weight of the book is the gravitational force exerted by the Earth on the book, while the normal force is the support force exerted by the floor on the book. They act on the same object (the book) and are not equal and opposite forces acting on two different objects.
Review the fourth option: 'Normal force on the book from the floor & normal force on the floor from the book'. These are not an action-reaction pair. The normal force on the book is exerted by the floor, while the normal force on the floor is exerted by the book, but they are not equal and opposite forces acting on two different objects in the context of Newton's Third Law.
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