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7 correct Dynamics without momentum.notebook 1 April 02, 2019 Newton's Third law of motion: There are always two forces acting and two objects involved According to Newton’s third law, when you exert a frictional force on the floor, it exerts an equal and opposite frictional force on you. In reality, the floor pushes on you, propelling you across the floor as shown in Figure 5.11.
Transcript of Newton's Third law of motionnordstromsjhs.weebly.com/uploads/1/2/1/7/121740634/newtons_thir… ·...
7 correct Dynamics without momentum.notebook
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April 02, 2019
Newton's Third law of motion:
There are always two forces acting and two
objects involved
According to Newton’s third law,
when you exert a frictional force on the floor, it exerts an equal
and opposite frictional force on you. In reality, the floor pushes on
you, propelling you across the floor as shown in Figure 5.11.
7 correct Dynamics without momentum.notebook
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April 02, 2019
Forces always occur in pairs, and one body cannot exert a force on another without experiencing a force itself. We sometimes refer to this law loosely as actionreaction, where the force exerted is the action and the force experienced as a consequence is the reaction. Consider a swimmer pushing off from the side of a pool, as illustrated below.
You might think that two equal and opposite forces would cancel, but they do not because they act on different systems. In this case, there are two systems that we could investigate: the swimmer or the wall. If we select the swimmer to be the system of interest, then Fwall on feet is an external force on this system and affects its motion. The swimmer moves in the direction of F. In contrast, the force Ffeet on wall acts on the wall and not on our system of interest. Thus Ffeet on wall does not directly affect the motion of the system and does not cancel Fwall on feet
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Identifying force pairs (action reaction pairs) is important! Can you
describe the other force in the actionreaction pair?
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The elephant's feet push backward on the ground; the ground pushes forward on its feet. The right end of the right rope pulls leftward on the elephant's body; its body pulls rightward on the right end of the right rope. The left end of the right rope pulls rightward on the man; the man pulls leftward on the left end of the right rope. The right end of the left rope pulls leftward on the man; the man pulls rightward on the right end of the left rope. The tractor pulls leftward on the left end of the left rope; the left end of the left rope pulls rightward on the tractor. etc.,
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on the the bus?
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Many people are familiar with the fact that a riffle recoils when fired.
This recoil is the result of action reaction force pairs.A gunpowder explosion creates hot gases that expand outward allowing the rifle to push foward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards against the rifle. The acceleration of the recoil is :
A.smaller than
b. greater than
c.same size as
the acceleration of bullet
7 correct Dynamics without momentum.notebook
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April 02, 2019
a.b.c.
7 correct Dynamics without momentum.notebook
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steps
a ,FP on T= mtotal x a,FA on B= mB xa,
FTotal on A = mAx a
Ftotal on A = FT on A FB on A
