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I am a high school student and I am studying the Newton's Laws of Motion. In my book, the physics of walking is described as follows:

We push the ground with our feet and due to Newton's third law, the reaction force from the ground pushes us forward. One day, my physics teacher told me that frictional force between the feet and the ground push us forward.

Am I wrong in concluding that the frictional force and the reaction force from the ground are the same? Moreover, it seems counter intuitive that friction pushes us forward. Friction tries to oppose motion, right?

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    $\begingroup$ If you happen to be thinking that "reaction force" and "normal force" are the same it is likely because you have heard or read the phrase "normal reaction force", but friction is—by definition—the part of the contact force directed along the plane of contact while the normal force is that perpendicular to the plane of interaction. So, yes, the contact forces on the person are the 3rd law pair of the contact forces on the ground, but they are not necessarily strictly normal: friction points along the ground. $\endgroup$ – dmckee May 15 '17 at 5:23
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    $\begingroup$ Related: physics.stackexchange.com/q/7657 and links therein. $\endgroup$ – dmckee May 15 '17 at 5:24
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Since you exert your weight (the force with which the Earth pulls you downwards) on the ground, the ground exerts another force (normal force) on you. In case you are not accelerating upward (vertically 0 acceleration), the normal force equals you weight (in magnitude). But since both of them are opposite in direction, they cancel each other. So, the net force vertically is zero. enter image description here

Now, the normal force is one of the reaction forces. Now, coming to friction, your thought that friction always opposes motion is a misconception. When a car accelerates on a road, what helps the car to accelerate on the road is friction.

Now, coming to walking on a road, what you do is push the ground backward with your feet to help you move forward. So, here the ground tries to oppose the force exerted by your feet by applying a frictional force in the opposite direction.enter image description here

But, since the net horizontal force "acting on you" (that is frictional force) is in the forward direction (that is your direction of motion or the direction in which you want to move), you are able to move forward.

Now, the reaction forces (talking individually) are two and both are vectors. One is the normal force (which is always perpendicular to the surface, and the other is the frictional force. So, the vector sum of the two is called the "contact force".This is the one basically that you consider as the "reaction force".

enter image description here

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  • $\begingroup$ Sorry, in the first diagram its " rest or uniform motion " and not " rest of uniform motion " :) $\endgroup$ – Physicpsycho May 15 '17 at 5:37
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The Newton's third law pair of force, action and reaction, are:

  • The frictional force on the walker due to the ground which enables the walker to move in a forward direction.

  • The frictional force on the ground (Earth) due to the walker which makes the ground move in a backwards direction.

The force on the ground shows itself if part of the surface is loose, for example the sand/stones/pebbles are "thrown" backwards when a person or a car tries to move forward.
Get on a merry-go-round in a children's playground and walk one way on the outside of the merry-go-round and the merry-go-round will move in the opposite direction.

If there are no frictional forces then when the walker tries to move there would be relative movement between the sole of the shoe and the ground.
It is that relative movement between the sole of the shoe and the ground which the frictional forces try to or do prevent happening.


Update as the explanation above would result in a continuous acceleration.

As with all things to do with the human body a detailed explanation is very difficult and walking is no exception.

This sequence of photographs illustrates the forces acting when a person is walking.

enter image description here

The vertical force due to the ground (often called the normal reaction) is shown in red and the frictional force is shown in blue.

The graph below illustrates how the forces vary with % gait cycle.

enter image description here

Thinking of the horizontal axis as something to do with time then the area under the frictional force graph (blue) is related to the impulse (change in momentum) which will be approximately zero if the person is walking at an unchanging average speed.

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Am I wrong in concluding that the frictional force and the reaction force from the ground are the same?

Not the same, but equal and opposite. Forces are vectors.

Your feet push against the ground and the force has a backward direction to the direction you want to walk. The frictional forces push back with an equal and opposite force "to every action there is an equal and opposite reaction" third law.

Think: Push against a wall, it pushes back. In friction the atoms and molecules of the shoe push against the atoms and molecules of the ground, and they push back. See also this answer.

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protected by Qmechanic May 15 '17 at 6:37

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