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?
Best Answer
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.
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.
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.