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9.1 The First Condition for Equilibrium
Simple machines are described.
Explain the forces exerted by muscles.
A bad posture can cause back strain.
Skeletal muscles are attached to joints.
They are normally motionless relative to the Earth.
Their acceleration is zero because they are motionless.
Something in common with a car moving at a constant velocity is that it has an acceleration of zero.
Newton's second law states that the net external force is zero for all stationary and moving objects.
The forces are balanced.
They are in equilibrium.
The study of forces in equilibrium is called statics and it's a special case ofNewton's second law.
In this chapter, we cover the topic more thoroughly, including consideration of the possible effects of the forces acting on the object.
Two conditions must be satisfied to achieve equilibrium.
The topics of the first two sections are about these conditions.
The net external force on the system must be zero in order to achieve equilibrium.
The net external force in any direction is zero if the net is zero.
The net external forces along the x- and y-axes are zero.
A person is motionless.
The forces are acting on him.
The forces are both vertical.
The car is moving at a constant rate.
The net external force in any direction is zero.
The weight of the car is supported by the normal forces and the applied force between the tires and the road is balanced.
It is not enough for the net external force of a system to be zero.
The net external force is zero in both situations, but in one case, equilibrium is achieved, whereas in the other, it is not.
We know that the point at which a force is applied is a factor in determining equilibrium.
This will be explored in the next section.
An ice hockey stick is lying flat on ice with two different horizontal forces applied to it.
The force is balanced by the support of the ice.
static equilibrium is achieved in this case.
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