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Static friction is a force that prevents objects from slipping or sliding across a surface. When you press the picture against the wall, static friction acts to stop it from moving downward. It acts parallel to the contact surface and in the opposite direction of the potential motion.

The static frictional force has a limit, meaning it can only hold objects still up to a certain point. After this, the object begins to slide. This limit is defined by the coefficient of static friction (\(\mu_s\)), which is a measure of how "grippy" the two surfaces are when touching. The equation for the maximum static friction is \( F_{friction} = \mu_s \cdot N \), where \(N\) is the normal force.

For the picture to remain in place and not slide down, the static friction created must be at least equal to the gravitational force pulling the picture downward.

Gravitational force is the force that attracts two bodies toward each other, commonly the Earth pulling objects toward itself. When we talk about the gravitational force acting on the picture, we refer to its weight.

The weight of any object is calculated using the formula \( F_g = m \cdot g \), where \( m \) is the mass of the object, and \( g \) is the acceleration due to gravity, which is approximately \( 9.8 \ m/s^2 \) on Earth. In this exercise, with the mass of the picture being \( 1.10 \ kg \), its gravitational force is \( F_g = 1.10 \ kg \times 9.8 \ m/s^2 = 10.78 \ N \).

This gravitational force is what makes the picture want to slide down the wall, thus needing counteraction by an equivalent static frictional force provided by pressing against the wall.

Normal force is a perpendicular force that surfaces exert to support the weight of an object resting on them. In this context, it's the force exerted by the wall when the picture is being pressed against it.

Think of the normal force like a support that prevents the picture from moving through the wall. It's vital because static friction depends directly on it through their relationship: \( F_{friction} = \mu_s \cdot N \).

In this exercise, the pressing force you apply against the wall becomes the normal force. To create enough static friction to counteract the gravitational force, we calculated that the minimum normal force needed is \( 16.33 \ N \). This ensures that the picture doesn't slide down, keeping it in place without movement.

An object is said to be in equilibrium when all the forces acting upon it are balanced, meaning the object remains in its current state, either at rest or moving at a constant velocity. For the picture pressed against the wall, equilibrium means it stays in place, not sliding down or moving up.

To achieve equilibrium with the picture, the static frictional force must balance the gravitational force. Hence, the pressing force must be enough to create sufficient static friction.

To keep the picture from moving, the forces must satisfy the condition \( \mu_s \cdot N = F_g \). This balance is necessary, ensuring the picture's position is stable and it is effectively in equilibrium. By calculating the correct amount of pressing force (normal force), the person ensures the picture remains where it should be, resting securely against the wall.