Suppose you press a book against the wall with your hand. The book is not moving and you are pushing horizontally. Now suppose you decrease your push, but not enough for the book to slip. What happens to each of the following forces? DO they increase in magnitude, decrease, or not change?
F(push)
Weight
Normal
Friction Force
F(push) of course decreases
The weight, m g, does not change
Now the normal force against the wall is exactly your push force.
The maximum friction force is mu, the coefficient of friction, times the normal force. Until the book slips, it is equal to mg, the weight. When mu * F decreases to equal the weight, mg, then the book will start to slip.
Here is a way to think about this:
Think of the book attached to the wall by a frictional hook.
Until the hook breaks, the hook(friction)holds the book up and the hook (friction) force is exactly equal to the weight because the friction and the weight are the only vertical forces on the book and the book is not accelerating.
However when the normal force decreases so that mu F = mg, the frictional hook breaks and the book slides down.
When you press a book against the wall with your hand, and the book is not moving, the following forces are acting:
1. F(push) - This is the force that you are exerting on the book horizontally. If you decrease your push, but not enough for the book to slip, the magnitude of the horizontal pushing force will decrease. Therefore, F(push) decreases in magnitude.
2. Weight - This is the force due to gravity acting vertically downward. The weight of the book remains unchanged as long as there is no vertical motion.
3. Normal - This is the force exerted by the wall on the book perpendicular to the wall's surface. The normal force depends on the weight of the book and counteracts it to keep the book from sinking into the wall. Since the weight of the book doesn't change, the normal force remains the same.
4. Friction Force - This is the force of friction between the book and the wall, opposing the motion between the two surfaces. As long as the book is not slipping, the force of friction adjusts to match the applied force. If you decrease your push, the friction force also decreases to match the reduced applied force. So, the friction force decreases in magnitude.
When you press a book against the wall with your hand and apply a horizontal force, several forces come into play. Let's analyze the situation and observe the changes in each force:
1. F(push) - This is the force you apply by pressing the book against the wall with your hand. When you decrease your push but not enough for the book to slip, the magnitude of the applied force, F(push), will decrease. So, F(push) decreases.
2. Weight - Weight is the force exerted by gravity on an object due to its mass. It acts vertically downward through the center of mass. The weight of the book remains constant as long as there are no external factors affecting the gravitational force. So, weight does not change.
3. Normal force - The normal force is the force exerted by a surface perpendicular to it. In this case, the wall exerts a normal force on the book, preventing it from passing through the wall. When you decrease your push, the normal force will also decrease since it is directly influenced by the applied force. So, normal force decreases.
4. Friction force - The friction force opposes the relative motion or tendency of motion, between two surfaces in contact. In this scenario, the friction force prevents the book from sliding downwards. As you decrease your push, the force of friction will also decrease to match the reduced applied force, keeping the book from slipping. So, friction force decreases.
To summarize:
- F(push) decreases
- Weight does not change
- Normal force decreases
- Friction force decreases