First law: Objects at rest tend to stay at rest and objects in motion tend to stay in motion if no net force is acting on them. Second law: If there is a net force on an object, the object accelerates. F = ma The acceleration is directly proportional to the net force and inversely proportional to mass. Third law: For every action force, there is an equal and opposite reaction force. F A on B = -F b on A

Frictional force: Objectives: Introduction to: The force of friction: static friction and kinetic friction Coefficient of friction. Calculating friction.

Force of Friction: We know that the force of friction always acts in the direction _______________ the motion of the object and acts to slow the object down.

Observation 1: Sliding a book across the desk. When you start pushing the book doesn’t ________, but when you push harder it starts to move. Before the book starts to move, there is a force of friction present which is equal and opposite to the applied force. If there was no friction then with the force applied the book would've been ____________________. Since the book is not moving this force of friction is called static friction. Static means ______________________, and this type of force of friction is given the symbol . move in motion /moving stationary (not moving)

Observation 2: Sliding a book across the desk. . After the book starts to move, the force you need to move it __________________. Once the book starts to move, the amount of force needed move it at constant velocity ____________. This means the force of friction must also decrease, since these are the only forces in the x direction. Because the book is moving it has a different type of force of friction acting on it called kinetic friction. Kinetic means __________________, and the force of kinetic friction is given the symbol . decreases decreases

The static frictional force you must overcome to start an object is larger than the kinetic frictional force.

Question: What happens to the force of friction when you push down on the book? When you push down on the book the amount of force needed to start it moving ___increases_____, meaning the amount of ____force of_________ friction must also increase. Similarly, once the book is moving it now takes more force to keep it moving at a constant velocity meaning the force of kinetic friction has also increased.

To understand how pushing down on the book increases static and kinetic friction we need to draw a FBD. If we calculate the net force that acts on the book it is still ________ because the acceleration is zero. zero But since there is a greater force down, the normal force that balances this force must be greater. We can conclude that the normal force is proportional to the force of friction. where means proportional

Question: Do objects with the same normal force always have the same force of friction? Not all objects with the same normal force have the same force of friction. Different materials slide easier than others. For example, your normal force does not change if you are standing on ice or on concrete but the amount of frictional force that you experience does change.

Coefficient of friction The force of friction that an object experiences can be expressed with the following equation: Where μ (mu) is called the coefficient of friction and is different for all surfaces. Mu is a positive number and has no units. All combinations of surfaces have two μ values, one is for if the object is moving, , and one for if it is static or not ________ , . * It is important to note that once an object is in motion, the force of friction that it experiences is constant. * moving

Direction: opposite to the motion of the friction. Ex 1: A ferret with a weight of 535 N is pushed along a flat surface and experiences of frictional force of -53.5 N. What direction is the motion of the ferret? What is the coefficient of kinetic friction? Solution: Direction: opposite to the motion of the friction. = 53.5 N /535 N = 0.1

F app. = Ff =1800.0 N Fn = Fg = (183.6 kg)X(9.81m/s²) = 1801.116 N Ex 2: An emu must pull with a force of 1800.0 N to start a donkey cart moving. If the donkey cart has a mass of 183.6 kg what is the coefficient of static friction between the cart and the cement? Solution: draw FBD Given : F app. = Ff =1800.0 N Fn = Fg = (183.6 kg)X(9.81m/s²) = 1801.116 N We know: = 1800N /1801.116 N = 0.9994

  Ex 3: Once the emu starts the donkey cart moving the force it needs to keep it moving at a constant velocity decreases to 1440.0 N. a) Draw a free-body diagram for the cart b) What is the friction force acting on the cart? The applied force just overcome the frictional force. Therefore the applied force must be equal to the frictional force. Ff = 1440.0 N

  Ex 3: Once the emu starts the donkey cart moving the force it needs to keep it moving at a constant velocity decreases to 1440.0 N. c) What is the net force acting on the cart? d) What is the coefficient of kinetic friction? Hint: constant velocity Net force = zero = 1440.0/1801.116 N = 0.7995