1. 4-4: Everyday Forces Objectives: Explain the difference between mass and weight Find the direction and magnitude of the normal force Describe air resistance as a form of friction Use coefficients of friction to calculate frictional force.

2. Weight Weight is the magnitude of the force of gravity acting on an object. Mass does not change as you go up in an airplane, but weight can.

3. The Normal Force When an object on a table is being acted on by gravity the object also has an equal and opposite reaction. This contact force is called normal force, F n. The normal force is always perpendicular to the contact surface, but it is not always in the opposite direction of gravity. – Recall the book on the architects desk. – If it is the opposite of gravity it will be mg.

4. The force of friction IF you have a jug of juice on a table it will not move because the only forces acting on it are the force of gravity and the normal force. If you push lightly on the jug it still will not move, as long as the net forces are zero. As you apply more force to the jug it will suddenly move until you remove the force. The friction that opposes an object to keep it at rest (in this case friction) is called static friction F s. – Until the force is greater than the static force the object will remain at rest. (F s = -F)

5. When the applied force is as great as it can be without causing the jug to move the force o static friction reaches it maximum value, F s,max. The jug will start to move with an acceleration. The net external force acting on the jug is equal to the difference between the applied force and the maximum force of static friction (F- F s,max ). The force of friction

6. Kinetic friction is less than static friction Kinetic friction is the force exerted on a moving object (F k ). In our case the friction on the moving jug. The net external force on a moving object is equal to the difference between the applied force and the force of kinetic friction. (F- F k )

7. Friction depends on the surface in contact A sled will move much easier on an icy surface than it will on a gravel road. To express the relationship between frictional forces and the properties of different surfaces, we use a symbol that relates the normal force to the frictional force for different surfaces. This symbol, μ the lower case Greek letter mu, is called the coefficient of friction, and is used to find the value for the friction force between two surfaces.

8. Coefficient of Static Friction μ s = F s,max / F n μ k = F k / F n

9. Coefficient of friction Page 144 Table 4-2 gives the coefficients of friction between common materials. The value for the coefficient of kinetic friction is always lower than the value for static friction. Static friction of 1.0 = normal force.

10. In the example of the juice jug earlier lets assume that μ s = 1.0. To move the jug you would need to apply a force equal to the normal force. This means we would have to apply a force equal to the weight of the juice jug to move it. If the coefficient of friction were 1.5 you would need to apply a force 1.5 times the weight of the jug. Coefficient of friction

11. A 24 kg crate initially at rest on a horizontal surface requires a 75N horizontal force to set it in motion. Find the coefficient of static friction (μ s ) between the crate and the floor. – Given F s,max = 75N, m= 24 kg – Find μ s = ? – μ s = F s,max / F n

12. Air Resistance is a form of Friction Any time an object moves through a fluid medium such as air or water, the fluid provides a resistance to the object’s motion. The force of air resistance, F R, acts in the direction opposite the direction of the objects motion. Resistance is proportional to the objects speed. When the FR is equal to the force acting in the opposite direction, the acceleration is zero and the car moves at a constant speed. When a free falling objects force downward due to gravity is equal to the upward force of resistance is balanced to zero and the acceleration is zero. – Once this is reached it will move with a constant maximum speed called terminal velocity, or terminal speed.