1. Force

2. Normal force Illustration The normal force acts on any object that touches surface (either directly or indirectly). The normal force would be applied on a ball on a table, but not on a ball in the air, for instance. It always acts perpendicularly to the surface.

3. The normal force is F N = - mg where: F N is the normal force in Newton (N), m is the mass in kg, and g is the gravitational force in m/s 2.

4. Friction force and Coefficient of friction

5. Force of friction F f can be calculated by the formula F f = µ F N where: F f is the force of friction in N, µ is the coefficient of friction, and F N is the normal force in N.

6. The value of µ depends on surface you are dealing with. The following table shows some example of µ. SurfaceValue of µ rubber on dry asphalt~1 rubber on wet asphalt0.95 steel on steel0.18 steel on ice0.010 rubber on ice0.005

7. Torque in rigid body Friction torque is a measure of how much a force acting on an object causes that object to rotate.

8. Static friction – friction force while it is greater than applied force, as a result the applied force give no motion to the subject. Kinetic friction – friction force that acting while the subject/object is in motion because of applied force.

9. Torque of friction Force acting on the moving object result to the torque of friction. The direction of the rotation due to the resultant force of friction. – Refer to the figure slide 3

10. Moment Force about a pint – Moment is a force of measure of its tendency to cause a body to rotate about a specific point or axis. – Perpendicular force to the moment arm Example of simple beams

11. Momentum Force that causes it hard to stop an object. If you were running, you would have momentum. Because of this, if you tried to stop really fast, you might fall because the momentum wants you to keep going forward. Momentum is define by: » mv where m and v are the mass and the velocity of the object respectively Figure: In a crash test. Much of the car’s initial kinetic energy is transformed into the energy it took to damage the vehicle

12. Conservation of momentum Isolated system – When no net external force acts on a system, the total momentum of the system remains constant in time. Σ m i v i = Σ m f v f

13. Concept