Force

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.

The normal force isFN = - mgwhere: •FN is the normal force in Newton (N), •m is the mass in kg, and •g is the gravitational force in m/s2.

Friction force and Coefficient of friction

Force of friction Ff can be calculated by the formula

Ff = µFNwhere:

• Ff is the force of friction in N,• µ is the coefficient of friction, and • FN is the normal force in N.

• The value of µ depends on surface you are dealing with. The following table shows some example of µ.

Surface Value of µrubber on dry asphalt ~1rubber on wet asphalt 0.95 steel on steel 0.18steel on ice 0.010rubber on ice 0.005

Torque in rigid body

• Friction torque is a measure of how much a force acting on an object causes that object to rotate.

• 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.

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

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

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

respectivelyFigure: In a crashtest. Much of thecar’s initial kineticenergy istransformed intothe energy it took todamage the vehicle

Conservation of momentum

• Isolated system– When no net external force acts on a system, the

total momentum of the system remains constant in time.

Σmivi = Σmfvf

Concept