Newtons 3rd Law of Motion and Friction

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RANDOLF L. FLORESNewtons 3rd Law of Motion and FrictionNewtons 3rd Law of Motion:For every ACTION, there is an EQUAL and OPPOSITEREACTION.

Newtons 3rd Law simply means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.So, if someone pushes horizontally against a wall with a force of 100 N, then the wall will push horizontally against the person with a force of 100N.

IMPORTANT: Do not confuse this situation with equilibrium of forces in which several forces act on the SAME OBJECT, with a zero resultant force. Here the forces act on DIFFERENT OBJECTS, i.e., the person on the wall and the wall on the person.

3rd Law of Motion (Animation)

Example # 1While driving down the road, a fly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The fly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?

Each force is the SAME size. Remember: for every action, there is an EQUAL and OPPOSITE REACTION. Example # 2In the diagram on the right, a 60 kg roller skater pushes a 30 kg roller skater with a force of 5 N. The force exerted by the 30 kg roller skater on the 60 kg roller skater is ___ N.

According to Newtons third law of motion, for every action force, there is an equal and opposite reaction force. So, the force exerted by the 30 kg roller skater on the 60 kg roller skater is equal to the force exerted by the 60 kg roller skater. Therefore, the answer is 5 N.FRICTION

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.Concepts on FrictionFriction is independent of the velocity (below certain limits) between two sliding bodies.If the NORMAL FORCE (force perpendicular to the surface) remains the same, friction is independent on the area of the rubbing surfaces.The force of friction is proportional to the total force pressing one surface against another.The force of friction is slightly greater at the start than after the motion has begun.Three Types of FrictionSLIDING FRICTIONROLLING FRICTIONSliding friction is the friction between two solid surfaces as shown below. If it werent for friction, students would slip right off their chairs and pencils would slide right out of their hands.Rolling friction is much less than sliding friction. It is the resistance that occurs when an object rolls. People use wheel barrels and dollies to help move large or heavy objects.

FLUID FRICTIONFluid friction occurs when a solid object is in contact with a fluid, such as a liquid or gas. When a force is applied to either the object or the fluid, there is a friction force that resists the motion. Examples where fluid friction occurs are water flowing through a hose, an airplane flying through the atmosphere, and oil lubricating moving parts. Air resistance is a type of fluid friction. As the force of gravity pulls an object down, fluid friction from the air pushes back up on the object. The diagram below illustrates the amount of force acting on a skydiver throughout their fall. If in free fall long enough, the person will reach terminal velocity when the force of air resistance is balanced with the force of gravity. To increase fluid friction, a person uses a parachute to increase surface area. This allows the person to catch air.

Calculating Frictional Force

Where is the coefficient of friction; and Fn is the Normal Force. Two types of coefficient of friction :A. Coefficient of Static Friction (s) - is defined as the ratio of the force necessary to start motion to the force pressing the bodies together, that is, the Normal Force.B. Coefficient of Kinetic Friction (k) - is the ratio of the force necessary to move the body uniformly, to the Normal Force. Calculating Frictional Force on a HORIZONTAL SURFACESample Problem: What is the (a) STATIC FRICTIONAL FORCE for a 30 kg mass on a horizontal surface where the coefficient of static friction is 0.525? And; (b) If you push the object with the INITIAL STATIC FORCE, what acceleration does the object experience when it is moving where the coeffiecient of kinetic friction is 0.325?

Calculating the COEFFICIENT OF FRICTION ON A HORIZONTAL SURFACESample Problem: A 24 kg crate is moving with a constant velocity along the ground due to a 53 N force applied to it in the direction of motion. What is the coefficient of Kinetic Friction?