FORCES

A force is an influence on a system or object which, acting alone, will cause the motion of the system or object to change. If a system or object at rest is subjected to a non-zero force it will start to move. What is a force?

Weight Weight is a force caused (on Earth) by the gravitational attraction of a mass to the Earth’s centre. The weight of a body, of mass m, is defined to be the force, W, with which it is attracted to the Earth. On Earth, W = mg, where g is the acceleration due to gravity (g ≈ 9.81 m s−2 on Earth).

Tension Many mechanics problems involve objects being pulled, pushed or suspended from a string, spring, rod or something similar. The force that the string (or similar) exerts on the object in these types of problems is called tension.

Friction A force that prevents, or tries to prevent, the slipping or sliding of two surfaces in contact.

Air Resistance Acts in the opposite direction to the motion.

Normal reaction force A mass lies on a horizontal surface. The weight of the mass pulls it downwards. The reason it does not fall is because the horizontal surface exerts an equal and opposite force on the mass called the normal reaction force. The normal reaction force always acts perpendicularly to the surface that is causing it.

Example A block lies at rest (not moving) on a rough inclined slope; what forces act on the block?

Example A ball, attached to the ceiling, is swinging on the end of a string in a simple pendulum motion; modelling the ball as a particle, what forces act on it?

Example What forces act on a trailer attached to a tow rope being pulled by a car?

Force Diagrams

Forces We know that a force can be a push or a pull acting on an object There is a good chance that 2 forces can be acting on an object at any one time Examples: –Lifting something –Dragging something –Floating Globe

Balanced Forces If two equal forces are applied to an object in opposite directions, the object does not move / or moves as a constant speed. This is called balanced forces.

Unbalanced forces If two unequal forces are applied to an object in opposite directions, the object does move. This is called unbalanced forces. And someone will win!!

Unbalanced Forces If the forces acting on an object are not balanced then the object with either: –Speed up –Slow down –Change direction –Chance its shape

We can use force diagrams to show this: 4N7N The block has 2 opposing forces being applied to it: 7N to the right and 4N to the left. They are Unbalanced Forces. To Calculate the Resultant Force (Fr), subtract one from the other: 7N – 4N = 3N to the right 3N

Problem: A man of mass 75kg has a weight of 750N. This 750N will act downward on the table and the table will exert a 750N force upwards on the man. The forces are balanced. What would happen if the upward force were less than 750N? Force down 750N Force up 750N

Newton’s Laws

Newton’s First Law Law of Inertia Newton’s Second Law F = ma Newton’s Third Law Action Reaction Law of Universal Gravitation

Mass …is measured in kilograms. …is the measure of the inertia of an object. Inertia is the natural tendency of a body resist changes in motion.

Force …the agency of change. …changes the velocity. …is a vector quantity....measured in Newton’s.

Newton ’ s First Law Law of Inertia “A body remains at rest or moves in a straight line at a constant speed unless acted upon by a force.”

Newton ’ s First Law F No mention of chemical composition F No mention of terrestrial or celestial realms F Force required when object changes motion F Acceleration is the observable consequence of forces acting

Newton ’ s Second Law The Sum of the Forces acting on a body is proportional to the acceleration that the body experiences F  aF  a  F = (mass) a

Net Force

Newton ’ s Third Law Action-Reaction For every action force there is an equal and opposite reaction force

The Law of Gravity Every mass exerts a force of attraction on every other mass. The math… FG = 6.67  N·m 2 /kg 2

Gravity Questions Did the Moon exert a gravitational force on the Apollo astronauts? What kind of objects can exert a gravitational force on other objects?

Gravity Questions The constant G is a rather small number. What kind of objects can exert strong gravitational forces? If the distance between two objects in space is doubled, then what happens to the gravitational force between them?

Weight The weight of an object F W is the gravitational force acting downward on the object. F W = m g

Tension (Tensile Force) Tension is the force in a string, chain or tendon that is applied tending to stretch it. F T

Normal Force The normal force on an object that is being supported by a surface is the component of the supporting force that is perpendicular to the surface. F N

Coefficient of Friction  Kinetic Friction F f =  k F N Static Friction F f   s F N In most cases,  k <  s.