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FORCES FORCES • Force is a vector quantity and is measured in newtons (1N) • There are different type of forces: – – – – – – – – – weight friction force normal reaction force tension force buoyancy (upthrust) force lift force thrust force compression force air resistance force WEIGHT vs MASS • Weight is is the pull of gravity on a body W=mg • If that body is on the surface of the earth, weight is directed towards centre of the earth (usually we say it is directed vertically down) • Mass and weight are not the same thing – weight describes force, mass describes amount of matter in a body TENSION Tension (T) is the force that arises in any body when it is stretched. NORMAL FORCE If a body touches another body, there is a reaction force (R) between the two bodies. This force is perpendicular to the body exerting the force DRAG FORCES Drag forces are forces that oppose the motion of a body through a fluid (gas or liquid). They are directed opposite to the velocity of the body and generally depend on the speed of that body. Higher speed equals higher drag force. UP THRUST FORCE An object placed in a fluid medium will experience up thrust. If the up thrust force on a body is equal to the weight, the body will float in the fluid. FRICTIONAL FORCE Frictional forces (f) are forces that oppose the motion of a body f. FREE BODY DIAGRAM • Problems involve more than one body and more than one force. • We will describe each body separately and only the forces acting on that body, not the forces that body exerts something else Normal force Weight ADDING FORCES • Force is a vector quantity • If two or more forces act on the same body, you must add them vectorially 70N 60N 150N 100N BALANCED FORCES • If the resultant force on a body is zero, the forces are said to be balanced • If a body is in translational equilibrium, it means that the forces acting on the body, in all three dimensions, are balanced 100N 100N NEWTON’S LAWS • 1st LAW – The velocity of a body remains constant unless the body is acted upon by an external force Body will either remain at rest, or move with constant velocity, unless acted by an unbalanced force • 2nd LAW – The acceleration a of a body is parallel and directly proportional to the net force F and inversely proportional to the mass m F=ma The rate of change of momentum of body is proportional to the applied force and takes place in direction in which the force acts • 3rd LAW – The mutual forces of action and reaction between two bodies are equal, opposite and collinear. If body A exerts a force on body B, then body B exerts an equal and opposite force on body A.
