Chapter 4 Physics

Section 4-1 I. Forces A. Def- a push or pull; the cause of acceleration. B. Unit: Newton Def- amt. of force when acting on a 1 kg mass produces an acceleration of 1 m/s 2 1N = 1 kg  m/s 2

Cont: C. Conversions 1 lb = N 1N = lb D. Kinds of forces 1. Contact force- physical contact 2. Field- exists, even in absence of contact ex. Gravity, electricity Especially important in nuclear & particle physics

II. Force Diagrams A. Force is a vector. B. Def- a diagram of the objects involved in a situation and the forces exerted on the objects.

III. Free – body diagram A. Shows forces on 1 object. B. Does not show forces exerted by the object. C. Drawing a free-body diagram. 1. Simple diagram visualizing the object. 2. Draw and label vector arrows representing all external forces.

Section 4-2 Newton’s 1 st Law I. Inertia- 1 st realized by Galileo in 1630’s A. An object at rest will stay at rest in the absence of external forces. -An object in motion will stay in motion….. B.Further developed by Newton in st law C. When net external force on an object is zero, its acceleration is zero.

Cont: II. Forces A. External force- single force acting on the object. B. Net External Force- vector sum of all the forces acting on a body. (resultant force) ‏ C. Sample problem 4A 1. Make a free body diagram 2. Find x & y components of all forces. 3. Find net external forces 4. Evaluate your answer.

III. Inertia A. Def- the reluctance of any body to change its state of motion. B. Mass is a measurement of inertia. *1. Mass is not weight. a. Mass- amt of matter in a body b. Weight- pull of gravity on an object. 1 Kg. = 9.8 Newtons W = mg

IV. Equilibrium A. An object is in =ibrium when all net forces = 0. Ef y = 0 Ef x = 0 B. At rest or moving w/ constant velocity.

Section 4-3 I. Newton’s 2 nd Law- the accel. of an object is directly proportional to net external force acting on the object, and inversely proportional to its mass.F= ma

II. Newton’s 3 rd Law A. Forces always exist in pairs. B. The force exerted on object A by B is equal and opposite to the force on B by A. C. One force is the action the other force is the reaction. D. The forces do not balance each other- they act on different objects.

Section 4-4 I. Everyday Forces A. Weight- the pull of gravity on an object. 1. Weight will change; mass will not change w= mg B. Normal Force- a contact force acting  to the surface of contact. 1. Opposes gravity, but not necessarily parallel to gravity.P. 142 fig  to surface.

C. Friction Forces 1. Static Friction- force exerted on a motionless body by its environment to resist an external force. 2. Kinetic Friction- the force exerted on a moving object. a.  than static friction. 3. Depends on the 2 surfaces.

II. Coefficient of Friction Coefficient of Friction- ratio between static friction and the normal force.  s = F s max/ F n  k = F k / F n

III. Air Resistance = F r A.  To car’s speed When F r = speed, accel = 0.