Chapter 4

Physics

Section 4-1

I. ForcesA. 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/s2

1N = 1 kgm/s2

Cont:C. Conversions 1 lb = 4.448 N1N = 0.225 lbD. Kinds of forces

1. Contact force- physical contact2. 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 1st Law

I. Inertia- 1st 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 1687 • 1st 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. InertiaA. 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 bodyb. Weight- pull of gravity on an object.1 Kg. = 9.8 NewtonsW = mg

IV. Equilibrium

A. An object is in =ibrium when all net forces = 0. Efy = 0 Efx = 0

B. At rest or moving w/ constant velocity.

Section 4-3

I. Newton’s 2nd 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 3rd LawA. 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 ForcesA. 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 4-20• 2. 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 = Fsmax/ Fn

k = Fk / Fn

III. Air Resistance = Fr

A. To car’s speedWhen Fr = speed, accel = 0.