WEEK 7 PHYSICS

MUST DO Unit 2 Cover Page Write three questions Write a statement of what you already know. Unit 2: Molecules to Cells My Questions:  Why does science need evidence?  Why don’t we know all the answers?  Will we ever know all the answers? What I already know: I know that science has a method. Scientists can work in labs. Science can lead to new discoveries. Unit 2: Molecules to Cells My Questions:  Why does science need evidence?  Why don’t we know all the answers?  Will we ever know all the answers? What I already know: I know that science has a method. Scientists can work in labs. Science can lead to new discoveries.

THE CONCEPT OF FORCE

FORCES AND THEIR DIRECTION Forces are measured in Newtons (N) It is important to identify the direction of a force (vector) There are 6 force types we will discuss. Weight Tension Normal (contact) Drag Upthrust Frictional

WEIGHT Force resulting from gravitational attraction W = mg g is the gravitational constant of the planet Units are N/kg or m/s 2 Weight is always directed vertically downward If mass is in kg and g in N/kg then Weight has units of N

TENSION A force that arises when any body is stretched A tension force in a string is created when two opposite forces are applied at both ends. In magnetic forces the “string” is the force between two molecules. Force pulls down where the string attaches to the top and pulls up on the object hanging. For the sake of idealized physics, the string is massless so tension is the same at all points on the string.

NORMAL (CONTACT FORCE) When two objects touch there is a force of reaction or contact This force is perpendicular to the object exerting the force.

DRAG FORCE Force that opposes motion of a moving body through a fluid (gas or liquid) A marble falling through honey. Drag forces are directed in the opposite direction of the velocity vector. The higher the speed the higher the drag force.

UPTHRUST An upward force on an object placed in a fluid If upthrust equals the weight of the body, the body will float in the fluid. If upthrust is less than the weight the body will sink. Upthrust id caused by the upward force the fluid places on the body.

FRICTIONAL FORCE Force opposing the motion of a body. Usually occurs when two bodies are in contact. Direction is opposite to the motion. Object on an inclined plane

TRY THIS A brick of weight 50N rests on a horizontal surface. The maximum frictional force between the brick and the surface is 30N. When the brick slides on the surface the frictional force is 10N. A horizontal force is applied to the brick, its magnitude increasing slowly from zero. Find the frictional force on the brick for various values of F. F0N12N28N29N30N30.01N40N f0N12N28N29N30N10N No motion motion

FREE-BODY DIAGRAMS Diagram showing all forces on a chosen body. The body is shown on its own free from other objects it may be in contact with. In mechanics it is important that the length of the arrow representing a force is proportional to the magnitude of the force.

HOOKE’S LAW When a spring is extended, a force pulls it back to its original length; if we try to compress a spring, again a force tries to pull the string back to its original position Hooks law is the relationship to the amount a spring is extended or compressed. Represented by x The tension T is proportional to x so the greater distance you pull/push the spring the more force is required. T = kx (k is a constant of proportionality known as the spring constant which varies from spring to spring) Units are force over extension or N/m This law only works in a range of extensions. Outside of that “elastic limit” the math becomes more complicated.