Unit 3: Forces and Laws of Motion

Transcription

1 Unit 3: Forces and Laws of Motion Feb 8 6:06 PM Objectives: Describe how force affects the motion of an object Interpret and construct free-body diagrams Solve problems using kinematic equations and Newton's 2nd and 3rd law. Feb 8 6:07 PM 1

2 Force is a push or pull on an object. Force is measured in Newtons (N). the amount of force needed to accelerate a mass of 1 kg at a rate of 1 m/s 2 So a Newton = 1kg m s 2 Feb 8 6:09 PM For comparison's sake, a Newton is similar to a pound in the Imperial system. 1 lb. is equal to N Pounds also measure force. Weight is a force. Both pounds and Newtons are a measure of the pull of gravity on an object's mass. Feb 8 6:13 PM 2

3 4 Fundamental Forces 1. Gravity; weak, but very long ranged; works on any two bodies with mass. 2. Electromagnetism; long-ranged, but much weaker than the strong force; only acts on matter with an electrical charge 3. Strong force; very strong, but very short-ranged. holds nuclei together. 4. Weak force; responsible for radioactive decay and neutrino interactions; weak and short range Feb 8 6:26 PM Field Forces vs. Contact Forces Act from a distance gravity electromagnetic forces Must make contact springs a hammer on nails Force is a vector. It has both magnitude and direction. Feb 8 6:17 PM 3

4 Free Body Diagrams We can use vectors to analyze force. A force vector points in the direction of the force. The tail of the vector is attached to the object that the force is acting on, Assumption: all forces act at the center of the object regardless of shape. Feb 8 6:33 PM Example: Draw a free body diagram for a person pulling a wagon. The forces are 6 N pulling on the handle. 8N by gravity and 5 N up. F ground Identify the forces F handle Draw a diagram Pictures don't have to be fancy. This could be a wagon. Wagon Draw and label the vectors F gravity Feb 8 6:39 PM 4

5 Unit 3: Force Newton's 1st Law - Inertia Feb 8 7:05 PM An object at rest remains at rest, and an object in motion continues in motion with constant velocity unless the object experiences a net external force.... or things keep doing what they are doing unless acted on by an outside force. Feb 8 7:06 PM 5

6 Inertia: the tendency of an object to resist a change in motion. Eureka! The simple version of inertia. (You can find it Youtube) Feb 8 7:47 PM Net force - the sum of all the forces acting on a body. If the net force is balanced there will be no change in motion. Equilibrium! If the net force is unbalanced there will be a change in motion.acceleration! Objects with more mass have more inertia... it takes a greater external force to change its motion. Feb 8 7:52 PM 6

7 Why Does It Matter? Scary Video No Seatbelts! No Excuses! psa (also on Youtube) Feb 8 8:02 PM Determining Net Force 1. Identify the forces 2. Draw a free-body diagram 3. Break all vectors into x and y components. 4. Find the net force by adding all the x forces and all the y forces. 5. The resultant vector is the net force Feb 8 7:54 PM 7

8 Newton's Second Law The acceleration of a body is directly proportional to the net force on it and inversely proportional to its mass. acceleration = net force mass a = F net m Feb 8 8:09 PM If a is constant F M F M If m is constant F a F a If F is constant m a m a Feb 16 5:10 PM 8

9 F net = ma How do we use F net?( Find the resultant of all the forces) For 1 dimension F net = ΣF(in one direction) - ΣF(in the opposite direction) Example: In a tug of war, the red team pulls to the right with a force of 250N, while the green team pulls to the left with a force of 68N. What is F net? F net = 250 N - 68 N = 182 N Feb 16 5:13 PM In the last problem, if the rope accelerates at 2.7 m/s 2, what mass is involved? m = F net = 182 N = 67.4kg a 2.7m/s 2 Feb 16 5:33 PM 9

10 Newton's 2nd Law resultant F net = ma F net = 140 N a = 19 m/s 2 m =? m = F net = 140 N = 140 kg m/s 2 a 19 m/s 2 19 m/s 2 = 7.4 kg Feb 16 5:37 PM m = 275 kg a = m/s 2 F net = ma = (275 kg)(-4.50 m/s 2 ) = N the force acting on an object is in the opposite direction to the motion of the object (as you would expect for an object slowing down). Feb 16 5:44 PM 10

11 Remember: F net = m a N kg m/s 2 1 Newton = kg m F w = W = mg weight weight (mass)(gravity) N N kg 9.8 m/s 2 s 2 Feb 16 5:49 PM 11