AP Physics C Chapter 4

Newton’s First Law Stations In groups of two, you will go from station to station to perform each activity. Provide a description using terms like inertia, force(s), and gravity, but not terms like energy and momentum. Provide a sketch to illustrate the forces acting on the object(s). Have fun

Newton’s First Law of Motion /fun-science-demonstrations/what-is- newtons-first-law /fun-science-demonstrations/what-is- newtons-first-law An object remains at rest or in constant motion unless a net external force acts on it.

Applying Newton’s 1 st Law of Motion If an object remains at rest or in constant motion, what is the net force that is acting on it? Three forces act upon a 3.0 kg body moving at constant velocity. F1 = (4i – 6j + k) N and F2 = (i – 2j - 8k) N. Find F3.

Tug-of-War A tug-of-war team ties a rope to a tree and pulls hard horizontally to create a tension of 30,000 N in the rope. Suppose the team pulls equally hard when, instead of a tree, the other end of the rope is being pulled by another tug-of-war team such that no movement occurs. What is the tension in the rope in the second case? ures_Smurf_Tug-Of-War.jpg

Free-body diagrams Diagrams that show the chosen body by itself, “free” of its surroundings, with vectors drawn to show the magnitudes and directions of all the forces applied to the body by the various other bodies that interact with it.

What is a normal force? Determine the normal force acting on a 5.0 kg box sitting on a flat table.

Practice Problem #1 Now determine the normal force acting on a 5.0 kg box sitting on a flat table. 16 N 30 o

Practice Problem #2 Now determine the normal force acting on a this 5.0 kg box sitting on a ramp at angle =30 o. 20 N 30 o

What if the net force is not zero? Two forces, F1 = (4i – 6j + k) N and F2 = (i – 2j - 8k) N, act upon a body of mass 3.0 kg. No other forces act upon the body at this time. What do you know must be true? What is the acceleration?

Practice Problem #3 Draw a free-body diagram for each the blocks shown. Assume that the surface is frictionless. What is the acceleration of this system?

Practice Problem #4 Draw a free diagram for each block shown below. Assume that the surface is frictionless. If the blue block is 5.0 kg, the red block is 7.5 kg, and the applied force is 125 N, what is the acceleration of the system? What is the tension between the two blocks? Applied Force Tension

Sample Problem #5 A 5.00-g bullet leaves the muzzle of a rifle with a speed of 320 m/s. The bullet is accelerated by expanding gases while it travels down the m long barrel. Assuming constant acceleration and negligible friction, what is the force on the bullet?

Sample Problem #6 A 3.00 kg mass undergoes an acceleration given by a = (2.50i j) m/s 2. Find the resultant force F and its magnitude and direction.

Sample Problem #7 Now determine the acceleration of this 5.0 kg box sliding down a ramp at angle =30 o if the ramp is frictionless. 30 o

Sample Problem #8 What is the tension in the cable attached to a 5,000 kg elevator that starts on the ground floor at rest and accelerates upward, reaching a speed of 3.0 m/s in 2 seconds?

Sample Problem #9 How high up the frictionless ramp will the block slide? 12 m/s 20 o 5.0 kg

Newton’s Third Law of Motion If body A exerts a force on body B (an “action force”), then body B exerts a force on body A (a “reaction force”). These two forces have the same magnitude but are opposite in direction. These two forces act on different bodies and are referred to as an action-reaction pair.

Check your understanding: While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?

Check your understanding: For years, space travel was believed to be impossible because there was nothing which rockets could push off of in space in order to provide the propulsion necessary to accelerate. This inability of a rocket to provide propulsion is because... a.... space is void of air so the rockets have nothing to push off of. b.... gravity is absent in space. c.... space is void of air and so there is no air resistance in space. d.... nonsense! Rockets do accelerate in space and have been able to do so for a long time.

Check your understanding: Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action- reaction force pairs. A gunpowder explosion creates hot gases which expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is... a. greater than the acceleration of the bullet. b. smaller than the acceleration of the bullet. c. the same size as the acceleration of the bullet.