Download Newton's Laws of Motion: Understanding Force, Mass, and Gravity and more Study notes Physics in PDF only on Docsity! The Laws of Motion The Concept of Force Newton’s First Law Mass Newton’s Second Law Gravitational Force and Weight Newton’s Third Law Friction The Concept of Force Eg: Push, pull, throw an object; gravity; magnetic attraction Only a force can cause a change in velocity … which results in acceleration Force is a vector quantity Newton’s First Law In the absence of an external force … an object in motion remains in motion with constant velocity. an object at rest remains at rest This means acceleration is zero. The tendency of an object to resist any change in its velocity is called inertia. Inertial Frame of Reference A frame of reference that is not accelerating Examples: A stationary observer, a car moving with a constant speed in a straight line The first law is only valid in inertial reference frames Mass It is an inherent property, independent of the environment and measurement. It can be thought of as the resistance of an object to any changes in its velocity. Don’t confuse mass with weight which is the gravitational force exerted on an object. Example – 5.1 What is the acceleration of the puck? m = 0.30 kg xx maF =∑ yy maF =∑ 22 yx aaa += ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ =∠ x y a a a arctan = 34 m/s2 = 30° Gravity and Weight Gravity is the attractive force every object feels towards Earth. The magnitude of the gravitational force is the weight. gF mg = mgg =F Gravitational Force Weight Weight can change from planet to planet but mass is constant. Newton’s Third Law If two objects interact, the force F12 exerted by object 1 on object 2 is equal in magnitude and opposite in direction to the force F21 exerted by object 2 on object 1. 2112 FF −= Fnh Fhn Fnh = - Fhn Example – 5.4 Conceptualize the problem (diagram) Categorize the problem (equilibrium) Isolate an object and identify the forces acting upon it. Establish a convenient coordinate system Apply Newton’s second law to each diagram Solve the equations simultaneously Check your answer 03 =+TFg 0321 =++ TTT 0=∑F The Normal Force Consider a person standing in an elevator that is accelerating upward. The upward normal force N exerted by the elevator floor on the person is 1. larger than 2. identical to 3. smaller than the downward weight W of the person. Example – 5.6 a) a = ? b) If x = d then t = ?, vf = ? xx mamgF ==∑ θsin yy mamgnF =−=∑ θcos θsingax = 2 2 1 tatvxx xxiif ++= θsin 2 g dt = tavv xxixf += θsin2dgvxf = 0=ya Friction Friction is proportional to the normal force. Friction is independent of area. μs is generally larger than μk. The coefficients do not depend on weight or speed and generally range between 0.003 and 1. Static Friction nf ss μ≤ Kinetic Friction nf kk μ= Example - Friction A person pulls a block across a rough horizontal surface at a constant speed by applying a force F. The arrows correctly indicate the directions, but not necessarily the magnitudes of the forces on the block. Which of the following relations among the force magnitudes of W, k, N, and F must be true? 1. F = k and N = W 2. F > k and N < W 3. F = k and N > W 4. F > k and N = W 5. None of the above choices Example – 5.12 μs = ? 0cos 0sin =− =− θ θ mgn fmg s nf ss μ=max, cs θμ tan=
