Dynamics - 1 - Newton's 1st and 2nd Laws

University:
High School
Course:
High School Physics
Academic year:

2024
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161

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10
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Quantum Chapter 14 1    Dynamics is the study of the effects of forces on matter. Galileo did some work on dynamics, but it was Newton who was the real “Father of Dynamics”. Galileo did a thought experiment in which he imagined a ball rolling on a smooth frictionless surface: 2 Galileo’s Thought Experiment    Galileo said that on a frictionless surface the ball would roll up the far side to the same height as it started at. If the slope is less, the ball will roll farther. If the slope is zero, the ball will roll forever. Inertia is an objects tendency to retain it’s state of motion. The ball will resist a change in its motion unless it is acted upon by an external force, such as friction. 3 “An object at rest will remain at rest and an object in motion will continue in uniform motion in a straight line unless it is acted upon by a net external force.” 4 To determine the net external force, or resultant force, free body diagrams can be very useful. FN Non-zero resultant forces cause accelerations! FF Fg FN=mgcosθ Fa=mgsinθ   “The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass.” Stated mathematically: F a= m or… F = ma 6 Since we will use the resultant force / net force we will write the equation as: Fnet = ma  Where: •F = Force (N) •m = mass (kg) •a = acceleration (m/s2) You may see it written as:    Fnet = Fapplied + F friction ie. Net force is vector sum of all forces or  Fnet = Fapplied − F friction ie. Net force is sum of forward minus backward forces 7 Convention: We will usually write Fnet = Fapplied − Ffriction and we define that - Forward forces will make up Fapplied - Backward forces will make up Ffriction - So positive Fnet create positive “a” and negative Fnet create negative “a”. 8  The wind-up motor of a toy car provides 1.20N of forward force. The force of friction is 0.200N. The toy has a mass of 0.800kg. What is the acceleration? • Fapp=1.20 N • Ff=0.200 N • m=0.800 kg • a=? F = F net app Fnet = ma − Ff = 1.20 N − 0.200 N = 1.00 N Fnet a= m 1.00 N = 0.800 kg = 1.25 m/s 2 9   HW p 308 Q 2, 3 P 314 Q 2, 8, 10, 12 10