# 2: Introduction to Kinematics

{'English - US': '/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/lecture-2/lec2.srt'}

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Topics covered: This lecture is an introduction to kinematics which ultimately leads (in Lecture 4) to trajectories in 3 dimensions.

Instructor/speaker: Prof. Walter Lewin

Date recorded: September 10, 1999

## Video Index

Please make sure you play the Video before clicking the links below.

• Introduction to 1-Dimensional Motion
Professor Lewin describes 1D motion of a particle. He talks about average velocity, the importance of "+" and "-" signs, and our free choice of origin.

• Average Speed vs. Average Velocity
The two are VERY different. The average velocity can be ZERO, while the average speed is LARGE.

• Instantaneous Velocity
Considering the incremental change in position x with time t, we arrive at v=dx/dt. The instantaneous velocity is the derivative of the position with respect to time. Professor Lewin reviews when the velocity is zero, positive and negative; he distinguishes speed from velocity.

• Measuring the Average Speed of a Bullet
Professor Lewin shoots a bullet through two wires. The average speed can be calculated from the distance between the wires and the elapsed time. All uncertainties in the measurements are discussed; they have to be taken into account in the final answer.

• Introducing Average Acceleration
The average acceleration between time t1 and t2 is the vectorial change in velocity divided by (t2-t1).

• Instantaneous Acceleration
The acceleration, dv/dt, is the derivative of the velocity with time. It is the second derivative of the position x with time. Professor Lewin shows how to find the sign of the acceleration from the slope in an x-t plot.

• Quadratic Equation of Position in Time
When the position is proportional to the square of the time, the velocity depends linearly on time, and the acceleration is constant.

• 1D Motion with Constant Acceleration
Professor Lewin writes down a general quadratic equation for the position as a function of time, and he relates the constants in this equation to the initial conditions at time t=0. The gravitational acceleration is a constant (9.80 m/s^2 in Boston), and it is independent of the mass and shape of a free-falling object, if air drag can be ignored (see Lecture #12). You can use this result to measure g using the free fall time measurements from the falling apples in lecture 1.

• Strobing an Object in Free Fall
Professor Lewin drops an apple from 3.20 m and takes a polaroid picture of the falling apple which is illuminated by a strobe light. First two light flashes per second, and then ten flashes per second.