 # Mechanics - Kinematics

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Mechanics - Kinematics
Define: Displacement – distance & direction from reference point Dx , Dy, Dz. Pos or neg sign gives direction. Velocity – rate & direction object changes in position Dx /t or s/t. sign follows displacement. Acceleration – rate change of velocity Dv /t. Sign give change in vel – not direction.

Velocity Average velocity = s tot/ t tot.
Instantaneous vel is a limit. Can find from slope of tangent to point.

Ex 1: A bus travels north 180 km in 2. 2 hours
Ex 1: A bus travels north 180 km in 2.2 hours. It stops for 30 minutes turns and drives 60 km south in 45 min. What is its average velocity in km/h. vav = stot / t tot. 34.8 km/h.

Kinematics Equations IB Tables pg 6
Uniform/constant acceleration: Dv same increment. u = vi v = vf s = displ. a = Dv/t

Acceleration of gravity g remarks.
Constant/uniform m/s2 on Earth. Negative sign is arbitrary! Often use 10 m/s2 as estimate. Air resistance in atmosphere causes objects to reach terminal fall velocity after falling for a time. Wind/Air resistance increases with speed. Fuel efficiency in cars better at slower speeds.

Relative Motion Ex 2: You are standing on a railway platform. A is train moving at 25 m/s to the right. On it is a passenger walking to the right at 2 m/s. What is his velocity relative to you? 27 m/s. If the velocity of object A, relative to a fixed point is VA, the velocity of object B, relative to the same fixed point is VB, then the velocity of A relative to B is the vector difference: is VA - VB.

Ex 3: Car A is traveling 15 m/s and is overtaken by car B which is moving in the same direction at 18 m/s. What is the velocity of car B relative to car A? Vb – Va = 3 m/s. What is the velocity of Car A relative to Car B? Va – Vb - 3 m/s

Ex 4: A boat heads north across a river 90 m wide
Ex 4: A boat heads north across a river 90 m wide. The speed of the boat relative to the shore is 1.8 m/s but the river pushes the boat east at 1.2 m/s. A) What is the speed of the boat relative to the water? B) How far will it land downstream when it arrives at the opposite shore? 2.2 m/s 60-m.

Practice Sheet “IB Kinematics Questions”

IB Motion Graph Review

Distance/Position/Displacement Time Graphs Constant vel.
Starting point Slope = Velocity Slower Velocity

Positive Negative Quadrants Where object is w/ref to starting point.

What is the object’s: a. displacement distance 10 30

Constant Velocity/Speed

Constant / Uniform Acceleration. On velocity time graph accl
Constant / Uniform Acceleration. On velocity time graph accl. is slope of straight line. + or neg a shown?

What’s going on here physically?

Sign of velocity is direction of motion shown by the quadrant.
What does this show?

What does -v mean?

What does the sign of slope mean?

Displacement = Area Under Curve v = d/t then, vt = d.

To find displacement, calc area of triangle + rectangle.

When is object back at start point?
+ area must = - area

Acceleration – time graphs
Graph d-t, v-t, a-t tossed ball & caught at same height.

In Class. Kerr pg 39 Ex 2.1 #1 Hwk Kerr pg 71 Topic 2.1 #1-4.