Chapter 2One-Dimensional Kinematics

Dr. Haykel Abdelhamid ElabidiWebsite: uqu.edu.sa/staff/ar/4331237

Email: haelabidi@uqu.edu.sa

Mobile: 0564518933

4th week of September 2013/DhQ 1434

Units of Chapter 2

• Position, distance and displacement

• Average speed and velocity

• Instantaneous velocity

• Average and instantaneous acceleration

• Motion with constant acceleration

• Application of the equation of motion

• Freely falling objects

Position, distance, and displacement

• Kinematics is a part of mechanics that studies the motion of an object without considering the forces acting on it.

Before describing motion, you must set up a coordinate system: define an origin and a positive direction.

Position, distance, and displacement

The distance is the total length of travel; if you go from your house to the groceries or from the groceries to your house, you cover the same distance of 4.3 mi. Distance is always positive (it is a scalar).

house

friend’s house

Average speed and velocity

Description of motion needs to consider how rapidly an object moves.

This can be done by introducing the average speed ([L]/[T]):

The average velocity tell us not only how fast an object is moving, it tells us the direction the object is moving.

calculate the average velocity every 10 mn is better than every 20 mn, every 2 mn is better than every 10 mn and so one. It is desirable to know the velocity at each instant of time.

Instantaneous velocity

The speedometer of your car indicates the instantaneous speed and not the instantaneous velocity.

it can not give information about the direction of motion.

Average and instantaneous acceleration

The average acceleration is the change in velocity divided by the change in time:

• Velocity and acceleration of an object have the same sign the object accelerates

• Velocity and acceleration of an object have opposit signs the object decelerates

Average and instantaneous acceleration

Exercise 2-1 p27 (modified):

a) A car goes from 0 to 60.0 mi/h in 6.2 s. What is the average acceleration of this car in m/s2?b) An airplane has an average acceleration of 5.6 m/s2 during takeoff. How long does it take for the plane to reach a speed of 65 m/s? (1 mi=1609 m)

Motion with constant acceleration

Goal: Give the different equations for motion with constant acceleration and apply them.

Motion with constant accelerationExample 2-5 p32 (modified):

A boat moves slowly with a constant speed of 1.50 m/s. After, it accelerates at 2.40 m/s2. a) How fast is the boat moving after accelerating for 5.00 s?

b) How far has the boat traveled in this time?

c) At what time is the boat’s speed equal 10.0 m/s?

Motion with constant acceleration

Example 2-6 p34:

A drag racer starts from rest and accelerates at 7.40 m/s2. How far has it travaled in 2.00 s?

See also the Example 2-8 page 36

Freely falling objectsFree fall is the motion of an object subject only to the influence of gravity

positive direction

positive direction

Example 2.10 p40:

A person steps off the end of a 3.00-m-high diving board and drops to the water below.

(a)How long does it take for the person to reach the water?

(b)What the person’s speed on entering the water?

Solution: (a) t=0,782 s, (b) v=7,67 m/s (from eq 2-7 or 2-12)

Freely falling objects

Freely falling objects

Thank you for your attention

See you next time Inchallah

Homework: