1. University Physics: Mechanics
Lecture 2
Ch2. STRAIGHT LINE MOTION
University Physics: Mechanics
Dr.-Ing. Erwin Sitompul

2. Solution for Homework 1: Truck
You drives a truck along a straight road for 8.4 km at 70 km/h, at which point the truck runs out of gasoline and stops. Over the next 30 min, you walk another 2.0 km farther along the road to a gasoline station.
(a) What is your overall displacement from the beginning of your drive to your arrival at the station?
(b) What is the time interval Δt from the beginning of your drive to your arrival at the station?

3. Solution for Homework 1: Truck
(c) What is your average velocity vavg from the beginning of your drive to your arrival at the station? Find it both numerically and graphically.

4. Solution for Homework 1: Truck
(d) Suppose that to pump the gasoline, pay for it, and walk back to the truck takes you another 45 min. What is your average speed from the beginning of your drive to you return to the truck with the gas?

5. Instantaneous Velocity and Speed
Instantaneous velocity (or simply velocity) is the average velocity over a very short period of time interval
Velocity v at any instant is the slope of the position-time curve.
Instantaneous speed (or simply speed) is the magnitude of velocity, that is, speed is velocity without any indication of direction

6. Motion with Constant Velocity
Taking at time t0 = 0 the position is at x0  c = x0
Motion with constant velocity on x-t graph

7. Acceleration
Average acceleration is the ratio of change in velocity to the time interval.
Instantaneous acceleration (or simply acceleration) is the derivative of the velocity with respect to time.

8. Motion with Constant Acceleration
Taking at time t0 = 0 the velocity equals v0  c = v0
Motions with constant acceleration on v-t graph

9. Motion with Constant Acceleration
Taking at time t0 = 0 the position is at x0  c = x0
Motions with constant acceleration on v-t graph

10. Motion with Constant Acceleration
From these two equations, the following equations can be derived:

11. Questions What is velocity in intervals A, B, C, D
What is acceleration in intervals A, B, C
1 m/s
0 m/s2
0 m/s
–1.5 m/s
2 m/s2
–0.5 m/s2
2 m/s

12. Example: Porsche
Spotting a police car, you brake a Porsche from a speed of 100 km/h to a speed of 80.0 km/h during a displacement of 88.0 m, at a constant acceleration.
(a) What is that acceleration?

13. Example: Porsche
Spotting a police car, you brake a Porsche from a speed of 100 km/h to a speed of 80.0 km/h during a displacement of 88.0 m, at a constant acceleration.
(b) How much time is required for the given decrease in speed?

14. Car accelerating and decelerating
Example: Porsche
Car accelerating and decelerating

15. Questions
Which time periods indicates that an object moves at constant speed?
What are
(a) Initial direction of travel?
(b) Final direction of travel?
(c) Does the particle stop momentarily?
(d) Is the acceleration positive or negative?
(e) Is the acceleration constant or varying? – +
Yes +
Constant
E, where a = 0

16. Questions
How far does the runner travel in 16 s?

17. Example: Race
A caravan moves with a constant velocity of 60 km/h along a straight road when it passes a roadster which is at rest. Exactly when the caravan passes the roadster, the roadster starts to move with an acceleration of 4 m/s2.
(a) How much time does the roadster need to catch up the caravan?
Thus, the roadster will catch up the caravan after 8.33 s.

18. Example: Race
A caravan moves with a constant velocity of 60 km/h along a straight road when it passes a roadster which is at rest. Exactly when the caravan passes the roadster, the roadster starts to move with an acceleration of 4 m/s2.
Distance traveled
(b) How far does the roadster already move when it catches up the caravan?
Both the vehicles travel m before they pass each other again.

19. Example: Particle’s Movement
The position of a particle is given by x = 4t2 – 2t + 10, where x is the distance from origin in meters and t the time in seconds.
(a) Find the displacement of the particle for the time interval from t = 1 s to t = 2 s.
(b) Find also the average velocity for the above given time interval. .
(c) Find the instantaneous velocity of the particle at t = 0.5 s.
At t = 0.5 s,

20. Homework 2: Aprilia vs. Kawasaki
An Aprilia and a Kawasaki are separated by 200 m when they start to move towards each other at t = 0.
200 m
The Aprilia moves with initial velocity 5 m/s and acceleration 4 m/s2. The Kawasaki runs with initial velocity 10 m/s and acceleration 6 m/s2.
(a) Determine the point where the two motorcycles meet each other.
(b) Determine the velocity of Aprilia and Kawasaki by the time they meet each other.

21. Homework 2
New
You come late to a running exercise and your friends already run 200 m with constant speed of 4 m/s.
The athletic trainer orders you to catch up your friends within 1 minute.
(a) Determine the minimum speed you have to take so that you can fulfill the trainer’s order.
(b) If you run with minimum speed, determine the point where you catch up your friends.