Mechanics Unit 01 Motion Speed Velocity and Acceleration

Mechanics Unit 01 Motion, Speed, Velocity and Acceleration

Motion – an object’s change in position relative to a reference point

Reference Point The Earth’s surface is used as a common reference point A moving object can be used as a reference point as well

Some Physics Quantities Vector - quantity with both magnitude (size) and direction Scalar - quantity with magnitude only Vectors: • Displacement • Velocity • Acceleration • Momentum • Force Scalars: • Distance • Speed • Time • Mass • Energy

Speed

Ex: Farmer Jones drives 6 km down a straight road. She turns around and drives 4 km back. What was her average speed for this trip if it took 1 hour? 6 km 4 km

The total distance traveled by Farmer Jones is 10 km. Therefore her average speed is 10 kmh-1 The net distance traveled by Farmer Jones is 2 km. Therefore, her average speed is 2 kmh-1 There are good reasons to use either interpretation - it's mostly a matter of preference. We will interpret "distance traveled" to be net distance (also called displacement). Farmer Jones' average speed was 2 kmh-1 6 km 6 -4 =2 km 4 km

Velocity

Velocity appears to be very similar to speed, however, when describing the velocity of an object you need to provide a magnitude and a direction Magnitude – the speed of the object Direction – the direction the object is moving

Example Imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. While this might result in a frenzy of activity, it would result in a zero velocity. Because the person always returns to the original position, the motion would never result in a change in position. Since velocity is defined as the rate at which the position changes, this motion results in zero velocity.

Velocity Ex: A bus has a velocity of 35 kmh-1 heading west

Resultant Velocity An object can have a resultant velocity if it is experiencing more than one motion. For example if a person walks down the center of a bus while it is in motion there are two velocities occurring. 1. The movement of the bus 2. The movement of the person inside the bus

Example 15 ms-1 + 1 ms-1 = 14 ms-1

Example 15 ms-1 + 1 ms-1 = 16 ms-1

Exercise You are on a bus traveling 25 m/s forward. You go to the back of the bus to visit your friend you are walking at a speed of 3 m/s. What is your resultant velocity? Answer =25 -3 =22 m/s

Acceleration is the rate at which velocity changes over time ◦ An object accelerates if its speed, direction, or both change Average acceleration = final velocity – starting velocity time it takes to change velocity

Acceleration cont: Example ◦ A car on the highway is traveling 55 kmh-1 and it passes another car. In order to pass, the car has to accelerate to 65 kmh-1. The car reaches this velocity 40 seconds later. What is the average acceleration of the car? Answer: 1 st method Acceleration =(final velocity-initial velocity)/ time =(65 km/h-55 km/h)/(40/3600)h =900 km/h

2 nd method: you can convert the velocity (km/h to m/s) and then find the answer using acceleration equation Difference of velocity =(65 -55) km/h convert to m/s Convert 10 kmh-1 to ms-1 10 km x 1000 m = 2. 78 ms-1 1 h 3600 s Acceleration =(final velocity-initial velocity)/ time =(2. 78 m/s ) /40 s =0. 0695 m/s

Acceleration and Velocity As velocity increases, so does acceleration As velocity decreases, so does acceleration When direction changes, so does acceleration When there is a constant velocity, there is no acceleration

Q 1 A car travels from point A to B in 3 hours and returns back to point A in 5 hours. Points A and B are 150 km apart along a straight highway. Which of the following statements most accurately describes the motion of the car? Please choose from one of the following options. The average velocity is 40 km per hour. The average speed is 37. 5 km per hour. The car travels at 50 mph for the first half and 30 mph for the second half. The total displacement of the trip is 300 km.

Exercise: 01. A car travels 10 km northwards, followed by 4 km southwards. What is the magnitude of its distance and displacement? 02. A man traveled a total distance of 35 km. His trip took 7 hours. What was his average speed?

Thank You