Mechanics

Unit 5: Motion and Forces

5.6a Linear Motion

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Fundamentals of physics - Mechanics Motion Is Relative

---------------------------------------------------------------------------------------- Everything in the world moves – even apparantly stationary objects. Everything moves relative to the Sun and stars. You're moving at about 1.1 X 105 kmh-1 relative to the Sun, and even faster relative to the center of our galaxy. When can only describe the motion of an object relative to something else. When you walk down the aisle of a moving bus, your speed relative to the floor of the bus is probably different from your speed relative to the road. When we say a high speed train reaches a speed of 300 kmh-1, we mean relative to the track. The speeds of things in our environment is measured relative to the surface of Earth.

Fundamentals of physics - Mechanics Speed

---------------------------------------------------------------------------------------- Galileo defined speed as the distance covered per unit of time.

Speed is measured in kilometers per hour (kmh-

1) For shorter distances, eters per second (ms-1) is more appropriate.

Driving 100m in 10s is a speed of 100m/10s = 10ms-1

When sitting, your speed is zero relative to Earth but 30 kms-1 relative to the Sun.

Fundamentals of physics - Mechanics Instantaneous Speed

---------------------------------------------------------------------------------------- When things move their speed may change. A car, for example, may travel along a street at 25 kmh-1, slow to 0 kmh-1 at a red light, and speed up to 120 kmh-1 on a highway. When you look at the speedometer it gives you an instantaneous picture of the speed as it may move up and down. A speed of 120 kmh-1 means if the car travelled for one hour it would have covered 120km...

A speed of 100 kmh-1 means if the car travelled for one hour it would have covered 100km... In ½ hr it would only have travelled 50km

Fundamentals of physics - Mechanics Average Speed

---------------------------------------------------------------------------------------- We are often more concerned with the average speed for the trip. For example in getting to work it is the time it takes for the entire trip and the entire distance that matters.

I drive 110km from Sangre Grande to Corinth Campus in 2 hours. My average speed is 110km/2hr= 55kmh-1. Along the way my instantaneous sped may vary from 0 (at traffic lights) to 160kmh-1 (on the Highway)

Fundamentals of physics - Mechanics Average Speed

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We can rearrange this to get:Total distance covered = average speed X time.

What is the average speed of a runner who covers 100m in 10s?Average Speed = 100m/10s

= 10ms-1

If you get a ticket is it for the average speed or instantaneous speed?

Fundamentals of physics - Mechanics Velocity

---------------------------------------------------------------------------------------- Velocity is both the speed and the direction of an object. For example, if a car travels at 10ms-1, we know its speed. But if we know it moves at 10ms-1 to the east, we now know its velocity. Speed is simply how fast an object moves but velocity is how fast and also in which direction.

Recall Speed is a scalar...Velocity is a vector...

Fundamentals of physics - Mechanics Constant and Changing Velocity

---------------------------------------------------------------------------------------- Constant speed means unchanging speed or no speeding up or slowing down. Constant velocity, however, means both constant speed and constant direction.

So how can we change Velocity?

1. Change speed.2. Change direction.

When velocity changes we call that acceleration...

The velocity of these cars are constantly changing because their direction changes constantly... Even if the speed is constant...

Fundamentals of physics - Mechanics Constant and Changing Velocity

---------------------------------------------------------------------------------------- On the highway one car moves to the east at 80 kmhr-1. Another car that moves to the east at 80 kmhr-1. Same speed Same velocity

3. Around a bend, the speedometer of a car reads a constant 60 kmhr-

1. Constant speed constant velocity

Fundamentals of physics - Mechanics Acceleration

---------------------------------------------------------------------------------------- We can change the velocity of something by changing its speed, by changing its direction, or by changing both its speed and its direction. How quickly (rate) the velocity changes is called acceleration:

If a car on a level straight road changes its velocity from 10ms-1 to 30ms-1 in 10s. What is the acceleration?Acceleration = (30ms-1 - 10ms-1 ) / 10s = 2ms-2

Fundamentals of physics - Mechanics Acceleration

---------------------------------------------------------------------------------------- Note when an object moving in a straight line slows down it also experiences an acceleration! Sometimes this is called a deceleration. However acceleration is a vector quantity and thus we can call a deceleration a negative acceleration. If a motorbike on a level straight road

changes its velocity from 30ms-1 to 10ms-1 in 10s. What is the acceleration?Acceleration = (10ms-1 - 30ms-1 ) / 10s = -2ms-2 The motorbike experiences an acceleration of -2ms-2 or a deceleration of 2ms-2 .

Fundamentals of physics - Mechanics Free Fall

---------------------------------------------------------------------------------------- Objects fall towards the centre of the earth because of the force of gravity. When a falling object falls under the influence of gravity alone, the object is in a state of free fall. At this stage we ignore air resistance. Free fall acceleration on earth is approximately 10ms-2. We refer to this free fall acceleration as g.

Time /s Velocity/ms-1 0 01 102 203 304 405 50t 10t