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# Revision: Describing Motion

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### Revision: Describing Motion

1. 1. Revision for Forces and Motion  What is Distance? Distance is a measure of how far an object has travelled from its starting point. Distance is the same as length and is measured in metres.  What is the Difference between Distance and Displacement? Displacement (like distance above) is also a measure of how far an object has travelled from its starting point but displacement also tells you the direction of motion of the object. Distance is a scalar and displacement is a vector.  What is Speed? Speed is a measure of how fast an object is moving. It is measured in metres per second, written as (m/s), or (ms-1). Speed = Distance ÷ Time.  What is Velocity? Velocity is similar to speed. It is also a measure of how fast an object is moving with the direction of motion, and is measured in m/s. Velocity = Distance ÷ Time.  If an object is not moving then it is called stationary. The speed or velocity of a stationary object is zero.  What is the Difference between Velocity and Speed? The difference between velocity and speed is that velocity is speed in a certain direction. Speed is a scalar and velocity is a vector.
2. 2.  If an object is moving in a straight line, then its speed and velocity will be the same.  If the moving object stays at the same speed but changes direction then we say that the velocity has changed, (because the direction has changed) but the speed has stayed the same.  If the velocity in one direction is positive (for example: forwards at 4 m/s) then the velocity in the opposite direction is negative (for example, backwards at -4 m/s).  How is the Velocity of an Object Calculated? Either Speed or Velocity = Distance ÷ Time. For example, if an object moving in a straight line travels 25 metres in 5 seconds, then its velocity = 25 ÷ 5 = 5 m/s. You must always say what the units are! (in this case m/s, called "metres per second"). Always check what units are given in the question. If the time is given in hours or minutes, then convert it to seconds before doing the calculation.  What is Constant Velocity? If an object’s velocity does not change, we say that it has a constant velocity. In the above example, we are not told whether the object has a constant velocity, or whether its velocity has changed during the 5 seconds. If the velocity has changed, then the answer we have calculated is an average velocity of 5 m/s. If the velocity has not change then the object had velocity of 5 m/s.  What is Acceleration? A change in velocity is called acceleration.
3. 3.  What is the Equation for Acceleration? Acceleration = Change in Velocity ÷ Time. This equation is written as: a = (v2-v1) ÷ t where: a = acceleration v2 = final velocity (the one it ended up with) v1 = initial velocity (the one it started with) t = time  What are the Units of Acceleration? The units of acceleration are m/s2  What is Negative Acceleration? If an object gets faster, it will have a positive acceleration. If an object gets slower, it will have a negative acceleration (this is sometimes called "deceleration"). Example 1. If a car changes from 10 m/s to 30 m/s in 8 seconds, what is its acceleration? v2 (or v) = 30, v1 (or u) = 10, t = 8 a = (30 - 10) ÷ 8 = 20 ÷ 8 = 2·5 m/s2 Example 2. If a bicycle moving at 15 m/s takes 10 seconds to stop, what is its acceleration? In this example, the final velocity is zero because the bicycle has stopped. v2 (or v) = 0, v1 (or u) = 15, t = 10 a = (0 - 15) ÷ 10 = -15 ÷ 10 = -1·5 m/s2 The acceleration is negative because the bicycle has slowed down.
4. 4.  What is Constant Acceleration? Acceleration is the rate of change of an objects velocity. The object is said to have constant acceleration if it gets faster (or slower, or its direction changes) at the same rate.  What is a Distance - Time Graph? A distance - time graph shows how an object’s distance changes with time. 1) The figure below shows a distance - time graph for an object that is stationary (at rest). This graph shows an object which is not moving The straight horizontal line shows that its distance stays the same as time goes by because it is not moving. 2) The figure below shows a distance - time graph for an object with a constant velocity. 
5. 5.      The straight line sloping upwards in the graph shows that the objects distance increases as time goes by. The object has velocity because it is moving. The straight line shows that it is a constant velocity. The gradient (the slope) of the line shows how fast the object is going. The greater the slope, the bigger the velocity is.  What is the Distance - Time Graph for Acceleration?   The curve in the above graph shows that the objects velocity is changing as time goes by. Changing velocity is called acceleration. How can a Distance - Time Graph be used to show the Motion of an Object? Ex. In the graph below, describe the motion of the object.
6. 6.    In region A the object is moving with constant velocity. In region B the object is at rest (not moving). In region C the object is moving with a constant velocity but when compared with region velocity in A: 1. the object is moving more slowly (slope is less steep) 2. the object is moving in the opposite direction because distance is decreasing as time goes by.  What is a Velocity - Time Graph? A velocity - time graph shows how an object’s velocity changes with time. 1) Velocity - time graphs for constant velocity : The straight horizontal line in the graph above shows that the objects velocity does not change as time goes by. 2) Velocity - time graph for an object with a velocity that increases at a constant rate: 
7. 7.     The straight line sloping upwards in the graph above shows that the objects velocity is increasing with time. Increasing velocity is called acceleration. The straight line in the graph above shows that the velocity of the object has a constant acceleration (velocity of the object is changing at a constant rate). The slope of the line in the graph above shows: a) the acceleration is positive because the line slopes upwards. b) how fast the acceleration is: the greater the slope , the faster the acceleration is. 3) Velocity - Time graphs for constant negative acceleration:     The straight line sloping downwards in the graph above shows that the objects velocity is decreasing with time. This is called negative acceleration. The straight line shows that it is constant acceleration. The slope of the line shows 1. the acceleration is negative because the line slopes downwards. 2. how fast the negative acceleration is: the greater the slope, the faster the acceleration is.
8. 8. 4) The picture below shows a velocity - time graph for an object with an acceleration that increases.   The curve in the above graph shows that the acceleration is not constant because the slope is getting steeper. It is changing acceleration that increases as time goes by.  How can a Velocity - Time Graph be used to show the Motion of an Object? Ex. In the graph below describe the motion of the object.   In region A the object is moving with constantacceleration. In region B the object is moving with constant velocity.
9. 9. In region C the object is again moving with constant acceleration, but when compared with region A: 1. the acceleration is slower because the slope is lesssteep 2. the acceleration is negative because the slope is downwards.  The total distance travelled by the object can be calculated by measuring the area under the graph.  This area can be divided into two triangles and one rectangle. The area of triangle A = ½ base x height = 0·5 x 10 x 20 = 100. The area of triangle C= ½ x (70 - 30) x 20 = 400. The area of rectangle B = (30 - 10) x 20 = 400. The distance travelled is the total area = A + B + C = 100 + 400 + 400 = 900 m. Amal Sweis & Amre Al Maghawish