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Physics A Level

Chapter 1: Kinematics 1.8 Other examples of scalar and vector quantities

Physics A Level

Chapter 1: Kinematics 1.8 Other examples of scalar and vector quantities

2022-09-27
210
Crash report

 Physics (9702)

Direction matters when vectors are combined. You can use this to decide whether a quantity is a vector or a scalar. For example, if you walk for 3 minutes north and then 3 minutes in another direction, the total time taken is 6 minutes whatever direction you choose. A vector of 3 units added to another vector of 3 units can have any value between 0 and 6 but two scalars of 3 units added together always make six units. So, time is a scalar.
Mass and density are also both scalar quantities.
Force and acceleration, as you will see in later chapters, are both vector quantities. This is because, if an object is pushed with the same force in two opposite directions, the forces cancel out.
Work and pressure, which you will also study in later chapters, both involve force. However, work and pressure are both scalar quantities. For example, if you pull a heavy case along the floor north and then the same distance south, the total work done is clearly not zero. You just add scalar quantities even if they are in the opposite direction.

EXAM-STYLE QUESTIONS

 

1) Which of the following pairs contains one vector and one scalar quantity? [1]
A: displacement : mass
B: displacement : velocity
C: distance : speed
D: speed : time

2) A vector P of magnitude $3.0 N$ acts towards the right and a vector Q of magnitude $4.0 N$ acts upwards. [1]
What is the magnitude and direction of the vector (P − Q)?
A: $1.0 N$ at an angle of ${53^ \circ }$ downwards to the direction of P
B: $1.0 N$ at an angle of ${53^ \circ }$ upwards to the direction of P
C: $5.0 N$ at an angle of ${53^ \circ }$ downwards to the direction of P
D: $5.0 N$ at an angle of ${53^ \circ }$ upwards to the direction of P

3) A car travels one complete lap around a circular track at a constant speed of $120\,km\,{h^{ - 1}}$.
a: If one lap takes 2.0 minutes, show that the length of the track is $4.0 km$. [2]
b: Explain why values for the average speed and average velocity are different. [1]
c: Determine the magnitude of the displacement of the car in a time of $1.0 min$. [2]
(The circumference of a $circle = 2\pi r$, where R is the radius of the circle.) [Total: 5]

4) A boat leaves point A and travels in a straight line to point B. The journey takes $60 s$.

Figure 1.18

Calculate:
a: the distance travelled by the boat [2]
b: the total displacement of the boat [2]
c: the average velocity of the boat. [2]
Remember that each vector quantity must be given a direction as well as a magnitude. [Total: 6]

5) A boat travels at $2.0\,m\,{s^{ - 1}}$ east towards a port, $2.2 km$ away. When the boat reaches the port, the passengers travel in a car due north for 15 minutes at $60\,k\,{m^{ - 1}}$
Calculate:
a: the total distance travelled [2]
b: the total displacement [3]
c: the total time taken [2]
d: the average speed in $m\,{s^{ - 1}}$ [2]
e: the magnitude of the average velocity. [2]
[Total: 11]

6) A river flows from west to east with a constant velocity of $1.0\,m\,{s^{ - 1}}$. A boat leaves the south bank heading due north at $12.4\,m\,{s^{ - 1}}$. Find the resultant velocity of the boat. [3]

7) a: Define displacement. [1]
b: Use the definition of displacement to explain how it is possible for an athlete to run round a track yet have no displacement. [2]
[Total: 6]

8) A girl is riding a bicycle at a constant velocity of $3.0\,m\,{s^{ - 1}}$ along a straight road. At time $t = 0$, she passes her brother sitting on a stationary bicycle. At time $t = 0$, the boy sets off to catch up with his sister. His velocity increases from time $t = 0$ until $t = 5.0s$, when he has covered a distance of $10 m$. He then continues at a constant velocity of $4.0\,m\,{s^{ - 1}}$.
a: Draw the displacement–time graph for the girl from $t = 0$ to $t = 12s$. [1]
b: On the same graph axes, draw the displacement–time graph for the boy. [2]
c: Using your graph, determine the value of t when the boy catches up with his sister. [1]
[Total: 4]

9) A student drops a small black sphere alongside a vertical scale marked in centimetres. A number of flash photographs of the sphere are taken at $0.10 s$ 
intervals:

Figure 1.19

The first photograph is taken with the sphere at the top at time $t = 0 s$.
a: Explain how Figure 1.19 shows that the sphere reaches a constant speed. [2]
b: Determine the constant speed reached by the sphere. [2]
c: Determine the distance that the sphere has fallen when $t = 0.80 s$. [2]
d: In a real photograph, each image of the sphere appears slightly blurred because each flash is not instantaneous and takes a time of $0.0010 s$.
Determine the absolute uncertainty that this gives in the position of each position of the black sphere when it is travelling at the final constant speed.
Suggest whether this should be observable on the diagram. [2]
[Total: 8]

10) a: State one difference between a scalar quantity and a vector quantity and give an example of each. [3]
b: A plane has an air speed of $500k\,m\,{h^{ - 1}}$ due north. A wind blows at $100k\,m\,{h^{ - 1}}$ from east to west.
Draw a vector diagram to calculate the resultant velocity of the plane. Give the direction of travel of the plane with respect to north. [4]
c: The plane flies for 15 minutes. Calculate the displacement of the plane in this time. [1]
[Total: 8]

11) A small aircraft for one person is used on a short horizontal flight. On its journey from A to B, the resultant velocity of the aircraft is $15\,m\,{s^{ - 1}}$ in a direction ${60^ \circ }$ east of north and the wind velocity is $7.5\,m\,{s^{ - 1}}$ due north.

Figure 1.20

a: Show that for the aircraft to travel from A to B it should be pointed due east. [2]
b: After flying $5 km$ from A to B, the aircraft returns along the same path from B to A with a resultant velocity of $13.5\,m\,{s^{ - 1}}$. Assuming that the time spent at B is negligible, calculate the average speed for the complete journey from A to B and back to A.

SELF-EVALUATION CHECKLIST

 

After studying the chapter, complete a table like this:

Ready to move on Almost there Needs more work See topic I can
      1.1, 1.2, 1.3 define and use displacement, speed and velocity
      1.4 draw and interpret displacement–time graphs
      1.1 describe laboratory methods for determining speed
      1.2 understand the differences between scalar and vector quantities and give examples of each
      1.6, 1.7 use vector addition to add and subtract vectors that are in the same plane.