Describing Motion Flashcards

All 71 cards in this deck

What is distance?

How far an object moves. It does not involve direction, so it is a scalar quantity.

What is displacement?

The distance from the start point to the finish point measured in a straight line, together with the direction of that line. It is a vector quantity.

How do you express a displacement fully?

e.g. a walker ends up 5 km from the start, towards the north

Give a magnitude and a direction: 5 km north.

True or false? An athlete who runs one complete lap of a track has travelled a distance of 400 m and has a displacement of 400 m.

False. The displacement is zero because the finish point is the same as the start point.

True or false? Distance is a vector quantity.

False. Distance has no direction, so it is a scalar.

What is speed, and is it a scalar or a vector?

How fast an object is moving (distance travelled per unit time). It does not involve direction, so it is a scalar.

What are the typical speeds of a person walking, running and cycling?

Walking about 1.5 m/s, running about 3 m/s, cycling about 6 m/s.

What is a typical value for the speed of sound in air?

330 m/s.

What are typical speeds for a car, a train and an aeroplane?

Car about 25 m/s, train about 55 m/s, aeroplane about 250 m/s.

Name four factors that affect the speed at which a person can walk, run or cycle.

Age, terrain, fitness and distance travelled.

True or false? A person's walking speed is exactly the same whatever the ground is like.

False. Terrain is one of several factors (with age, fitness and distance travelled) that change the speed.

Which two measurements are needed to find the speed of a sled using a single light gate?

The length of the sled and the time for the sled to pass through the light gate.

What are the steps to measure the speed of a trolley using a light gate?

e.g. a card 0.05 m long fixed to the trolley

1. Measure the length of the card: 0.05 m
2. Let the card pass through the light gate; the data logger records the time: 0.10 s
3. Speed = length ÷ time = $0.05 \div 0.10 = 0.5$ m/s

What are the steps to measure the average speed of a person walking?

e.g. walking along a corridor

1. Measure the distance with a tape measure or trundle wheel: 20 m
2. Time the walk with a stopwatch: 16 s
3. Speed = distance ÷ time = $20 \div 16 = 1.25$ m/s

Why does a light gate and data logger give a more accurate speed than a stopwatch?

There is no human reaction time in starting and stopping the timing.

What is the equation linking distance travelled, speed and time?

$distance = speed \times time$, or $s = vt$

In the equation $s = vt$, what does each symbol stand for and what is its unit?

$s$ = distance travelled in metres (m), $v$ = speed in metres per second (m/s), $t$ = time in seconds (s).

What are the steps to calculate the time taken for a journey from distance and speed?

e.g. travelling 1500 m at 20 m/s

1. Write $s = vt$
2. Rearrange: $t = s \div v$
3. $t = 1500 \div 20 = 75$ s

How do you calculate average speed for non-uniform motion?

e.g. a car travels 300 m in 3 s, then 200 m in 7 s

Total distance ÷ total time = $500 \div 10 = 50$ m/s.

What must you do with a distance given in km before using $s = vt$ with a speed in m/s?

Convert it to metres by multiplying by 1000.
e.g. 1.5 km = 1500 m

True or false? If a car covers 1000 m in 50 s, it must have been travelling at 20 m/s the whole way.

False. 20 m/s is its average speed; its actual speed may have varied.

What is velocity?

The speed of an object in a given direction. It is a vector quantity.

True or false? An object can have a constant speed but a changing velocity.

True. If its direction changes, its velocity changes even though its speed stays the same.

Why can the average speed and the average velocity of a car between two speed cameras be different?

Velocity is a vector and includes direction; if the road is not straight the direction changes, so the velocity differs from the speed.

How do you state a velocity fully?

e.g. a train moving at 30 m/s along a track heading east

Give the speed and the direction: 30 m/s east.

What is a vector quantity?

A quantity with both magnitude and direction.
e.g. displacement, velocity

What is a scalar quantity?

A quantity with magnitude only, no direction.
e.g. distance, speed

Of distance, displacement, speed and velocity, which are scalars and which are vectors?

Scalars: distance and speed. Vectors: displacement and velocity.

True or false? Acceleration is a scalar quantity.

False. Acceleration has a direction, so it is a vector.

Why does an object moving in a circle at a steady rate have a changing velocity?

Its direction is constantly changing, and velocity is a vector, so the velocity changes even though the speed is constant.

True or false? A car going round a roundabout at a constant 10 m/s has a constant velocity.

False. Its speed is constant but its direction, and so its velocity, is always changing.

Is an object moving in a circle at constant speed accelerating?

Yes. Its velocity is changing because its direction changes, and changing velocity means acceleration.

What does the gradient of a distance–time graph represent?

The speed of the object.

What motion does a horizontal line on a distance–time graph show?

The object is stationary.

What motion does a curve of increasing gradient on a distance–time graph show?

The object is accelerating (speeding up).

What are the steps to find the speed from a straight section of a distance–time graph?

e.g. the line rises from 0 m to 20 m between 0 s and 4 s

1. Read the change in distance: 20 m
2. Read the change in time: 4 s
3. Speed = gradient = $20 \div 4 = 5$ m/s

When drawing a distance–time graph from measurements, which quantity goes on each axis?

Time on the x-axis (horizontal), distance on the y-axis (vertical).

True or false? A steeper line on a distance–time graph means a greater speed.

True. Speed is the gradient, so a steeper line means faster motion.

What is instantaneous speed?

The speed of an object at one particular moment in time, rather than averaged over a journey.

True or false? On a curved distance–time graph, the speed at a point is found by dividing the total distance by the total time.

False. That gives the average speed; the speed at a point needs the gradient of a tangent.

What is a tangent to a curve on a distance–time graph?

A straight line drawn touching the curve at one point, with the same gradient as the curve there.

What is the equation for average acceleration?

$acceleration = \dfrac{change\ in\ velocity}{time\ taken}$, or $a = \dfrac{\Delta v}{t}$

What is the unit of acceleration?

Metres per second squared, m/s².

What is deceleration?

An object slowing down — a negative acceleration.

What are the steps to calculate acceleration from a change in velocity?

e.g. a car speeds up from 0 m/s to 20 m/s in 5 s

1. Find the change in velocity: $20 - 0 = 20$ m/s
2. Write $a = \Delta v \div t$
3. $a = 20 \div 5 = 4$ m/s²

What is the acceleration of any object falling freely under gravity near the Earth's surface?

About 9.8 m/s².

True or false? Acceleration can always be found by dividing the velocity by the time.

False. You must divide the change in velocity by the time, $a = \Delta v / t$.

What does the gradient of a velocity–time graph represent?

The acceleration of the object.

What motion does a horizontal line on a velocity–time graph show?

Constant velocity (steady speed in a straight line), with zero acceleration.

What does a negative gradient on a velocity–time graph show?

The object is decelerating (slowing down).

What are the steps to determine acceleration from a velocity–time graph?

e.g. the line rises from 0 m/s to 12 m/s between 0 s and 4 s

1. Read the change in velocity: 12 m/s
2. Read the time taken: 4 s
3. Acceleration = gradient = $12 \div 4 = 3$ m/s²

What does the velocity–time graph look like for an object that is stationary for 10 s and then accelerates?

A horizontal line along the time axis up to 10 s, then a line with a positive gradient from 10 s.

True or false? A horizontal line on a velocity–time graph means the object is stationary.

False. It means constant velocity; the object is stationary only if that line lies on the time axis.

What does the area under a velocity–time graph represent?

The distance travelled (or the displacement of the object).

What are the steps to find distance travelled from a velocity–time graph made of straight lines?

e.g. velocity rises 0 to 10 m/s in 4 s, then stays at 10 m/s for 6 s

1. Split the area into simple shapes
2. Triangle = $\tfrac{1}{2} \times 4 \times 10 = 20$ m; rectangle = $10 \times 6 = 60$ m
3. Add them: distance = $20 + 60 = 80$ m

What are the steps to find the distance travelled under a curved velocity–time graph by counting squares?

e.g. each grid square is 1 m/s by 1 s

1. Work out the distance one square represents: $1 \times 1 = 1$ m
2. Count the squares under the line, counting part-squares over half as whole: 46
3. Distance = $46 \times 1 = 46$ m

True or false? The area under a distance–time graph gives the distance travelled.

False. It is the area under a velocity–time graph that gives distance travelled.

What is the equation for uniform acceleration linking final velocity, initial velocity, acceleration and distance?

$v^2 - u^2 = 2as$

In $v^2 - u^2 = 2as$, what does each symbol stand for?

$v$ = final velocity (m/s), $u$ = initial velocity (m/s), $a$ = acceleration (m/s²), $s$ = distance (m).

What are the steps to find the final velocity using $v^2 - u^2 = 2as$?

e.g. starting from rest, $a = 2$ m/s², $s = 9$ m

1. Substitute: $v^2 - 0^2 = 2 \times 2 \times 9$
2. $v^2 = 36$
3. $v = \sqrt{36} = 6$ m/s

Which value of acceleration do you use in $v^2 - u^2 = 2as$ for an object dropped near the Earth's surface?

9.8 m/s², the acceleration of free fall.

True or false? $v^2 - u^2 = 2as$ can be used for any journey at all.

False. It only applies when the acceleration is uniform (constant).

What is terminal velocity?

The constant maximum velocity reached by an object falling through a fluid, when the resultant force on it is zero.

Why does an object accelerate when it first starts to fall through a fluid?

Its weight is greater than the air resistance (drag), so there is a resultant downward force.

Why does a falling object eventually reach terminal velocity?

As its velocity increases, air resistance increases until it equals the weight, so the resultant force becomes zero.

True or false? At terminal velocity there are no forces acting on the falling object.

False. Weight and drag still act, but they are equal and opposite so the resultant force is zero.

How does opening a parachute change the forces on a falling skydiver?

Air resistance increases greatly and becomes bigger than the weight, giving a resultant upward force, so the skydiver slows down.

What shape is the velocity–time graph for an object falling through a fluid until it reaches terminal velocity?

A curve starting steep, with the gradient decreasing, which levels off into a horizontal line.

On a velocity–time graph of a falling object, what does the horizontal section represent?

Terminal velocity — constant velocity with zero resultant force.

How does a velocity–time graph show a skydiver opening a parachute?

The line falls (negative gradient) as they decelerate, then levels off at a new, lower terminal velocity.

True or false? On the velocity–time graph of a falling object, decreasing gradient means the object is slowing down.

False. It means the object is still speeding up, but its acceleration is decreasing as drag increases.