Describing Motion Flashcards

All 50 cards in this deck

What is a scalar quantity?

A quantity that has magnitude (size) but no specific direction.

What is a vector quantity?

A quantity that has both magnitude (size) and a specific direction.

Name four vector quantities from the Physics specification.

Displacement, velocity, acceleration, force, weight, momentum (any four).

Name four scalar quantities from the Physics specification.

Distance, speed, mass, energy (any four).

True or false? Force is a scalar quantity.

False. Force is a vector — it has a size and a direction.

True or false? Mass is a scalar but weight is a vector.

True. Weight is a force acting downwards, so it has direction; mass has size only.

What is the equation for average speed?

average speed = distance ÷ time, in m/s (distance in m, time in s).

What is the equation for distance travelled in terms of speed and time?

distance travelled (m) = average speed (m/s) × time (s).

What are the steps to calculate an average speed?

e.g. a runner covers 100 m in 20 s

1. Write the equation: speed = distance ÷ time
2. Convert to metres and seconds if needed: 100 m, 20 s
3. Divide: $100 \div 20 = 5$ m/s

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

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

What is the typical speed of sound in air?

About 330 m/s.

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

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

What is velocity?

Speed in a stated direction — it is a vector.

True or false? An object moving at constant speed in a circle has a constant velocity.

False. Its direction keeps changing, and velocity is speed in a stated direction, so velocity changes.

What two things can change to make an object's velocity change?

Its speed, or its direction (or both).

What is the difference between distance and displacement?

Distance is the total path length (scalar); displacement is the straight-line distance in a stated direction (vector).

True or false? A velocity can be given as a negative number.

True. The minus sign shows the object is moving in the opposite direction to the one chosen as positive.

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

The speed of the object.

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 40 m between 0 s and 8 s

1. Read the change in distance: 40 m
2. Read the change in time: 8 s
3. Gradient = distance ÷ time = $40 \div 8 = 5$ m/s

What does a horizontal (flat) line on a distance–time graph show?

The object is stationary (not moving).

On a distance–time graph, what does a steeper line show?

A greater speed.

What does a curved line on a distance–time graph show?

The speed is changing — getting steeper means speeding up, less steep means slowing down.

True or false? On a distance–time graph, a line sloping downwards means the object is slowing down.

False. It means the object is moving back towards its starting point.

What is acceleration?

The rate of change of velocity (how quickly velocity changes).

What is the equation for acceleration in terms of change in velocity and time?

acceleration (m/s$^2$) = change in velocity (m/s) ÷ time taken (s), i.e. $a = \dfrac{v-u}{t}$

What are the steps to calculate an acceleration?

e.g. a car speeds up from 4 m/s to 20 m/s in 8 s

1. Find the change in velocity: $20 - 4 = 16$ m/s
2. Divide by the time taken: $16 \div 8$
3. Acceleration = 2 m/s$^2$

What does a negative acceleration tell you about an object's motion?

It is decelerating (slowing down), if it is moving in the positive direction.

What is the value of the acceleration in free fall, $g$?

10 m/s$^2$ (downwards).

Roughly what is the acceleration of an everyday family car pulling away from rest?

About 2 m/s$^2$ — much less than free fall (10 m/s$^2$).

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

$v^2 - u^2 = 2 \times a \times x$, where $x$ is the distance in metres.

When should you use $v^2 - u^2 = 2ax$ instead of $a = (v-u) \div t$?

When the question involves distance and the time taken is not known.

True or false? In the equation $v^2 - u^2 = 2ax$, $x$ stands for the time taken.

False. $x$ is the distance travelled in metres.

What value of initial velocity $u$ do you use for an object that starts from rest?

$u = 0$ m/s.

What are the steps to find the distance travelled using $v^2 - u^2 = 2ax$?

e.g. a cyclist accelerates from rest to 10 m/s at 2 m/s$^2$

1. Substitute: $10^2 - 0^2 = 2 \times 2 \times x$
2. Rearrange: $x = \dfrac{100}{4}$
3. Distance = 25 m

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

The acceleration of the object.

What are the steps to calculate acceleration from a straight section of 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. Gradient = $12 \div 4 = 3$ m/s$^2$

On a velocity–time graph, what does a steeper line show?

A greater acceleration.

What does a line sloping downwards on a velocity–time graph show?

The object is decelerating (velocity decreasing).

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

False. It means constant velocity (zero acceleration); the object is only stationary if that line is at zero velocity.

On a velocity–time graph, what shape of line shows uniform (constant) acceleration?

A straight sloping line.

What does the area between a velocity–time graph line and the time axis represent?

The distance travelled.

What are the steps to find the distance travelled from a velocity–time graph?

e.g. the line rises from 0 to 8 m/s in 5 s, then stays at 8 m/s for 10 s

1. Split the area into triangles and rectangles
2. Triangle: $\frac{1}{2} \times 5 \times 8 = 20$ m; rectangle: $8 \times 10 = 80$ m
3. Add the areas: distance = 100 m

What is the formula for the area of a triangular section under a velocity–time graph?

Area = $\frac{1}{2} \times$ base (time) $\times$ height (velocity).

True or false? The area under a velocity–time graph gives the acceleration.

False. The area gives the distance travelled; the gradient gives the acceleration.

What does a light gate connected to a timer actually measure?

The time for which an object (or a card on it) blocks the light beam.

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

e.g. a card of length 0.05 m blocks the beam for 0.02 s

1. Measure the length of the card on the trolley: 0.05 m
2. Let the card interrupt the beam and record the time: 0.02 s
3. Speed = length ÷ time = $0.05 \div 0.02 = 2.5$ m/s

What is the main advantage of using light gates (or a data logger) instead of a stopwatch to measure speed?

It eliminates human reaction time, so the timing is more accurate.

How can two light gates be used to find the acceleration of a trolley?

Measure the velocity at each gate and the time between them, then use $a = (v-u) \div t$.

What are the steps to measure the average speed of a trolley using a metre rule and a stopwatch?

e.g. a trolley runs along a 2 m bench in 4 s

1. Measure the distance with the metre rule: 2 m
2. Time the whole run with the stopwatch: 4 s
3. Average speed = distance ÷ time = $2 \div 4 = 0.5$ m/s

Apart from using electronic timing, how can the effect of reaction time on a stopwatch measurement be reduced?

Time over a longer distance (longer time), and repeat the measurement to find a mean.