Motion along a straight line Flashcards – AQA A-level Physics (7408)

What is displacement?

The vector change in position from an object’s initial position to its final position.

How is signed displacement calculated for motion along a straight line?

Choose a positive direction, then calculate $\Delta s = s_{\text{final}} - s_{\text{initial}}$.

What unit is displacement measured in?

Metres.

What is distance travelled, and is it a scalar or vector?

The total length of the path taken; it is a scalar.

When can the magnitude of displacement be smaller than the distance travelled?

When the object changes direction.

What is acceleration?

The rate of change of velocity.

What is the formula for average acceleration over a time interval?

$a = \frac{\Delta v}{\Delta t}$

Is acceleration a scalar or a vector?

A vector.

What unit is acceleration measured in?

Metres per second squared, $\mathrm{m/s^2}$.

What determines the sign of acceleration?

The chosen positive direction.

In straight-line motion, what can acceleration change about an object's motion?

Its speed, its direction, or both.

How does acceleration affect speed when it is in the same direction as velocity versus the opposite direction?

In the same direction, it increases speed; in the opposite direction, it decreases speed.

What is the difference between uniform and non-uniform acceleration?

Uniform acceleration is constant; non-uniform acceleration changes with time.

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

Velocity.

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

Acceleration.

How do an interval gradient and a tangent gradient differ on a motion graph?

An interval gradient gives an average rate; a tangent gradient gives an instantaneous rate.

What does a straight line, rather than a changing gradient, on a velocity–time graph indicate about acceleration?

A straight line indicates uniform acceleration; a changing gradient indicates non-uniform acceleration.

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

Displacement.

What does the signed area between an acceleration–time graph and the time axis represent?

Change in velocity.

Why is the signed area under a velocity–time graph not necessarily the total distance travelled?

Areas below the time axis contribute negatively.

What happens to a bouncing ball’s velocity at a bounce?

Its sign changes as the ball reverses direction.

What changes a bouncing ball’s velocity between bounces?

Gravity.

What does the rapid change in a bouncing ball’s velocity during ground contact imply about its acceleration?

The ball has a large acceleration over a short time.

What is the acceleration due to gravity, $g$?

The acceleration of an object in free fall caused by gravity.

What are the approximate magnitude and direction of gravitational acceleration near Earth’s surface?

$9.81\ \text{m s}^{-2}$, directed downwards.

When is gravitational acceleration approximately constant?

When air resistance is negligible.

How does gravitational acceleration compare for an object moving upwards and one moving downwards when air resistance is negligible?

It is approximately the same in both cases.

What sign is used for gravitational acceleration when upwards is positive versus when downwards is positive?

• Upwards positive: $-g$
• Downwards positive: $+g$

At the highest point of an object’s upward motion, what are its velocity and the direction of its gravitational acceleration?

Its velocity is momentarily zero, while its gravitational acceleration remains downwards.