Projectile motion Flashcards – AQA A-level Physics (7408)

How are a projectile’s horizontal and vertical motions combined to find its position and velocity?

Treat the components independently using the same time, then combine their positions and velocities. Uniform-acceleration equations apply separately to each component.

What are a projectile’s horizontal and vertical accelerations under gravity alone?

Horizontal acceleration is zero; vertical acceleration is downwards with constant magnitude $g$.

A projectile is launched at speed $u$ and angle $\theta$ above the horizontal. What are its initial horizontal and vertical velocity components?

Horizontal: $u\cos\theta$. Vertical: $u\sin\theta$.

How does a projectile’s horizontal velocity change under gravity alone?

It remains constant.

For a projectile launched at speed $u$ and angle $\theta$ above the horizontal, what are its horizontal and vertical displacements after time $t$? Take upwards as positive.

$x=(u\cos\theta)t$

$y=(u\sin\theta)t-\frac{1}{2}gt^2$

For a projectile launched at speed $u$ and angle $\theta$ above the horizontal, what is its vertical velocity after time $t$? Take upwards as positive.

$v_y=u\sin\theta-gt$

What are a projectile’s velocity components at the highest point of its flight?

The vertical velocity is zero; the horizontal velocity need not be zero.

What is the shape of a projectile’s trajectory under gravity alone in a uniform gravitational field?

A parabola.

What is friction?

A contact force between surfaces that opposes their relative motion or tendency to move relative to each other.

In what direction does friction act relative to the surfaces in contact?

Parallel to the surfaces.

What two factors affect the magnitude of friction?

The nature of the surfaces and how strongly they are pressed together.

What is lift, and is it necessarily directed vertically upward?

Lift is the component of a fluid’s force on an object that acts perpendicular to the relative flow. It is not necessarily vertically upward.

How can pressure differences around an object produce lift?

Different pressures on parts of the object’s surface produce a resultant force.

What three factors does lift depend on?

The flow and the object’s shape and orientation.

What is drag?

A force exerted by a fluid that opposes an object's motion relative to the fluid.

What is air resistance?

Drag exerted by air.

How does air resistance change as an object's speed relative to the air increases?

Its magnitude increases.

Besides relative speed, which two properties of an object affect the magnitude of air resistance?

Its shape and the area presented to the airflow.

Which aspects of an object's velocity can drag change?

Its speed and its direction.

In what direction does air resistance act on a projectile?

Opposite to its velocity relative to the air.

In still air, how does air resistance affect a projectile’s horizontal velocity during flight?

The horizontal velocity is no longer constant.

How does air resistance affect a projectile’s vertical acceleration throughout its flight?

It is not simply a constant $g$ throughout the flight.

Compared with the same launch under gravity alone, how does air resistance affect a projectile’s maximum height and horizontal distance?

It reduces both the maximum height and the horizontal distance travelled.

How does air resistance change the shape of a projectile’s trajectory compared with motion under gravity alone?

The trajectory is no longer parabolic or symmetric about its highest point.

Can the uniform-acceleration equations describe a projectile’s complete flight with air resistance using a single constant acceleration?

No. The motion cannot be described using a single constant acceleration.