Quick questions on Kinematics: equations of motion and projectiles - WJEC A-Level Physics

Displacement is the vector distance from a defined start point, measured in metres. Velocity is the rate of change of displacement, a vector. Acceleration is the rate of change of velocity, also a vector.

A curved displacement-time graph means changing velocity, so you take the gradient of a tangent at a point to find the instantaneous velocity. On a velocity-time graph, splitting the area into triangles and rectangles is the quickest way to get displacement when the acceleration is not uniform.

For motion with constant acceleration in a straight line, WJEC uses four equations (often called the $suvat$ equations):

A projectile moves under gravity alone. Resolve the launch velocity into horizontal and vertical components using $u_x = u\cos\theta$ and $u_y = u\sin\theta$. The horizontal velocity is constant because no horizontal force acts; the vertical motion has constant acceleration $g \approx 9.81\,\text{m s}^{-2}$ downward. Time is the bridge: solve the vertical motion for the time of flight, then feed that time into the horizontal equation for the range.

A car accelerates from rest at $2.0\,\text{m s}^{-2}$ for $5.0\,\text{s}$. Find its final velocity and the distance travelled. [3 marks]

Explain why the horizontal velocity of a projectile stays constant. [2 marks]

A ball is thrown vertically upward at $20\,\text{m s}^{-1}$. Find the maximum height reached. [3 marks]