Physics - Recall for Chapter 3: Motion
OCR A-Level Physics 2022
Recall Questions
Displacement, velocity and acceleration
Define instantaneous speed?
Displacement $\Delta s$, divided by time for a very short time interval, $\Delta t$.
Define average speed?
Total displacement divided by the total time for journey.
Motion graphs
What does the gradient of a displacement-time graph represent?
Instantaneous velocity.
What assumption is made when using the SUVAT equations?
Acceleration is constant/uniform.
Stopping distances
How is the total stopping distance of a car calculated?
stopping distance = thinking distance + braking distance
Define thinking distance?
The distance travelled by a vehicle between the driver recognising a reason to stop and applying the brakes.
Define braking distance?
The distance travelled by a vehicle from the instant the brakes are applied to when the vehicle stops.
How does thinking distance depend on the speed of the vehicle?
Thinking distance depends on reaction time, so thinking distance is proportional to speed.
How does braking distance depend on the speed of the vehicle?
Braking distance is proportional to $\text{speed}^2$.
Measuring g
Describe how acceleration due to gravity $g$ can be determined by experiment (long)?
Drop an object from a known height and time how long it takes to fall using a stopwatch/light gates/electromagnet and trap door. Repeat for different heights and then plot a graph of height against $\text{time}^2$. Gradient is $\frac{g}{2}$.
Projectile motion
What is a projectile?
An object acted upon only by weight, so its acceleration is $9.8ms^{-2}$.
How can we solve projectile problems?
By treating horizontal and vertical motion separately.
What assumption is usually made when solving projectile motion problems on Earth?
Air resistance is ignored.
Describe and explain the horizontal motion of a projectile?
Horizontal velocity remains constant as no forces are acting horizontally.
Describe and explain the vertical motion of a projectile?
Vertical acceleration is $9.8ms^{-2}$ downwards as the only force acting is the weight of the object.
What is the horizontal component of the initial velocity of a projectile launched at velocity $u$ and angle $\theta$ to the horizontal?
$u\cos \theta$
What is the vertical component of the initial velocity of a projectile launched at velocity $u$ and angle $\theta$ to the horizontal?
$u\sin \theta$