Motion and forces Revision Notes

Basic Speed Measurement

Speed is defined as distance travelled per unit time. For objects moving at a relatively constant visible speed, a ruler or measuring tape can be used to measure the distance, and a stopwatch can be used to measure the time taken. The formula Speed = Distance / Time is then applied. This method is straightforward but may lack precision for fast-moving objects or when instantaneous speed is required.

Light Gate Method

Light gates offer a more accurate way to measure speed. A single light gate can determine the instantaneous speed of an object by measuring the time it takes for a known length of the object (e.g., a card attached to it) to pass through the beam. Speed = Length of object / Time interval. Two light gates can be used to measure average speed over a set distance. The first gate starts a timer when the object breaks its beam, and the second gate stops the timer when the object breaks its beam. Speed = Distance between gates / Time interval. This method is effective for precise measurements and can be automated.

Ticker Timer Method

A ticker timer is a device that makes dots on a paper tape at a constant frequency (e.g., 50 dots per second). As an object pulls the tape, the distance between consecutive dots indicates its speed. A greater distance between dots means higher speed. By counting the number of dots to determine the time interval (e.g., 10 dots = 0.2 seconds for a 50 Hz timer) and measuring the tape length, changes in speed (acceleration) can also be observed. This method is useful for studying short intervals and changes in motion.

Human and Animal Speeds

Typical human speeds vary depending on the activity. A person walking generally moves at about 1.5 m/s. Running speeds are higher, around 3 m/s for a typical person. A cyclist on a level road usually maintains a speed of approximately 6 m/s. These values are averages and can differ based on factors like fitness, terrain, and effort.

Environmental and Transport Speeds

The speed of sound in air is approximately 330 m/s to 340 m/s, depending on temperature and atmospheric conditions. Wind speeds are highly variable; a gentle breeze might be a few meters per second, while strong winds or gales can exceed 10-20 m/s. Cars on urban roads typically travel around 13 m/s (30 mph), while trains can reach much higher speeds, often over 50 m/s. Aeroplanes cruise at speeds around 250 m/s.

Definition and Units of Acceleration

Acceleration describes how quickly an object’s velocity changes over time. It is calculated as the change in velocity divided by the time taken, and its SI unit is metres per second squared (m/s²), showing how many metres per second velocity increases each second.

Value of g and Comparing Real-World Accelerations

On Earth, g is about 10 m/s² for free-falling objects with negligible air resistance. Estimating the size of accelerations in activities like driving, sprinting, or taking off in a rocket allows you to make informed comparisons to g, recognising how everyday and extreme situations relate to this standard value.

Definition of Momentum

Momentum is a measure of the 'quantity of motion' an object has. It depends on an object's mass and its velocity. The greater the mass or the greater the velocity, the greater the momentum. It is a vector quantity, so direction is important.

Momentum Equation and Units

Momentum (p) is calculated by multiplying an object's mass (m) by its velocity (v). The equation is: p = m × v. The standard unit for mass is kilograms (kg), and for velocity is metres per second (m/s). Therefore, the unit for momentum is kilogram metre per second (kg m/s).

Calculating Momentum

To calculate momentum, ensure all values are in their standard SI units: mass in kg and velocity in m/s. If units are provided differently (e.g., mass in grams or velocity in km/h), they must first be converted before applying the formula p = m × v. For example, 1 kg = 1000 g, and 1 m/s = 3.6 km/h (or 1 km/h = 1000/3600 m/s).