China 
Africa 
Explore chapters and articles related to this topic
Units and Measurements
Published in Daniel H. Nichols, Physics for Technology, 2019
A radar gun measures the speed of a moving car by determining the difference in frequency of the outgoing and incoming waves it emits. The signal out of the gun is equal to 10mphHz. Suppose the gun measures a frequency difference of 9 Hz. How fast is the car moving in mph?
Does ground-reaction force influence post-impact ball speed in the tennis forehand groundstroke?
Published in Sports Biomechanics, 2022
Raku Shimokawa, André Nelson, James Zois
Each ball was ejected from a ball machine (Spinfire Pro2) at an initial speed of 32 km/h. The ball machine was placed on a 70.2 cm high table, and the trajectory was adjusted for each participant to allow ball contact in their contact zone (between hip and shoulder height) (Stroke & Tactical Fundamental, 2010). The GRF of both feet during each of the forehand strokes was measured separately using two 600 × 900 mm force plates (BP600900 force plates, AMTI, Watertown, USA) at a sample rate of 480 Hz, and an anti-aliasing low-pass filter with a cut off set at 1000 Hz (MSA-6 MiniAmp, AMTI, Watertown, USA). BioWare software version 4.1.0.2 (Kistler, Switzerland) was used for the data acquisition and signal processing of the force plates. The force plates were covered with a Mondo (Mondo Sport & Flooring, Italy) track and field surface. Post-impact ball speed of each forehand was measured using a radar gun (Stalker PRO, Stalker Radar, Plano, TX, USA), which was placed 2 m directly behind the participant. According to the manufacturer’s specifications, the radar gun has an accuracy of ± 0.1 km/h. Each data collection session was filmed using a GoPro® (Hero 3) with a frame rate of 120 frames per second to detect any large and visible technical differences during testing. The GoPro® was placed 2 m from the participant on their right-hand side (90 degrees to the participant). The GoPro® footage was analysed using Kinovea software (www.kinovea.org).