What determines the flight path of a projectile in sport?
The factors affecting the horizontal distance of a projectile (speed, angle and height of release), the parabolic flight path and the resolution of forces into horizontal and vertical components.
A focused answer to OCR A-Level PE on projectile motion: the three factors affecting horizontal distance (speed, angle and height of release), why the optimum release angle is 45 degrees only when release and landing heights are equal, the parabolic flight path, and resolving the resultant force into horizontal and vertical components.
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What this dot point is asking
OCR wants you to state the factors affecting the horizontal distance of a projectile, explain the role of the release angle (and why 45 degrees is optimal only for equal release and landing heights), describe the parabolic flight path, and resolve the resultant force or velocity into horizontal and vertical components.
The three factors affecting horizontal distance
The optimum angle of release
The parabolic flight path
For a dense object such as a shot or a human body, air resistance is small relative to weight, so the path is very close to a true parabola. For a light object with a large surface area, such as a shuttlecock or a table-tennis ball, air resistance is large relative to weight, decelerating the horizontal motion strongly, so the path is asymmetrical (non-parabolic), dropping more steeply at the end.
Resolving forces into components
Once airborne, the release velocity (and any net force) can be resolved into a horizontal and a vertical component, drawn on a free body diagram (an arrow for weight acting down through the centre of mass, plus air resistance for a light object). The horizontal component stays constant because no horizontal force acts on a dense body (Newton's first law). The vertical component is reduced by gravity on the way up, reaches zero at the peak, then increased on the way down. Combining a constant horizontal velocity with a uniformly changing vertical velocity gives the parabolic curve.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksState the three factors that affect the horizontal distance a shot travels, and explain why the optimum release angle for a shot is slightly less than 45 degrees.Show worked answer →
A Component 01 Section C application question. One mark per factor and one for the angle explanation.
Award marks for: the three factors are the speed of release, the angle of release and the height of release. The optimum angle for maximum horizontal distance is exactly degrees only when the point of release is at the same height as the point of landing. A shot is released from above shoulder height, so the release height is greater than the landing height; this extra height adds flight time, so the optimum angle is slightly less than degrees (around to degrees) to favour horizontal velocity.
Markers reward the three named factors and the link between a release height above the landing height and an optimum angle below 45 degrees.
OCR 20216 marksUsing horizontal and vertical components, explain why a projectile such as a long jumper's centre of mass follows a parabolic flight path. Use a free body diagram in your reasoning.Show worked answer →
A Component 01 Section C extended-response question. Markers reward resolving the velocity into components and applying gravity and air resistance.
Award marks for: once airborne, the only significant forces on the jumper (ignoring air resistance for a dense body) are weight acting vertically downward, shown on a free body diagram. The release velocity is resolved into a horizontal component and a vertical component. The horizontal component stays constant because no horizontal force acts (Newton's first law), so the horizontal motion is uniform. The vertical component is constantly decelerated on the way up by gravity, reaches zero at the peak, then accelerates downward, so the vertical motion is symmetrical. Combining constant horizontal velocity with uniformly changing vertical velocity produces a symmetrical curved path, a parabola. For a light object such as a shuttlecock, large air resistance distorts the path so it is non-parabolic.
A top answer states that the horizontal component is constant and the vertical component changes uniformly under gravity, giving a parabola, and notes air resistance distorts the path for light objects.
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Sources & how we know this
- OCR A Level Physical Education (H555) specification — OCR (2016)