What is meant by the independence of perpendicular components of motion?

The horizontal and vertical motions can be treated as separate problems, because a force in one direction has no effect on the motion at right angles to it.

Describe the vertical motion of a projectile when air resistance is negligible.

It accelerates downwards at g throughout, exactly as an object in free fall does.

Two identical balls leave a table at the same moment, one dropped and one rolled off horizontally. Which lands first, and why?

They land together, because both start with zero vertical velocity and have the same downward acceleration, and the horizontal motion has no effect on the vertical.

A ball leaves a table 20 m high moving horizontally at 15 m s⁻¹. Taking g as 9.81 m s⁻², calculate the time before it lands.

t = √(2s/g) = √(40/9.81) = 2.0 s.

OCR A-Level Physics B H557

Planning

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Implementing

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Analysis

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Probing Deep Into Matter

Topic overview Particle accelerators and high-energy beams Scattering and the nuclear atom Paths of scattered particles and closest approach Discrete energy levels in atoms Electron standing waves and the confined electron Quarks, hadrons and leptons Conservation laws and nuclear equations Charged particles in detectors and magnetic fields Relativistic energy and the relativistic factor

Ionising Radiation and Risk

Topic overview The nature of ionising radiations Effects on living tissue and radiation safety Absorbed dose, effective dose and risk Binding energy and nuclear stability The Nuclear Valley and modes of decay Fission, fusion and nuclear power Activity, half-life and the decay constant Investigating absorption and half-life

Space, Time and Motion

Projectiles and motion in two dimensions

3 things to know
Components 01, 02 and 03

Overview

Recall questions on the independence of perpendicular motions, the horizontal and vertical motion of a projectile, its parabolic path, a dropped and a rolled ball landing together, time of flight and range for a ball leaving a table or launched at 30°, velocity at the highest point, air resistance, constant vertical acceleration, a boat crossing a current, and perpendicular forces.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe the horizontal motion of a projectile when air resistance is negligible.

Key term

Describe the shape of the path of a projectile launched at an angle above the horizontal.

Key term

A ball is launched at 20 m s⁻¹ at 30° to the horizontal. What is its initial vertical velocity?

Questions from this subtopic

Each one with the model answer underneath.

1
What is meant by the independence of perpendicular components of motion?
Model answer
The horizontal and vertical motions can be treated as separate problems, because a force in one direction has no effect on the motion at right angles to it.
2
Describe the horizontal motion of a projectile when air resistance is negligible.
Model answer
The horizontal velocity stays constant throughout the flight, because no horizontal force acts on the projectile.
3
Describe the vertical motion of a projectile when air resistance is negligible.
Model answer
It accelerates downwards at g throughout, exactly as an object in free fall does.
4
Describe the shape of the path of a projectile launched at an angle above the horizontal.
Model answer
The path is a parabola, symmetrical about the highest point when the launch and landing heights are the same.
5
Two identical balls leave a table at the same moment, one dropped and one rolled off horizontally. Which lands first, and why?
Model answer
They land together, because both start with zero vertical velocity and have the same downward acceleration, and the horizontal motion has no effect on the vertical.
6
A ball leaves a table 20 m high moving horizontally at 15 m s⁻¹. Taking g as 9.81 m s⁻², calculate the time before it lands.
Model answer
t = √(2s/g) = √(40/9.81) = 2.0 s.

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