Edexcel A Level Physics:复习笔记2.7 Projectiles

Components of Velocity

  • The trajectory of an object undergoing projectile motion consists of a vertical component and a horizontal component
    • These need to be evaluated separately

     

  • Some key terms to know, and how to calculate them, are:
    • Time of flight: how long the projectile is in the air
    • Maximum height attained: the height at which the projectile is momentarily at rest
    • Range: the horizontal distance traveled by the projectile

     

2.1.2-Projectile-Motion

How to find the time of flight, maximum height and range

 

Problems Involving Projectile Motion

  • There are two main considerations for solving problems involving two-dimensional motion of a projectile
    • Constant velocity in the horizontal direction
    • Constant acceleration in a perpendicular direction

     

  • The only force acting on the projectile, after it has been released, is gravity
  • There are three possible scenarios for projectile motion:
    • Vertical projection
    • Horizontal projection
    • Projection at an angle

     

Worked Example

To Calculate Vertical Projection (Free Fall)

A science museum designed an experiment to show the fall of a feather in a vertical glass vacuum tube.

The time of fall from rest is 0.5 s.

WE-Projectile-Motion-Worked-Example-1-question-image

What is the length of the tube, L?

 

WE-Projectile-Motion-Worked-Example-1-answer-image

Worked Example

To Calculate Horizontal Projection

A motorcycle stunt-rider moving horizontally takes off from a point 1.25 m above the ground, landing 10 m away as shown.

What was the speed at take-off? (ignoring air resistance)

WE-Projectile-Motion-Worked-Example-2-question-image

 

WE-Projectile-Motion-Worked-Example-2-answer-image

Worked Example

To calculate projection at an angle

A ball is thrown from a point P with an initial velocity u of 12 m s-1 at 50° to the horizontal.

What is the value of the maximum height at Q? (ignoring air resistance)

WE-Projectile-Motion-Worked-Example-3-question-image

 

WE-Projectile-Motion-Worked-Example-3-answer-image

Exam Tip

In almost every question using the SUVAT equations one component of velocity will have constant acceleration (for example, the vertical component of projectile motion) and the other will have no acceleration (for example the horizontal component of projectile motion when we ignore air resistance).

The trick which you nearly always have to remember is find the common value - time - first, then substitute it into the second part of your calculation.

As long as your working is clear, even if you forget, you are likely to still achieve half marks. But writing good clear Maths and remembering this tip should get you to the end of your calculation - and full marks!

And finally.... don't forget that deceleration is negative as the object rises.

 

转载自savemyexams

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