Motion
边玩边学
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给老师： 面向 Motion（Physics，CIE）的即用型课程幻灯片, 复习笔记, 图示——用在你的课堂上，或将该知识点作为学生可实时游玩的互动课堂活动来运行。
课程笔记
Speed & Velocity
- Speed = distance travelled per unit time; a scalar quantity (magnitude only).
- Equation: v = \frac{s}{t}, where v in m/s, s in m, t in s.
- Average speed = total distance / total time; useful when speed varies.
- Velocity = speed in a given direction; a vector quantity (magnitude and direction).
- Velocity uses displacement (vector) instead of distance.
- Same speed but different directions → different velocities.
A person walking at 2.0 m/s and a bee flying at 4.5 m/s, illustrating speed.
Acceleration (Extended)
- Acceleration = rate of change of velocity; a = \frac{\Delta v}{\Delta t}, units m/s².
- Change in velocity: \Delta v = v - u (final minus initial).
- Positive acceleration = speeding up; negative acceleration (deceleration) = slowing down.
- Any change in velocity (speed up, slow down, change direction) is acceleration.
- Formula triangle: cover the quantity to find; a = \frac{\Delta v}{\Delta t}, \Delta v = a \times \Delta t, \Delta t = \frac{\Delta v}{a}.
Examples of positive and negative acceleration
Distance-Time Graphs
- Distance-time graph: straight line = constant speed; horizontal line = stationary.
- Gradient = speed; steeper slope = faster speed.
- Curved line = changing speed (acceleration/deceleration); increasing gradient = speeding up, decreasing = slowing down.
- To calculate speed: draw a large gradient triangle, find \frac{\Delta y}{\Delta x}.
- Always check units (e.g., km → m, min → s).
The gradient of a distance-time graph, found from the change in distance (Δy) over the change in time (Δx), gives the speed.
Speed-Time Graphs
- Speed-time graph: straight line = constant acceleration; horizontal line = constant speed (zero acceleration).
- Gradient = acceleration; positive gradient = acceleration, negative = deceleration.
- Area under graph = distance travelled.
- Split area into triangles and rectangles: triangle area = \frac{1}{2}bh, rectangle area = bh.
- Total distance = sum of all areas under the line.
The area under a speed-time graph, split into a triangle and a rectangle (base × height), gives the distance travelled.
Calculating Acceleration from Speed-Time Graphs (Extended)
- For constant acceleration: gradient = \frac{\Delta v}{\Delta t} using a straight line.
- For changing acceleration: draw a tangent at the point; gradient of tangent = instantaneous acceleration.
- A tangent is a straight line that just touches the curve at that point.
- Use a large gradient triangle on the tangent to calculate \frac{\Delta y}{\Delta x}.
A tangent drawn to a curved speed-time graph at a point gives the gradient, and so the instantaneous acceleration, at that point.
Freefall
- In absence of air resistance, all objects fall with same acceleration: g = 9.8 \, \text{m/s}2 (acceleration of freefall).
- Weight W = mg, where g is gravitational field strength (N/kg).
- With air resistance: forces are weight (down) and air resistance (up).
- Air resistance increases with speed; when it equals weight, resultant force = 0 → terminal velocity.
- Skydiver: initially accelerates, then air resistance increases, eventually reaches terminal velocity; deploying parachute increases air resistance → deceleration to a lower terminal velocity.
- In a vacuum (no air resistance), objects never reach terminal velocity; they accelerate at g continuously.
A person dropping two objects of different sizes from the Leaning Tower of Pisa to demonstrate freefall.
幻灯片
练习题
免费预览——63 题中的 8 题。注册即可查看全部。
1.Which of the following is a scalar quantity?
Easy- Avelocity
- Bdisplacement
- Cacceleration
- Dspeed
2.What is the acceleration of free fall near Earth's surface?
Easy- A9.8 m/s
- B9.8 m/s²
- C9.8 m/s² downward
- D9.8 N/kg
3.On a distance-time graph, a horizontal line indicates that the object is:
Easy- Aaccelerating
- Bmoving at constant speed
- Cstationary
- Ddecelerating
4.Which of the following is the correct definition of velocity?
Easy- ADistance travelled per unit time
- BSpeed in a given direction
- CChange in displacement per unit time
- DRate of change of speed
5.The area under a speed-time graph represents:
Easy- Aacceleration
- Bvelocity
- Cdistance travelled
- Ddeceleration
6.An object accelerates from 4 m/s to 10 m/s in 3 s. What is its acceleration?
Medium- A2 m/s²
- B3 m/s²
- C4 m/s²
- D6 m/s²
7.On a speed-time graph, a straight line with negative gradient represents:
Easy- Aconstant acceleration
- Bconstant deceleration
- Cconstant speed
- Dincreasing acceleration
8.A skydiver falls from a plane. After reaching terminal velocity, the parachute opens. Which statement is correct?
Medium- AThe skydiver immediately stops falling.
- BThe skydiver accelerates upward.
- CThe skydiver decelerates to a lower terminal velocity.
- DThe skydiver continues at the same terminal velocity.
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