速度-时间
读数
发生了什么
摩擦力阻碍运动，大小为 μmg。施加的力小于它时，木块保持不动或逐渐减速。超过它之后，合力 F − μmg 使物体加速，速度-时间图变成一条直线，斜率正好是 a = Fnet/m。
- Newton's second law with frictiona = (F − μmg) / m
考试大纲
- GCSE Physics AQA 4.5.6, Edexcel, OCRresultant forces, Newton's second law and friction
- A-Level Maths mechanicsF = ma with friction
挑战
先预测：m = 5 kg、μ = 0.4 时，施加多大的力木块才会开始运动？先算出 μmg，再来验证。
μmg = 0.4 × 5 × 9.8 ≈ 19.6 N，所以施加的力超过约 19.6 N，木块就会开始运动。
系统地学
GCSE Combined and Separate Science 中讲这个主题的课：先看分步讲解，再做考试风格的题目，每次数字都不同。 课程为英文。
常见问题
What is Newton's second law?
The resultant force on an object equals its mass times its acceleration, F = ma. Double the resultant force and the acceleration doubles; double the mass and it halves.
Why doesn't the block move when I push gently?
Friction matches your push up to a limit of μs times the weight. Only when the applied force beats that limit does a resultant force appear and the block starts to accelerate.
Why is the velocity-time graph a straight line?
With a constant resultant force the acceleration is constant, and acceleration is the gradient of a velocity-time graph.
How much force do I need for a given acceleration with friction?
Add the force for the acceleration to the friction: F = m(a + μg). A 5 kg block with μ = 0.2 needs F = 5 × (2 + 0.2 × 9.8) = 19.8 N to accelerate at 2 m/s². That is more than the 9.8 N of static grip, so it does get going. Pick Force needed under Solve for to see each step.