Balanced forces and F = ma
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 2 worked examples, 7 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Why does a rolling ball stop?
- The resultant force
- A car on a road
Read the transcript of the teaching part
Last time
Before we start: last time, in Density and how to measure it, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Why does a rolling ball stop?
Push a ball on the ground and it rolls, then stops. Long ago people thought things stop on their own. The truth is that something stops them. A force is what changes motion, and the resultant force tells us exactly how.
The resultant force
Usually several forces act on an object at once. The resultant force is the single force that has the same effect as all of them together. For forces along a line, add the ones in the same direction, and subtract the ones in the opposite direction. Choose one direction as positive, and the answer will tell you which way the resultant points.
A car on a road
A car's engine pushes it forward with eight hundred newtons. Friction and air resistance push back with three hundred newtons. The resultant is eight hundred minus three hundred: five hundred newtons forward. The car speeds up.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Effects of forces and resultant force · Balanced forces and Newton's first law · Resultant force = mass × acceleration.
Chapter 1.5: Forces
- Balanced forces and F = ma 6 min
- Newton's third law 5 min
- Friction, drag and terminal velocity 5 min
- Thinking, braking and stopping distance 6 min
- Circular motion 5 min
- Springs: load–extension graphs 5 min
- Moments, balance and stability 5 min
Also for O Level Physics 5054
Which topics come up most in O Level Physics 5054 past papers, counted from every paper since 2004.