Newton's laws of motion GCSE physics lesson: resultant force, F = ma, third-law pairs

Newton's laws of motion: practice

Show your working on paper. Forces are in newtons (N), masses in kg, accelerations in m/s². Take g = 9.8 N/kg. The scenes are virtual experiments: move only the setting the question names. The stretch is optional.

Diagnostic question: choose and explain

1. A car drives along a straight, flat road at a steady 30 m/s. What is true about the forces on it?
   1. The driving force is bigger than the forces against it
   2. The driving force is equal to the forces against it
   3. There are no forces on it
   4. The forces against it are bigger than the driving force

Practice questions

1. In a tug of war, one team pulls left with 420 N and the other pulls right with 380 N. The resultant force is ___ N to the ___.
2. A 1200 kg car accelerates at 2.5 m/s². What is the resultant force on it, in newtons?
3. An astronaut has a mass of 80 kg on Earth. She goes to the Moon, where g = 1.6 N/kg. What is her mass on the Moon?
   1. 80 kg
   2. 128 kg
   3. 50 kg
   4. 0 kg
4. A book rests on a table. The Earth pulls the book down (its weight). Which force is the other half of this Newton's third law pair?
   1. The table pushes the book up
   2. The book pulls the Earth up
   3. The book pushes down on the table
   4. There is none: the book is still
5. A 5 kg block rests on a rough floor. Someone pushes it with 10 N and it doesn't move. What is the friction force on it, in newtons?
6. Virtual experiment. The same 5 kg block on the rough floor (the scene's slope at 0°: "up the slope" is the way you push). Push it from 0, 1 N at a time, until it just starts to move. Watch the friction arrow F as you push, and note what it reads once the block moves.They change an interactive scene (inclined plane) until it does it.
7. Once the block in the scene slides, friction on it is 9.8 N. What push makes the 5 kg block accelerate at 2 m/s²? Find the resultant force it needs first with F = m a. Give the push in newtons.
8. Virtual experiment. No push now, and a 2 kg block on a slope. Tilt it from 10°, a degree at a time, until the block just starts to slide down. Watch friction F grow as you tilt. Then try a different mass: does the angle change?They change an interactive scene (inclined plane) until it does it.
9. In the tilting scene, a block of the same material but twice the mass is put on the slope. At what angle does it start to slide? (You can try it there: the mass moves.)
   1. A smaller angle: it is heavier
   2. The same angle
   3. A larger angle: there is more friction
10. A 900 kg car has a driving force of 2400 N. Air resistance and friction add up to 600 N. What is its acceleration, in m/s²?
11. Stretch (Higher): the same resultant force of 6 N gives trolley A an acceleration of 3 m/s² and trolley B 2 m/s². A's inertial mass is ___ kg and B's is ___ kg, so trolley ___ has more inertia.

Support: For the scenes: in the first, the floor is the scene's slope at 0°, so "up the slope" means the way you push. Leave the μ settings: they say how rough the surfaces are. Move only what the question lets you, a little at a time, and watch the line under the picture (still or moving) and the friction arrow F.

Newton's laws of motion: exit

Show your working on paper for the first question.

1. A 1500 kg car has a driving force of 4000 N, and 1000 N of air resistance and friction act against it. What is its acceleration, in m/s²?
2. A rocket's engine pushes hot gas backwards. Which force is the Newton's third law pair of the engine's push on the gas?
   1. The gas pushes the rocket forwards
   2. The rocket's weight
   3. Air resistance on the rocket

Exit check

An acceleration from a resultant force and one third-law pair show who has the two laws.

Students draw the car as a box with its two forces before calculating.

Newton's laws of motion

Draw each object as a box with an arrow for every force. Find the resultant before using F = m a. Take g = 9.8 N/kg on Earth. Give units.