الأسبوع 11 · المحور الثاني، الوحدة الثانية، المفهوم 2.1 «الحركة والتوقف» (ب) والمفهوم 2.2 «الطاقة والحركة» (أ): توقف الأجسام والقوة والطاقة · 3 دروس
1
What makes things stop?
Discovery Education Science P4: Term 1, Unit 2, pp. 130–144
GoalTell balanced forces from unbalanced forces and explain how friction slows and stops moving things.
New words
balanced forces · forces that are equal and opposite, so the motion does not changeunbalanced forces · forces that are not equal, so the motion changesfriction · a force between two surfaces that rub, which slows or stops motion
Explanation
A book lying on a table does not move. Gravity pulls it down and the table pushes it up with an equal push. The two forces are balanced, so nothing changes. When the forces on an object are unbalanced, it can start to move, speed up, slow down or change direction. So how does a moving thing stop? Sometimes a clear force does it: a wall stops a rolling ball by pushing back on it. Often the force is friction. Friction acts where two surfaces touch and rub, and it pushes against the motion. A toy car that runs out of push on a floor slows down and stops because of friction between its wheels and the floor. Rough surfaces such as carpet have more friction than smooth tiles, so things stop sooner on them.
Situation
Forces
Result
Book on a table
gravity down, table up: equal
stays still
Toy car rolling on a floor
friction pushes back, nothing pushes on
slows and stops
Ball being kicked
the foot pushes it forward
starts to move
Worked examples
A cup rests on a shelf. Gravity pulls it down and the shelf pushes it up with an equal force. The forces are balanced, so the cup stays where it is.
Karim rolls a football along the grass. Nobody pushes it any more, but it slows down and stops. The grass rubs on the ball, and this friction pushes against the motion.
Marwan slides a tin along the tiles and it goes 4 m. On a thick rug it goes only 1 m. The rug has more friction than the tiles, so it stops the tin sooner.
Class activity
Slide test. A grown-up helps you choose three flat surfaces at home: a tiled floor, a rug and a wooden board. Give a small plastic box the same gentle push from the same line on each surface. Predict where it will stop soonest. Measure how far it goes each time with a tape and write the distances in a table. Which surface has the most friction? Keep the same box and the same push.
Practice
1
A book rests on a desk. Are the forces on it balanced or unbalanced? How do you know?
Balanced. Gravity pulls down and the desk pushes up equally, and the book does not move.
2
A bicycle rider stops pedalling on a flat road and the bicycle slowly stops. Which force slows it?
Friction between the tyres and the road (and with the air).
3
A box slides across a smooth surface. Which surface will stop it sooner? Say the letter.
B, the rubber. It has more friction than the glass.
4
Two children push a box from opposite sides with equal pushes. Does the box move? Why?
No. The forces are equal and opposite, so they are balanced.
