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Common Mistakes on Forces

(Open-ended Answers)

These are some of the common mistakes relating to forces.

When you write out the answers, take note of the keywords and the sentence structure.

Type of force: Frictional Force

Sentence structure:

The material Z is the roughest. There is the greatest frictional force between the ball and the material Z. The ball travels the shortest distance before it stopped.

Example

Tom wants to find out how different materials on the ramp affect the time taken for the wooden block to reach the bottom of the ramp. He creates the set up as shown below and repeats the experiment using materials A, B and C.

Ramp - Using the frictional force concept

He recorded the results in the table below.

Which material, A, B or C, is best used to make the flooring in the toilet?

PSLE - write out your answers clearly.
Combining the answer:

Material  C. The time taken for the wooden block to reach the bottom of the ramp is the greatest. The material is the roughest. There is the greatest frictional force between the wooden block and the material C. The person will not slip and fall easily.

 

Take note:

You have to mention two different surfaces.

Remember to compare!

Type of force: Elastic Spring Force

Sentence structure:

As the compressed spring is released, the spring exerts an elastic spring force onto the ball. The ball moves.

 

Wrong answer 1

When the compressed spring is released, the spring has elastic spring force. The ball moves.

 

Teachers’ Comments:

Force is exerted on another object. Force is not possessed in any object.

Remember: Force is an interaction between different objects.

 

Correct answer

When the compressed spring is released, the spring exerts an elastic spring force on the ball. The ball moves.

 

 

 

A spring is stretched by hanging a ball from it.

Wrong answer 2

The ball exerts an elastic spring force on the spring.

 

Teachers’ Comments:

When the spring is stretched, the elastic spring force is acting on / exerting on the ball.

It is not the other way round.

 

Correct answer

When the spring is stretched, the elastic spring force is acting on / exerting on the ball.

Type of force: Gravitational Force

MUST-KNOW factors affecting gravitational force:

  1. Mass of the object. The greater the mass of the object, the object does not change in mass.
  2. Distance between the object and Planet surface. If the object is in deep space, no gravitational force is acting on the object.

2.1) The gravitational force acting on the object is the same anywhere on the surface of the Earth (there is no change in the mass of the object.)

Situation 1

Tom and Jerry are playing a game of catching the giant paper aeroplane.

Jerry folded a giant paper aeroplane and threw it away from the 2nd floor balcony as shown below.

The giant paper aeroplane flew the path from position A to E before Tom catches the aeroplane.

Does the gravitational force acting on the paper aeroplane change at any position (A, B, C, D and E)?

 

No. Gravitational force acting on the paper aeroplane remains the same.

 

Teachers’ comments:

So long as there is no change in mass of the object and the paper aeroplane is on the Earth surface, rule 2.1 applies.

 

Take note:

The paper aeroplane cannot fly to deep outer space.
Jerry is not Superman!

 

Situation 2

 

Tom is determine not to be outdone by Jerry.

Tom creates a parachute that has attached to the box as shown below.

Tom drops the parachute and the box from the same balcony.

PSLE Gravitational force

Type of force: Magnetic Force

Sentence structure : Attraction

The magnet exerts a magnetic force of attraction on the steel pole.

The steel pole is attracted by the magnet.

OR

The magnet attracts the steel pole.

 

Sentence structure : Repulsion

The magnet exerts a magnetic force of repulsion on another magnet.

Both magnets repel each other.

 

Wrong answer 1

The magnet has a magnetic force of attraction on the steel pole.

 

Teachers’ Comments:

Force is exerted on another object. An object does not possess any force!

 

Correct answer

The magnet exerts a magnetic force of attraction on the steel pole. The steel pole is attracted to the magnet.

 

 

Wrong answer 2

The steel pole attracts the magnet.

 

Teachers’ Comments:

When the steel pole is not magnetised, the steel pole does not attract the magnet!

 

Correct answer:
The magnet attracts the steel pole.

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Andy Ling

 

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