P6 Science: Forces [Part 2]

A clear answer on Forces usually includes three things: the name of the force, its direction, and the effect.

Types of Forces:

• Gravitational force pulls objects downward

Gravity is a non-contact force. It pulls objects toward Earth. That is why a dropped eraser falls to the ground. It is also why objects have weight.

Point: A heavier object usually has a greater weight because gravity pulls on it with a larger force. However, Mass and Weight are not the same thing. Mass is the amount of matter in an object. It is measured in grams or kilograms. Weight is the force of gravity acting on that mass. It is measured in newtons.

• Friction (frictional force) acts against Motion

Friction is a contact force. It happens when two surfaces rub against each other. Friction acts in the opposite direction to motion.

Friction can be helpful. It allows shoes to grip the ground. It helps bicycle brakes slow the wheels. It also helps a pencil leave marks on paper. However, friction can also be a problem. It can cause moving parts to heat up or wear out. That is why oil or grease is used in some machines. The oil reduces friction between surfaces.

Rough surfaces usually produce more friction than smooth surfaces. A rubber sole on a dry floor gives more grip than a smooth sole on a wet floor.

Air resistance slows moving objects

Air resistance is a type of friction caused by air. It acts against the motion of objects moving through air, and can slow a falling object. Some examples:

i) A parachute works because it has a large surface area. More air pushes against it, so air resistance increases. This slows the fall.

ii) A flat sheet of paper falls more slowly than a crumpled paper ball because the flat sheet has a larger surface area. It experiences more air resistance.

• Magnetic force can attract or repel

Magnetic force is a non-contact force. A magnet can pull some materials toward it without touching them. These materials include magnetic materials such as iron and steel. Magnets do not attract all metals. For example, aluminum and copper are not attracted to ordinary magnets.

Magnets also have poles. Like poles repel each other, while unlike poles attract each other.

• North and North repel.

• South and South repel.

• North and South attract.

This is why two magnets may push away from each other even though they are not touching.

• Elastic spring force happens when objects stretch or compress

Elastic force appears when an elastic object changes shape and tries to return to its original shape.

A stretched rubber band pulls back. A compressed spring pushes outward. However, not every object is elastic. If an object is stretched too much and cannot return to its original shape, it has passed its elastic limit.

How to answer force questions well

Force questions often test explanation, not just memory. A strong answer usually names the force and describes its effect.

Use this simple pattern:

• Identify the object. 

  Decide which object the question is asking about.

  
  

• Name the force. 

  Use the correct science term, such as friction, gravity, magnetic force, or elastic spring force.

  
  

• State the direction. 

  Say whether the force acts upward, downward, forward, backward, toward, or away.

  
  

• Describe the effect. 

  Explain whether the object speeds up, slows down, changes direction, changes shape, floats, sinks, attracts, or repels.

  
  

• Compare the forces. 

  If the question involves motion, say whether the forces are balanced or unbalanced.

  
  

Here is an example:

A child pushes a box, but the box does not move. Why?

A good answer would say that the pushing force is balanced by friction acting in the opposite direction. Since the forces are balanced, the box remains at rest.

Another example:

A toy car rolls across the floor and then stops. Why?

A good answer would say that friction between the wheels and the floor acts against the motion of the toy car. Air resistance may also act against the motion. These forces slow the car down until it stops.