Friction and drag

• Physics
• Lower Secondary (Key Stage 3)

Friction and drag

Friction

 Friction is the force that acts between two solid objects sliding past each other.  It is a contact force  since the surfaces of each object must touch.

 Friction increases if the surfaces are rough. To make the  objects move, if there is more friction, they must be pushed or pulled apart with more force. 

Friction can be reduced using lubrication; this provides a layer allowing the surfaces to slide over each other.

What is the force that acts between two solid objects sliding past each other.

1. Drag
2. Upthrust
3. Friction
4. Magnetic

Friction is a ______ force - the surfaces of each object must touch.

1. non contact
2. contact

Friction increases if the surfaces are ___________.

1. Rough
2. Smooth

How can friction be reduced?

1. Rougher surface
2. Lubrication

Which tyre has the best ‘grip’ - the most friction against a road surface?

1. 
2. 
3. 
4. 

Where friction is useful

Where friction is not useful

Writing or drawing with a pen or pencil.

Applying brakes to stop.

When on a slide.

On the chain of a bike

Walking on the pavement

Drag

Drag is also a contact force, but occurs when a solid’s surface comes into contact with a fluid (liquid or gas). 

Water resistance, air resistance are forms of drag.

 Water resistance is greater than air resistance since liquid particles are closer together than gas  particles. So more force is exerted in liquids.

Drag can be reduced by making the solid object more streamlined, which reduces the number of particles that contact each other.

Drag, like friction is a __________ force.

1. Non contact
2. Contact

The word fluid refers to

1. Solid
2. Liquid
3. Gas

_______ occurs when a solid’s surface comes into contact with a fluid (liquid or gas).

1. Friction
2. Drag
3. Upthrust
4. Pressure

Effect of friction on motion.

If a force acts on a stationary object it will experience static friction that opposes the motion; once this is overcome, the object will accelerate. 

 The more acceleration the more drag and/or friction it will experience from the substance it is moving over or through - this is because there will be more collisions with the particles it encounters (this transfers energy to the thermal store of the surroundings) acceleration continues all the time the forces remain unbalanced. 

 If drag/frictional forces reach equilibrium with the forward force of the object in motion it will move at a constant speed - if the object is falling this is called terminal velocity.

What type of friction opposes the motion of a stationary object when a force is applied?

1. Rolling friction
2. Dynamic friction
3. Kinetic friction
4. Static friction

What happens once the static friction is overcome for a stationary object?

1. The object will decelerate
2. The object will accelerate
3. The object will remain stationary
4. The object will vibrate

Why does an object experience more drag and/or friction as it accelerates?

1. Reduced contact with surroundings
2. Decreased surface area
3. Less collisions with particles
4. More collisions with particles

What happens if drag/frictional forces reach equilibrium with the forward force of the object in motion?

1. The object moves at a constant speed
2. The object accelerates indefinitely
3. The object stops moving
4. The object changes direction

What is the term for the constant speed reached by a falling object when drag forces are balanced with the forward force?

1. Critical speed
2. Maximum speed
3. Final velocity
4. Terminal velocity

Explain why it is more difficult to walk through waist-high water than through air.

A penny is dropped from a very tall building. It starts to fall.
Describe how the penny can reach an equilibrium during its fall. [6]