Exploring Forces · Lesson 2 of 7
Contact Forces: Muscular Force and Friction
“Explore forces that require contact, from muscular force to friction between surfaces and through fluids.”
• Define contact force and identify examples of direct and indirect contact. • Explain how muscular force is produced and used by living beings. • Describe friction as a force that opposes motion or attempted motion. • Explain how surface irregularities produce friction. • Relate friction to surface roughness and to motion through air and water.
Forces That Need Contact
Many forces act only when objects are physically touching. Contact may be direct, such as a hand pushing a box, or indirect, such as a person pulling an object with a rope. In both cases, the force is transmitted through physical contact.
A force that acts only when the interacting objects are in physical contact is called a contact force.
Muscular Force
When we walk, run, lift, push, jump, stretch, or chew, muscles contract and relax to produce force. Animals, birds, fish, and insects also use muscular force for movement, feeding, defence, and survival.
The force produced due to the action of muscles is called muscular force.
Muscular force also works inside the body. Muscles help us chew food and move it through the alimentary canal. The rhythmic contraction and relaxation of heart muscle keeps blood circulating throughout the body.
For centuries, humans have used the muscular force of animals for carrying loads, pulling carts, ploughing fields, and other tasks.
Why Moving Objects Slow Down
A ball rolling on level ground eventually slows and stops. A bicycle also slows when the rider stops pedalling. Since a change in speed requires a force, some force must be acting even when no one appears to be pushing or pulling the moving object.
That force is friction. It acts between surfaces in contact and opposes the direction in which one surface moves, or tries to move, over another.
The force that comes into play when an object moves or tries to move over another surface is called friction. It acts opposite to the direction of motion or attempted motion.
Why Surfaces Produce Friction
Even a surface that looks smooth has tiny irregularities. When two surfaces touch, these irregularities can catch or interlock. As one surface tries to move across the other, the interlocking opposes motion.
If the same object is pushed across glass, cloth, wood, tile, and sand, it usually travels different distances before stopping. This shows that friction depends on the nature of the surfaces in contact. Rougher surfaces generally produce greater friction than smoother surfaces.
| Surface condition | Expected friction | Likely effect on sliding |
|---|---|---|
| Smoother surface | Less | Object tends to travel farther |
| Rougher surface | More | Object tends to stop sooner |
Friction in Air and Water
Friction is not limited to solid surfaces. Air and water also oppose the motion of objects moving through them. Aeroplanes, ships, boats, and high-speed trains are therefore given shapes that help reduce this resistance.
Problem
A bicycle is allowed to roll without pedalling on a rough road and then on a smoother road. On which road should it slow sooner, and why?
- 1.Friction opposes the motion of the bicycle.
- 2.Rougher surfaces generally produce greater friction.
- 3.Therefore, the bicycle should lose speed sooner on the rough road.
Quiz
Which of these is a contact force?
In which direction does friction act on a sliding object?
Why can a smooth-looking surface still produce friction?
What usually happens to friction when surfaces are rougher?
Which situation involves friction from a fluid?
Practice Problems
- Explain the difference between direct and indirect contact force, with one example of each.
- Describe two ways muscular force is used inside the human body.
- Why does a ball rolling on flat ground eventually stop?
- Explain how microscopic surface irregularities produce friction.
- Predict whether a sliding object will usually travel farther on polished tile or sand, and explain why.
- Why are vehicles such as aeroplanes and high-speed trains designed with streamlined shapes?
Key Takeaways
• Contact forces act only when objects are physically touching. • Muscular force is produced by the action of muscles. • Friction opposes motion or attempted motion between surfaces in contact. • Friction arises because even apparently smooth surfaces have tiny irregularities. • Rougher surfaces generally produce greater friction, and fluids such as air and water also exert friction.