How Forces Affect Motion · Lesson 3 of 8
The Force of Friction: Often Overlooked but Always Present
“Friction is the quiet force that resists slipping, stops moving objects, and sometimes makes motion possible.”
• Explain how friction affects stationary and moving objects. • Draw and interpret the forces acting on a box on a horizontal surface. • Relate surface nature to friction and stopping distance. • Use a spring balance to estimate friction. • Predict motion in an ideal frictionless situation.
Push a heavy cupboard gently and it may refuse to move. Push harder and it suddenly begins sliding. Stop pushing, and after some distance it comes to rest. The floor has not been passive during any of these stages. At the contact between the two surfaces, a force develops that opposes their tendency to slide. This force is friction.
Force of friction is a contact force that opposes relative motion, or the tendency of relative motion, between surfaces in contact.
Friction Before Motion Begins
When a stationary box is pushed weakly, friction can match the applied force in the opposite direction. The net horizontal force is then zero and the box remains at rest. Increasing the push allows friction to increase up to a limit. Once the applied force becomes greater than the greatest opposing friction available in that situation, a forward net force acts and the box begins to move.
This explains why an applied force does not always produce visible motion. The conclusion “a force was applied, therefore the object must move” ignores other forces. Motion changes only when the net force is non-zero.
Forces On A Box
Four forces are commonly shown for a pushed box on a horizontal floor. The applied force points in the push direction. Friction points opposite the actual or attempted sliding. Weight acts vertically downwards. The normal force from the floor acts vertically upwards, perpendicular to the surface. When there is no vertical acceleration, normal force balances weight.
Normal force is the contact force exerted by a surface perpendicular to itself.
Friction does not simply point opposite to every applied force. It opposes relative sliding or the tendency to slide at the contact. Later, while studying walking, friction will point forwards on the foot because the foot tends to slip backwards.
Why Moving Objects Slow Down
A rolling ball or gliding box continues for a while after the push ends because it already has velocity. Once the applied force disappears, friction may remain as the only unbalanced horizontal force. Since friction points opposite the motion, it produces acceleration opposite to the velocity. The speed decreases until the object stops.
Everyday experience once encouraged the mistaken belief that a continuous force is needed for continuous motion. What is actually needed in many everyday situations is a continuous applied force to counter friction. If the applied force exactly balances friction, the net force is zero and the object can continue with constant velocity.
Activity: Comparing Surfaces
Secure identical coins into a stack and use the same stretched rubber band to launch the stack across several level surfaces. Keep the launch arrangement and stretch the same each time. Measure the distance from the release point to where the stack stops and repeat each trial. A longer stopping distance indicates that the speed decreased more slowly, suggesting a smaller opposing frictional force.
A fair comparison changes only the surface. The coin stack, starting arrangement, stretch of the rubber band, and method of release should remain the same. Repetition reduces the influence of a single unusual trial. A table of surface type and repeated stopping distances makes the pattern easier to see.
Activity: Measuring Friction
Attach a wooden block to a spring balance and pull horizontally with gradually increasing force. Record the reading when the block just begins to move. Repeat on the surfaces compared earlier. The reading provides an approximate measure of the friction that had to be overcome. A larger reading indicates greater friction in that setup.
If the block is already moving at constant velocity during a steady pull, its acceleration is zero and the net horizontal force is zero. The pulling force then balances the frictional force in magnitude. The spring balance reading can therefore be used as an estimate of friction while the block is moving steadily.
Surface Nature And Stopping Distance
Roughness at the contact is microscopic. Even a surface that looks polished has tiny irregularities. Different pairs of surfaces interact differently, so friction is not a property of one surface alone. In the coin investigation, the surface producing greater friction removes the motion more quickly, giving a shorter stopping distance. The surface producing smaller friction allows motion to continue farther.
Problem
A block is pulled horizontally at constant velocity. The spring balance reads 8 N. What is the frictional force?
- 1.Constant velocity means acceleration is zero.
- 2.Zero acceleration requires zero net horizontal force.
- 3.The pull is 8 N in the direction of motion.
- 4.Friction must balance it with equal magnitude in the opposite direction.
- 5.The frictional force is 8 N opposite to the motion.
Problem
A student pushes a stationary box rightward with 45 N. Friction can oppose with 50 N before sliding begins. What happens?
- 1.Compare the applied force with the available opposing friction.
- 2.The required balancing friction is only 45 N, which is within the 50 N limit.
- 3.Friction matches the push with 45 N leftward.
- 4.The net horizontal force is zero.
- 5.The box remains at rest.
A World Without Friction
Imagine a horizontal surface on which friction is exactly zero. Give an object a brief push and then remove your hand. After contact ends, no horizontal force remains. Its velocity would not gradually decrease; it would continue in a straight line with constant speed. This thought experiment isolates the effect normally hidden by friction and prepares the way for the first law of motion.
Air Resistance
Objects moving through air also experience resistance. For a compact object moving slowly, air resistance may be small enough to ignore in a simple model. For a fast vehicle, a falling sheet of paper, or a parachute, it can be important. Saying that a force is neglected means that its effect is small for the question being studied, not that the force has vanished from reality.
Quiz
Which description best matches Force Of Friction?
Which description best matches Normal Force?
Which term matches this description: Force of friction is a contact force that opposes relative motion, or the tendency of relative motion, between surfaces in contact.
Which term matches this description: Normal force is the contact force exerted by a surface perpendicular to itself.
Which statement is a key takeaway from this lesson?
Practice Problems
- Explain why a bicycle slows after the rider stops pedalling.
- Describe what the spring balance reads when a block moves with constant velocity.
- Predict which of two surfaces has greater friction if the same launched object stops sooner on one surface.
- Draw the four main forces on a box pushed along a level floor.
Key Takeaways
• Friction opposes relative sliding or its tendency. • A push may be balanced by friction before motion begins. • Friction opposite to motion slows an object after the push ends. • Weight and normal force can balance vertically. • Smaller friction generally produces a longer stopping distance. • Without friction, an object already moving would not slow down on its own.
The frictionless thought experiment leads directly to a law describing motion when the net force is zero.