Friction: Test Answers #
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This happens because static friction is greater than kinetic (sliding) friction. When the box is stationary, static friction must be overcome to start the movement. Once the box is moving, only the smaller kinetic friction needs to be overcome to maintain the motion. This is why it always takes more force to start pushing an object than to keep it moving.
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The two types of friction acting on the bicycle are:
- Rolling friction between the wheels and the road – this creates resistance as the wheels roll along the surface
- Air resistance (fluid friction) acting against the cyclist and bicycle’s motion through the air – this increases with speed and depends on the cyclist’s position and shape
Both these forces act in the opposite direction to the motion, converting the bicycle’s kinetic energy into heat energy, eventually bringing it to a stop.
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The brakes don’t work as well in rainy conditions because water acts as a lubricant between the brake pads and the wheels, reducing the friction between these surfaces. Additionally, the water creates a thin film between the tires and the road surface, reducing the grip. This reduction in friction means the braking force is decreased, making it harder to stop the car quickly and safely.
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Using rollers converts sliding friction into rolling friction. Rolling friction is typically much less than sliding friction because the surfaces are not sliding directly against each other. Instead, the rolling motion means that at any instant, only a small point of the roller is in contact with the ground, and the rolling action reduces the resistance to motion. This is why less force is needed to move the crates.
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The student’s statement is incorrect. For solid friction, the surface area does not affect the amount of friction between two surfaces. This might seem counterintuitive, but it’s because when the surface area increases, the pressure decreases proportionally, so the overall friction force remains the same. The factors that do affect friction are:
- The types of surfaces in contact (roughness)
- The normal force (weight) pressing the surfaces together
Note: This only applies to solid friction. In fluid friction (like air resistance), surface area does affect the amount of friction.
