QUARTER FINAL STAGE 2024
YILO KROBO
KOFORIDUA SENIOR HIGH TECH
KNUST SHS
ROUND 4 - TRUE OR FALSE
FIRST SET OF QUESTIONS
PREAMBLE
1. The rate of change of linear momentum with respect to time equals force.
ANSWER: FALSE
EXPLANATION:
An object can move at a constant speed along a rough surface if an applied force balances the kinetic friction force exactly.
For example, a car driving down a highway at a constant speed still experiences rolling resistance and air friction.
QUESTION
Statements about the friction force on an object.
2. The object moves at constant velocity
ANSWER: FALSE
EXPLANATION:
According to Newton's first law, a constant velocity requires the net force to be zero, not individual forces.
A block sliding down an incline at a steady velocity has a kinetic friction force balancing the component of gravity pulling it down the slope.
QUESTION
Statements about the friction force on an object.
3. The net force on the object is zero
ANSWER: FALSE
EXPLANATION:
A net force of zero implies that all forces acting on the body are in equilibrium.
A heavy box resting on a floor with a horizontal push applied to it remains stationary because static friction acts to balance out that push exactly.
NOTE: Statement 1 repeats a momentum statement by mistake; its explanation is for “The object moves at constant speed.”
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PRACTICE QUESTIONS
1. QUESTION: The friction force on a block at rest on a level floor with no applied horizontal force is zero.
ANSWER: TRUE
EXPLANATION:
Nothing tends to make it slide, so no friction is needed.
2. QUESTION: The friction force on a block at rest on a rough incline is zero.
ANSWER: FALSE
EXPLANATION:
Static friction balances the component of weight down the slope.
3. QUESTION: Kinetic friction on a sliding block is zero if the block moves at constant speed.
ANSWER: FALSE
EXPLANATION:
It is balanced by another force, not zero.
4. QUESTION: Friction on a surface is zero when the surface is perfectly smooth.
ANSWER: TRUE
EXPLANATION:
A smooth surface is one with no friction.
5. QUESTION: Static friction always equals its limiting value.
ANSWER: FALSE
EXPLANATION:
It is only as large as needed, up to the limiting value $\mu_sR$.
6. QUESTION: Pushing a heavy box that does not move means static friction equals the push.
ANSWER: TRUE
EXPLANATION:
The box is in equilibrium.
7. QUESTION: Friction always opposes relative motion or the tendency to relative motion.
ANSWER: TRUE
EXPLANATION:
It acts along the surface against sliding.
8. QUESTION: Friction on a car's driving wheels acts backwards.
ANSWER: FALSE
EXPLANATION:
It acts forwards, pushing the car along.
9. QUESTION: The limiting friction between two surfaces is proportional to the normal reaction.
ANSWER: TRUE
EXPLANATION:
$F = \mu R$.