115 practice questions covering Exploring Forces, organised into 0 topics.
115 questions include a written explanation.
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Sample questions
Q1
medium
A brick weighs $30\ \text{N}$ in air. When it is fully immersed in water, a spring balance holding it reads $22\ \text{N}$. The apparent loss in weight of the brick is
A$8\ \text{N}$
B$22\ \text{N}$
C$30\ \text{N}$
D$52\ \text{N}$
Show answer and explanation
Correct answer: A - $8\ \text{N}$
The apparent loss is the drop in the reading, that is $30 - 22 = 8\ \text{N}$. Adding the two readings to get $52\ \text{N}$ has no physical meaning. The values $22\ \text{N}$ and $30\ \text{N}$ are the readings in water and in air, not the loss.
Q2
hard
A box is dragged along a floor by a rope pulling with $60\ \text{N}$ towards the right. Friction opposes the sliding with $22\ \text{N}$. A second rope now also pulls the box with $12\ \text{N}$ towards the left. The net force on the box is
A$50\ \text{N}$ towards the right
B$26\ \text{N}$ towards the right
C$38\ \text{N}$ towards the right
D$26\ \text{N}$ towards the left
Show answer and explanation
Correct answer: B - $26\ \text{N}$ towards the right
Both friction and the second rope act leftwards, so together they oppose the first rope with $22 + 12 = 34\ \text{N}$. The net force is $60 - 34 = 26\ \text{N}$ towards the right, the side of the stronger pull. Leaving out the second rope gives $38\ \text{N}$, leaving out friction gives $48\ \text{N}$, and $50\ \text{N}$ or a leftward answer cannot follow from these numbers.
Q3
hard
Assertion (A): Water exerts no upward force on an object that sinks to the bottom. Reason (R): A sinking object still pushes aside some water as it goes down.
ABoth A and R are true, and R is the correct explanation of A
BBoth A and R are true, but R is not the correct explanation of A
CA is true, but R is false
DA is false, but R is true
Show answer and explanation
Correct answer: D - A is false, but R is true
Water pushes up on every object placed in it, including one that sinks, so A is false; the object sinks only because its weight is larger than that upward push. A sinking object does push water aside, so R is true, and this is exactly why an upward force still acts on it.
Q4
medium
The same wooden block is dragged first over a smooth glass sheet and then over a sheet of sandpaper of the same size. The friction on the block is
Azero on the sandpaper
Bthe same on both, since the block is the same
Cgreater on the glass
Dgreater on the sandpaper
Show answer and explanation
Correct answer: D - greater on the sandpaper
Sandpaper is far rougher than glass, so its surface grips the block much more strongly and friction is greater there. Glass is smooth, which is why sliding on it is easy. Friction depends on the surfaces as well as on the block, so it is not the same on both, and it is certainly not zero on sandpaper.
Q5
easy
A tin is sealed with nothing but air inside and placed gently on water. It will most likely
Afloat on the surface
Bhang in the middle of the water without moving
Csink straight to the bottom
Ddissolve in the water
Show answer and explanation
Correct answer: A - float on the surface
The tin is very light for the space it takes up, so the water's upward push easily matches its weight and it floats. It would sink only if it were heavy for its size, for example if it were filled with sand. Tin does not dissolve in water, and an object does not hang still in mid-water in this simple case.
Q6
easy
Which of these is the clearest example of a muscular force?
AA magnet drawing a pin across a table
BA farmer lifting a sack of grain onto his shoulder
CAn apple falling from a branch
DA comb picking up bits of paper after being rubbed on dry hair
Show answer and explanation
Correct answer: B - A farmer lifting a sack of grain onto his shoulder
Muscular force is the force our muscles apply by touching an object, and lifting a sack is exactly that. The falling apple is pulled by gravity, the comb picks up paper by an electrostatic force, and the magnet uses a magnetic force. None of those three needs any muscles.
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