133 practice questions covering How Forces Affect Motion, organised into 1 topics.
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One rope pulls a sledge northward with a force of $25\ \text{N}$. A second rope pulls the same sledge eastward with a force of $25\ \text{N}$. Comparing the two pulls, which quantity is the same and which is different?
ABoth the magnitudes and the directions are different
BThe magnitudes are the same but the directions are different
CBoth the magnitudes and the directions are the same
DThe directions are the same but the magnitudes are different
Show answer and explanation
Correct answer: B - The magnitudes are the same but the directions are different
Both pulls measure $25\ \text{N}$, so their magnitudes match, but north and east are different directions. Saying the directions match ignores the words north and east in the description. Saying both quantities match would make the two pulls identical, which they are not, and saying both differ contradicts the equal readings.
Q2
medium
Why does a standing passenger lurch when a bus suddenly changes its speed?
AThe bus presses the air inside it against the passenger's body, and this push of the air throws the passenger forwards or backwards
BThe floor of the bus changes the motion of the passenger's feet at once, while the rest of the body tends to keep its earlier state of motion
CThe passenger's weight begins to act sideways instead of downward while the bus speeds up or slows down, and this sideways pull tips the body over
DThe passenger's mass suddenly increases while the bus is changing speed
Show answer and explanation
Correct answer: B - The floor of the bus changes the motion of the passenger's feet at once, while the rest of the body tends to keep its earlier state of motion
Friction from the floor acts directly on the feet and changes their motion immediately, but the upper body is only dragged along afterwards, so for a moment the two parts are moving differently and the passenger lurches. Air inside the bus is far too light to throw a person about. Mass is fixed, and weight always acts vertically downward, never sideways.
Q3
medium
An object of mass $3\ \text{kg}$ has a momentum of $12\ \text{kg m s}^{-1}$ as it moves along a straight line. How fast is it moving?
A$0.25\ \text{m s}^{-1}$
B$4\ \text{m s}^{-1}$
C$9\ \text{m s}^{-1}$
D$36\ \text{m s}^{-1}$
Show answer and explanation
Correct answer: B - $4\ \text{m s}^{-1}$
Since momentum is mass multiplied by velocity, the velocity is $12 \div 3 = 4\ \text{m s}^{-1}$. Multiplying instead of dividing gives $36$, and dividing the wrong way round gives $0.25$. Subtracting the mass from the momentum gives $9$, which mixes two quantities that are not even measured in the same unit.
Q4
medium
A flower pot stands at rest on a window sill. Which two forces form an action-reaction pair?
AThe weight of the pot and the weight of the sill
BThe downward push of the pot on the sill and the upward push of the sill on the pot
CThe downward push of the pot on the sill and the weight of the pot, since both act downward
DThe weight of the pot and the upward push of the sill on the pot, which cancel each other
Show answer and explanation
Correct answer: B - The downward push of the pot on the sill and the upward push of the sill on the pot
An action-reaction pair needs the two objects to act on each other, and here the pot presses down on the sill while the sill presses up on the pot. The weight of the pot and the upward push of the sill both act on the pot itself, so they are balanced forces on one object rather than a pair. The weight of the sill is a force from the Earth on the sill, and pairing two forces that both point downward is impossible.
Q5
medium
Two trolleys, one of mass $3\ \text{kg}$ and one of mass $5\ \text{kg}$, are joined by a light rigid rod and stand on a smooth level floor. A horizontal force of $16\ \text{N}$ is applied to the pair. What is their acceleration?
A$3.2\ \text{m s}^{-2}$
B$5.33\ \text{m s}^{-2}$
C$8\ \text{m s}^{-2}$
D$2\ \text{m s}^{-2}$
Show answer and explanation
Correct answer: D - $2\ \text{m s}^{-2}$
The rod makes the two trolleys move as one object of mass $3 + 5 = 8\ \text{kg}$, so the acceleration is $16 \div 8 = 2\ \text{m s}^{-2}$. Dividing by only one of the masses gives $3.2$ or $5.33$, which would be right only if that trolley were on its own. The value $8$ is the total mass in kilograms and is not an acceleration at all.
Q6
hard
A heavy truck runs head-on into a small parked car. Which statement about the forces during the collision is correct?
AThe two vehicles push on each other with forces of equal size, but the car's motion changes far more because its mass is smaller
BThe truck pushes on the car with a larger force than the car pushes on the truck, because the truck is heavier
CThe truck pushes on the car with a larger force, because it is the one that is moving
DThe car pushes on the truck with a larger force, because it is the lighter of the two
Show answer and explanation
Correct answer: A - The two vehicles push on each other with forces of equal size, but the car's motion changes far more because its mass is smaller
During the collision the two vehicles form an action-reaction pair, so the forces on them are equal in size and opposite in direction. Neither mass nor speed makes one force of a pair bigger than the other. The car is wrecked and thrown about far more only because the same force produces a much larger change in motion in a much smaller mass.
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