5.4 Resultant forces
9 exam questions worth 21 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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A box is pushed to the right with a force of 25 N. Friction of 10 N acts to the left. Calculate the resultant force on the box. Include its direction.
Mark scheme
25 − 10 (1); 15 N to the right (1)
Model answer
Resultant = 25 − 10 = 15 N to the right.
Three forces act on a trolley: 60 N and 20 N to the right and 45 N to the left. Calculate the resultant force.
Mark scheme
(60 + 20) − 45 (1); 35 N (1); to the right (1)
Model answer
Resultant = (60 + 20) − 45 = 35 N to the right.
A car travels along a straight road at a constant speed of 20 m/s. What is the resultant force on the car?
Choose one answer.
Correct Not quite: the answer is Zero
Mark scheme
C: Zero
Model answer
Zero (C).
A book rests on a table. Explain why the resultant force on the book is zero.
Mark scheme
weight (downwards) is equal to the normal contact force (upwards) (1); the forces are equal and opposite / balance each other (1)
Model answer
The weight of the book downwards is equal in size to the normal contact force upwards, so the forces are balanced and the resultant force is zero.
Describe the free body diagram for a parachutist falling at a steady speed.
Mark scheme
two arrows (1); weight downwards (1); air resistance upwards, of the same length / size as the weight (1)
Model answer
There are two arrows, weight downwards and air resistance upwards. They are the same length, since the forces are balanced.
A car's driving force is 2500 N. The resistive forces are 1800 N. Describe what happens to the motion of the car and explain why.
Mark scheme
the resultant force is 700 N forwards (1); the forces are unbalanced (1); so the car accelerates / speeds up (1)
Model answer
The resultant force is 2500 − 1800 = 700 N forwards. The forces are unbalanced, so the car accelerates.
A cyclist stops pedalling on a flat road and slows down. Explain why, using ideas about forces.
Mark scheme
there is no driving force (1); friction and air resistance still act backwards (1); so there is a resultant force opposite to the motion, which makes the cyclist decelerate (1)
Model answer
With no driving force, the friction and air resistance acting backwards give a resultant force opposite to the motion. The cyclist decelerates.
A student says that a resultant force is needed to keep an object moving at a constant speed. Explain why the student is wrong.
Mark scheme
a resultant force changes the velocity / causes acceleration (1); an object moves at constant velocity when the resultant force is zero (1)
Model answer
A resultant force changes the velocity. An object moving at constant velocity has a resultant force of zero, so the forces are balanced.
The arrows on a free body diagram must have lengths that are in proportion to the forces. Explain why this is useful.
Mark scheme
it shows the relative sizes of the forces (1); so you can see whether the forces are balanced / the direction of the resultant (1)
Model answer
It shows how big each force is compared with the others, so you can see if the forces are balanced and which way the resultant acts.
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