Practise › Questions › Newton's laws and the resultant force
Newton's laws and the resultant force questions
One theme runs through all three laws. Forces change motion, they never sustain it, and only the resultant counts. Add the free-body diagram habit and the third law's real meaning, and the rest of the topic follows.
19 original questions · 60 marks · the newton's laws and the resultant force notes · Mechanics
Every question here is written for this library rather than taken from a past paper. Write your answer out before opening a mark scheme: the schemes award marks point by point, and the marks are easier to see when you have something of your own to compare against.
State Newton's first law of motion.
Mark scheme
An object stays at rest, or continues at constant velocity (1), unless acted on by a resultant external force (1).A mass of 2.0 kg has an acceleration of 3.0 m s−2. Calculate the resultant force acting on it.
Mark scheme
F = ma = 2.0 × 3.0 (1)
F = 6 N (1)Explain what is meant by the resultant force on an object.
Mark scheme
The resultant force is the single force equivalent to all the individual forces acting on the object (1); it is their vector sum (1).A delivery van travels along a straight, level road at a constant velocity of 20 m s−1. State the magnitude of the resultant force acting on it.
Mark scheme
Zero: at constant velocity there is no resultant force (Newton's first law). (1)A model rocket has a mass of 250 g. At one instant during its flight its acceleration is 12 m s−2. Calculate the resultant force on the rocket at this instant.
Mark scheme
Mass = 0.250 kg (1). F = ma = 0.250 × 12 = 3.0 N. (1)State Newton's third law of motion.
Mark scheme
When body A exerts a force on body B, body B exerts a force on body A that is equal in magnitude and opposite in direction (1); the two forces are of the same type and act on different bodies. (1)A block of mass 4.0 kg is pushed to the right by a horizontal force of 12 N while a frictional force of 4.0 N acts to the left. Calculate the resultant force on the block and its acceleration.
Mark scheme
Resultant = 12 − 4.0 = 8 N to the right (1)
a = F/m = 8/4.0 (1)
a = 2.0 m s−2 (1)A person of mass 70 kg stands in a lift. Taking g = 9.81 m s−2, calculate (a) their weight and (b) the force the lift floor exerts on them when the lift accelerates upwards at 2.0 m s−2.
Mark scheme
(a) W = mg = 70 × 9.81 (1)
W = 686.7 N (1)
(b) N − mg = ma, so N = m(g + a) = 70 × (9.81 + 2.0) (1)
N = 826.7 N (1)A 3.0 kg block and a 2.0 kg block are in contact on a smooth surface. A horizontal force of 10 N pushes the 3.0 kg block, which pushes the 2.0 kg block. Calculate (a) the acceleration of the blocks and (b) the contact force between them.
A book rests on a table. Identify the Newton's third-law pair to the force of the table pushing up on the book, and explain why the book's weight is not that pair.
A crane raises a 350 kg load on a vertical cable. The tension in the cable is 3.65 kN. Taking g = 9.81 m s−2, calculate the acceleration of the load.
An ice hockey puck of mass 160 g slides in a straight line across level ice. Its speed falls from 18 m s−1 to 12 m s−1 in 4.0 s. Calculate the average frictional force acting on the puck.
Explain, using Newton's third law, how a swimmer is able to accelerate forwards through the water.
A 2.0 kg block slides down a smooth slope inclined at 30° to the horizontal. Taking g = 9.81 m s−2, calculate its acceleration down the slope.
A 3.0 kg mass hangs from a light string that passes over a smooth pulley and connects to a 2.0 kg mass on a smooth horizontal table. Taking g = 9.81 m s−2, calculate (a) the acceleration of the system and (b) the tension in the string.
A person stands at rest on horizontal ground. State the Newton's third-law pair to the force the person exerts on the ground, and explain why the person remains in equilibrium.
A packing crate of mass 12 kg rests on a ramp inclined at 25° to the horizontal. The maximum frictional force the ramp can exert on the crate is 55 N. Taking g = 9.81 m s−2, deduce whether the crate remains at rest on the ramp.
A student of mass 62.0 kg stands on scales, calibrated in kilograms, on the floor of a lift. While the lift is moving, the scales read 56.5 kg. Taking g = 9.81 m s−2, determine the magnitude and direction of the lift's acceleration, and state one possible motion of the lift at this moment.
A student says: “When a horse pulls a cart, Newton's third law means the cart pulls back on the horse just as hard, so the forces cancel and the pair can never accelerate.” Explain fully why the student's conclusion is wrong. Refer to the forces acting on the cart, the forces acting on the horse, and Newton's laws.
The same practice on paper: the printable workbook for this topic, questions and a worked answer book.
Practise newton's laws and the resultant force one question at a time
The player marks nothing for you. It shows one question, waits, then shows the scheme so you can mark yourself, and brings a question back sooner when it went badly.