Forces and Motion: Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Forces and Motion revision notes

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Section A

1. State the difference between a scalar and a vector, and classify: force, mass, weight, speed, acceleration. [4]

2. State Newton’s first law and give one everyday example. [2]

Section B

3. A car of mass 900 kg experiences a driving force of 3200 N and a total resistive force of 800 N.

(a) Calculate the resultant force. [1] (b) Calculate the acceleration. [2] (c) Describe what happens as the car speeds up, if the driving force stays constant. [3]

4. Explain the difference between mass and weight, and calculate the weight of a 6.0 kg object on Earth (g = 9.8 N/kg) and on the Moon (g = 1.6 N/kg). [5]

5. A box is pushed with a horizontal force of 45 N and moves at constant velocity.

(a) State the size of the friction force and justify your answer. [2] (b) Draw or describe a free-body diagram showing all four forces on the box. [3] (c) Explain what happens if the push increases to 60 N. [2]

6. Explain Newton’s third law, and explain why the two forces in a third-law pair never cancel each other out. [4]

7. Explain the difference between thinking distance, braking distance and stopping distance, and state one factor affecting each of the first two. [5]

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Section C

8. A skydiver jumps from a plane and falls, accelerating from rest.

(a) Explain, in terms of the forces acting, why the skydiver’s acceleration decreases as they speed up. [3]

(b) State the name given to the constant speed eventually reached, and explain why speed remains constant after this point. [2]

9. An object moves in a circular path at constant speed, held in the circle by a string.

(a) State the direction of the resultant force acting on the object. [1]

(b) The force applied increases, while the object’s mass and the radius of the circle stay the same. State the effect, if any, on its speed. [1]

(c) The object’s mass increases, while its speed and the radius of the circle stay the same. State what must happen to the force required. [1]

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Answers

1. A scalar has magnitude only; a vector has both magnitude and direction [1]. Vectors: force, weight, acceleration [1] [1]. Scalars: mass, speed [1].

2. An object remains at rest or moving at constant velocity unless acted on by a resultant force [1]. Example: a passenger continues forward when a bus brakes suddenly, because no force has acted on them to slow them [1].

3. (a) 3200 − 800 = 2400 N [1]. (b) a = F ÷ m = 2400 ÷ 900 [1] = 2.7 m/s² [1]. (c) As speed increases, air resistance increases [1], so the resultant force decreases and the acceleration falls [1]; when the resistive force equals the driving force the resultant is zero and the car travels at a constant top speed [1].

4. Mass is the amount of matter in an object, measured in kilograms, and is the same everywhere [1]. Weight is the force of gravity acting on that mass, measured in newtons, and varies with gravitational field strength [1]. On Earth: W = 6.0 × 9.8 = 58.8 N [1] [1]. On the Moon: W = 6.0 × 1.6 = 9.6 N [1].

5. (a) 45 N [1]; the box moves at constant velocity, so the resultant force is zero and friction must exactly balance the push [1]. (b) Weight downwards from the centre of the box [1]; normal contact force upwards, equal in size [1]; push of 45 N to the right and friction of 45 N to the left, all arrows labelled and drawn to a consistent scale [1]. (c) There is now a resultant force of 15 N in the direction of the push [1], so the box accelerates rather than moving at constant velocity [1].

6. If object A exerts a force on object B, then B exerts an equal and opposite force on A, of the same type [1] [1]. The forces never cancel because they act on two different objects [1]; forces only combine into a resultant when they act on the same object [1].

7. Thinking distance is the distance travelled during the driver’s reaction time, before the brakes are applied [1]. Braking distance is the distance travelled while the brakes are decelerating the vehicle [1]. Stopping distance is the sum of the two [1]. Factor affecting thinking distance: tiredness, alcohol, drugs or distraction, all of which lengthen reaction time [1]. Factor affecting braking distance: wet or icy roads, worn tyres or brakes, or a heavier load [1].

8. (a) As speed increases, air resistance (drag) increases [1]; weight stays constant, so the resultant force decreases [1], and since a = F ÷ m, acceleration decreases [1].

(b) Terminal velocity [1]. Drag has increased until it exactly balances weight, so the resultant force is zero and, by Newton’s first law, speed remains constant [1].

9. (a) Towards the centre of the circle [1].

(b) Speed increases [1].

(c) The force required must increase [1].

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Where marks are usually lost

• Calling weight a scalar.
• Saying a constant driving force gives constant acceleration when air resistance is present.
• Drawing a free-body diagram with forces acting on more than one object.
• Saying speed affects thinking distance but not braking distance — it affects both.
• Describing terminal velocity as the point where drag “overtakes” weight — at terminal velocity the two forces are exactly balanced, not one exceeding the other.
• Quoting F = mv²/r for circular motion at this level — the syllabus requires only the three qualitative force/speed/radius/mass relationships, not the equation itself.