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GCSE Questions - Forces

Q18.

(a) Objects can interact in many ways. Pairs of forces arise when objects interact. Write down one type of force involved when objects interact.

Any one from:

electrostatics or static electricity (but not just 'static')

gravity/weight

magnetism

(normal) contact force or reaction force

friction or drag or air resistance

[1 mark]

(b) A book rests on a table. Draw a free body force diagram to show the forces acting on the book. Use arrows to represent the forces.

Weight arrows are always drawn from the centre of mass/gravity of an object - vertically downwards.

Reaction force arrows always act from the point of contact of the object with a surface - at right angles to that surface and moving away from it.

 

But all the exam board expected from you was a free body diagram that correctly indicated the size and direction of the forces - not the point where they originated from - as that is awkward as the reaction force and weight arrows cross over - making correct size difficult!

So they wanted something simple - like this...

 

Two arrows drawn of equal length

One up and one down

The downwards arrow labelled weight/gravity

The upwards arrow labelled contact/reaction force

[4 marks]

(c) A teacher uses an air-track for motion experiments. Using the air-track means that there is no friction between the glider and the air-track.

The teacher places the glider on the horizontal air-track and gives it a small push to start it moving. Describe and explain the motion of the glider.

Any two from:

The glider accelerates during the push.

After release it has constant/steady/uniform speed/velocity.

The speed is steady because there are negligent external (unbalanced) forces acting on it.

There is, however very slight deceleration due to air resistance.

[2 marks]

(d) A presenter on a car TV programme says:

"The car maker has reduced the mass of this car so it now has better acceleration."

(i) Explain why the presenter is correct.

(According to Newton's Second Law) F = ma (force equals mass times acceleration) so reducing the mass means the same engine force will cause a greater acceleration

[2 marks]

(ii) A car accelerates from 5m/s to 25m/s in 4 seconds.

Calculate the acceleration of the car.

acceleration = change in speed/time

a = Δv/t

Δv = 25 - 5 = 20 m/s

a = 20 รท 4 m/s2

a = 5 m/s2

[3 marks]

(Total 12 marks)