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4 Forces
1 a) The resultant force acting on the system = 7000 N − (1000 + 1000) N =
5000 N.
Using Newton’s second law: a = F __
m = _______ 5000 N
4000 kg = 1.25 m s−2 – the answer
is A.
b) Resultant force on the trailer = (T − 1000) N = 2500 kg × 1.00 m s −2
T = 2500 N + 1000 N = 3500 N – the answer is B.
2 a) The weight is a downward gravitational force of the Earth on the man.
By Newton’s third law there must be an equal upward gravitational force
of the man on the Earth – the answer is C.
b) Only two forces act on the man; his weight pulling him down, and the
upward push of the table on his feet. For the man to be in equilibrium
these must be equal and opposite – the answer is B.
3 Examples of distant forces are: gravitational, electrostatic and magnetic
(electromagnetic) and nuclear.
Examples of contact forces are: friction, air resistance and normal reaction
forces.
4 Examples of gravitational forces are: sun and planets, satellites, weight of
objects on Earth.
Examples of electromagnetic forces are: motors, electrons orbiting the
nucleus, forces on static electrical charges.
Examples of nuclear forces are: strong forces binding protons and neutrons,
weak forces involved in beta decay.
5 A body is in equilibrium if the vector sum of all the forces acting on it is zero.
6 b) Weight
c) i) F = (4.0 kN) cos 15 = 3.86 kN ≈ 3.9 kN
ii) R + (4.0 kN) sin 15 = 800 kg × 9.8 m s −2 ⇒ R = 6.81 kN ≈ 7 kN
5 Work, energy and power
1 Work done by each force = F s cos θ = 100 N × 100 m × cos 60 = 5000 J
Total work done by both forces = 10 000 J – the answer is C.
2 At the lowest point the jumper is stationary, and the kinetic energy must
be zero.
The elastic cord will have elastic strain energy stored within it, but some
of the initial gravitational potential energy will have been converted
to internal energy as the molecules are displaced within the cord – the
answer is B.
3 Power = _________ work done mgh
time
= ____
t = 1000 kg × 9.8 m s−2 ______________________ × 2.4 m
= 2940 W ≈
8.0 s
3000 W – the answer is C.
4 Useful power output = 75% of 120 W = 90 W
P = F × v ⇒ F = P __
v = _______ 90 W
2.0 m s−1 = 45 N – the answer is A.
5 Work is the product of the force and the distance moved in the direction
of the applied force. Work is measured in joules (J).
6 a) W = F∆x = 60 N × 100 m = 6000 J
b) W = F∆x cos θ = 80 N × 50 m cos 30 = 3500 J (to 2 significant figures)
7 a) Potential energy is the ability of a body to do work by virtue of its
position or state.
Full Answers to Review Questions: 4 Forces
Edexcel Physics for AS © Hodder Education 2009
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