Physics Solution

PHYSICS SOLUTION
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1. a) State Isaac Newton’s three laws of motion
Law 1
It states that a body is in its state of rest or uniform motion unless an unbalanced force acts on it.
Law 2
It states that the rate of change of momentum of a body is proportional to the applied force and
takes place in the direction in which the force acts.
Law 3
It states that for every action or force, there is an equal and opposite force or reaction.
2. a) What is the formula for momentum and what are the units for momentum?
Momentum= velocity*mass.
Momentum=vm ; where v is the velocity and m is the mass
The SI units for momentum is kgm/s
b) What is meant by conservation of momentum?
Conservation of linear momentum refers to a situation when the total momentum of the
system stays constant when no outside forces act on a system of moving objects.
c) A gas molecule of mass 5*10- 2 6 kg travelling at 320m/s strikes the side of a
container perpendicularly. The collision is elastic. What is meant by elastic
collision?
Elastic collision occurs when bodies collide and bounce off each other after
collision. In elastic collision, both momentum and kinetic energy are conserved.
d) Use the information in part c to calculate the change in momentum of the
molecule during its collision with the side of the container.
Change in momentum= mv-mu
= (0)-(5*10 - 2 6 kg*320m/s) kgm/s
=-1.6*10 - 2 3 kgm/s
3. A racing car moving at 25m/s collides with a wall . The driver of mass 65kg is
brought to rest by its seatbelt in 0.2s. Find:
a) The driver’s change in momentum
Change in momentum=mv-mu; where v is the final velocity, u is initial velocity
=Since the driver is brought to rest,
v=0m/s and u=25m/s
= (0m/s*65kg)-(25m/s*65kg)
= (0-1625)kgm/s
=-1625kgm/s

b) The average force exerted by the seatbelt on the driver?
F=ma
a = v
t
= 25 m/s
0.2 s
=125ms- 2
Since it was a deceleration, a=-125ms - 2
F=ma
=-125ms- 2 * 65Kg
=-8125N. The negative sign illustrates it was a retardation.
c) How many times is this force greater than the driver’s weight?
Weight=mg
g=10N/kg
Driver’s weight= 65kg *10N/kg
=650N
Comparing with retardation force
= 8125
650
=12.5 times greater
4. a) i. What is happening to the speed of the ball bearing between points A and B?
The speed is increasing uniformly from point A to B.
ii. Explain in terms of forces X and Y why this happens.
Here, force X acts on the ball bearing uniformly i.e. the magnitude of force X is
higher than that of Y between these points and thus X acts on the ball bearing uniformly
pulling it down at an increasing speed.
iii. What is happening to the speed of the ball between C and D?
Between point C and D, the speed of the ball is constant.
iv) Explain in terms of forces X and Y why this happens.
The speed between C and D is constant because the magnitude of both forces X and Y are
constant.

b) Use the graph to help you calculate the distance travelled by the ball bearing between
points C and D.
There was no change in velocity between the two points and thus V is constant
V= 2.4cm/s
Time taken, t= (4-2) s
=2s
Distance=V * t
=2.4cm/s*2s
=4.8cm
5. a) Define the term resultant force?
A resultant force is a force obtained by combining all the magnitudes of the forces acting on a
body taking into consideration their directions to obtain a single force.
b) Two force vectors each have magnitude of 12 newton. When they are added together
which of the following magnitudes of the resultant vector could not be produced? Circle
your answer
The answer is 30N
c) Sketch 3 vector diagrams below to show how the other three resultant vectors could
be produced.
0N
12N 24N
d) Find the resultant magnitude and angle from the horizontal of a Force 7 N East
followed by a force of 9 N North
Magnitude=√ (F12 + F22)
=√ (92 + 72)
=√ (81+49)
=√130
=11.4 N
Angle =tan-1 Fy
Fx
=tan-1( 9
7 )
=tan-1 1.286
=52.13o