CHAPTER 4 ISAAC NEWTON ON THE ROAD
Name:
QUESTIONS
4.1 Isaac Newton and the Laws of Motion
1.
What will be the normal reaction force exerted on a 4.00 kg block by the table on which it
rests?
2.
Calculate the normal reaction force acting on a 1.20 kg book which is lying on a bench if
you
(a)
press on it with a force of 20.0 N or
(b)
pull upwards on it with a force of 4.0 N.
3.
An 8.0 kg wooden sled is pulled over the snow by means of a rope that makes an angle of
40° with the horizontal. If the rope has a tension of 70.0 N, what is the normal reaction
force acting on the sled?
4.
What would be the frictional force acting on a 3.0 kg box dragged over a surface with
which it has a coefficient of friction of 0.4?
5.
Calculate the coefficient of friction between a surface and a 2.0 kg object sliding across it
if there is a frictional force of 3.5 N acting on the object.
6.
A 12.0 kg pram is pulled over a footpath with a force of 130 N by means of a handle that
makes an angle of 25° with the horizontal. What is the coefficient of friction if there is a
frictional force of 8.0 N between the wheels of the pram and the footpath?
4.2 Motion on a level surface
7.
If a net force of 50.0 N acts on a 7 kg object sitting on a smooth surface, what will the
object’s acceleration be?
8.
What force will need to be exerted on a 1200 kg car to move it at a constant speed of
13.0 m s–1 across a surface that has a coefficient of friction of 0.78 with the wheels of the
car?
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QUEENSLAND PHYSICS
9.
A toy car with a mass of 3.0 kg is accelerated from rest at 2 m s–2 across a polished floor.
If the coefficient of friction between the wheels of the toy car and the floor is 0.24, what
was the force applied to accelerate the car?
4.3 Motion on an incline
10. A 100 N box is initially at rest on a perfectly smooth 15° slope. What will be the box’s
acceleration down the slope?
11. What force would need to be applied to prevent a 1400 kg car from sliding down a
slippery (µ = 0.12) 20° slope?
12. A force of 200 N is applied to push an 18.0 kg box up a 30° inclined ramp. What would
be the box’s acceleration if a frictional force of 35.0 N acts on it?
Review questions
Understanding
1.
Which of the following objects would you expect to have the smallest amount of inertia
and which would have the greatest?
(a)
A cricket ball after being struck by a bat
(b)
A ping-pong ball hit by a paddle
(c)
An 18-wheel truck travelling at 80 km h–1
(d)
A bullet fired from a gun
Explain your answer.
2.
Joseph has been asked by his mum to pull the garbage bin out to the kerb for the
collection next day. Joseph, not being a good physics student, points out to his mother
that there would be no point as Newton’s Third Law of Motion indicates that, when he
applies a force to the bin, the bin will apply a force back on him and so the bin would
remain stationary! Where is the flaw in his argument?
3.
The coefficient of friction between two metal surfaces becomes smaller as they are
polished, but increases again if they are polished further. Explain why this occurs.
4.
In a head-on collision between two cars, the drivers of both vehicles are thrown forwards
and then backwards. Explain why this happens.
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QUEENSLAND PHYSICS
5.
In November 1984, astronauts Joe Allen and Dale Gardner salvaged a Westar-6
communications satellite in space and placed it into the cargo bay of the space shuttle
Discovery. Describing the experience, Allen said of the satellite, ‘It’s not heavy, it’s
massive.’ What did he mean?
Application
6.
What would be the normal reaction force acting in each of the following cases?
(a)
(b)
(c)
(d)
7.
(a)
A car travels at a constant speed of 20 m s–1 along a dry road. If the engine provides
a force of 7000 N and the car has a mass of 1200 kg, what is the coefficient of
friction acting between the wheels and the surface of the road?
(b)
If the road were wet, would the car’s engine need to provide less force or more
force to keep the car travelling at that speed? Explain.
8.
Susan pulls her sled across the surface of packed snow at a constant speed by means of a
towrope that makes an angle of 40° with the horizontal. If the weight of the sled is 100 N
and the coefficient of friction is 0.14, what is the force with which she pulls the rope?
9.
A child drags a 3 kg boogie board across the sand by means of a rope that makes an angle
of 40° with the horizontal. If the tension in the rope is 4.0 N and the board is being
dragged with a constant speed, determine the coefficient of friction between the board
and the sand.
10. A 1200 kg car travelling at 22 m s–1 loses engine power and coasts to a halt on a dry road.
How long does the car take to stop if the coefficient of friction for the road and the tyres
is 0.3?
11. A 2-tonne truck accelerates from rest over a distance of 200 m to a speed of 15 m s–1.
What is the total friction acting on the car if its engine delivers a driving force of
10 000 N?
12. What is the force acting on a 120 g softball if it accelerates from rest to 28 m s–1 in 0.2 s?
13. A 12 kg box is pushed with a force of 100 N. What will happen to the acceleration if:
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QUEENSLAND PHYSICS
(a)
the size of the force is doubled?
(b)
the box has 8 kg removed from it?
14. How much force does a tennis racket exert on a 50 g ball if it strikes a racquet at 30 m s–1
and leaves it 0.5 s later with a speed of 35 m s–1 in the opposite direction?
15. A 10 kg trolley is pulled from rest across a carpeted floor. At some time during its
journey, a 4 kg weight is dropped onto it. The acceleration–time graph above right shows
its motion as it crosses the floor.
(a)
At what time was the weight dropped onto the trolley?
(b)
What was the trolley’s speed at that time?
(c)
If the trolley was pulled by means of a string exerting 150 N, what was the friction
constant throughout its journey? Explain.
(d)
What was the force of friction between the trolley and the carpet after 5 s?
16. A cyclist coasts down a 15° slope under gravity. If the bike and the rider together have a
mass of 110 kg and they have a net acceleration of 1.2 m s–2, determine:
(a)
the value of w||
(b)
the value of w
(c)
the net force acting down the slope
(d)
the total friction impeding their motion
(e)
the coefficient of friction.
Challenges
17. An 8-tonne truck loses brake pressure as it heads down past Cunningham’s Gap. The
driver points his rig at a safety ramp and crosses his fingers. If the truck stopped 10 m up
the 15° ramp and its speed was 35 km h–1 when he entered the ramp, what was the
frictional force acting on the truck?
18. A 50 kg crate is held on a frictionless ramp by means of a rope tied to a pole at the top
end of the ramp. If the ramp makes an angle of 40° with the floor:
(a)
What is the tension in the rope?
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QUEENSLAND PHYSICS
(b)
If the rope were to break, what would be the acceleration of the crate down the
slope?
19. A team of donkeys pull a cart full of tourists up a 20° hill in Greece. If the cart (including
tourists) has a mass of 600 kg and the coefficient of friction between the cart wheels and
the pathway is 0.56, how much force must be exerted by the donkeys if the cart:
(a)
travels at 3 m s–1?
(b)
accelerates at 1 m s–2?
20. The graph above right shows the acceleration experienced by a wooden block placed on a
concrete floor as it is pushed across the floor by a force FA.
From this graph, determine:
(a)
the limiting friction
(b)
the mass of the block
(c)
the sliding friction.
Notes:
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