12.6 CHAPTER 12. FORCE,MOMENTUM AND IMPULSE
The diagram shows a crate being held stationary on the ramp by means of a rope
parallel to the ramp. The tension in the rope is 5000 N.
15 ◦
Elephants
5000 N
(a) Explain how one can deduce the following:“The forces acting on the crate are
in equilibrium”.
(b) Draw a labelled free-body diagram of theforces acting on the crane and ele-
phant. (Regard the crate and elephant as oneobject, and represent them as a
dot. Also show the relevant angles between theforces.)
(c) The crate has a massof 800 kg. Determine the mass of the elephant.
(d) The crate is now pulled up the ramp at a constant speed. How does the crate
being pulled up the ramp at a constant speed affect the forces acting on the crate
and elephant? Justify your answer, mentioning any law or principle thatapplies
to this situation.
- [IEB 2002/11 HG1- Car in Tow]
Car A is towing Car B with a light tow rope. The cars move along a straight, horizontal
road.
(a) Write down a statement of Newton’s SecondLaw of Motion (in words).
(b) As they start off, Car A exerts a forwards force of 600 N at its endof the tow
rope. The force of friction on Car B when it starts to move is 200 N. Themass of
Car B is 1 200 kg. Calculate the acceleration of Car B.
(c) After a while, the cars travel at constant velocity. The force exertedon the tow
rope is now 300 N while the force of friction on Car B increases. What is the
magnitude and directionof the force of friction on Car B now?
(d) Towing with a ropeis very dangerous. A solid bar should be used inpreference
to a tow rope. This is especially true should CarA suddenly apply brakes. What
would be the advantageof the solid bar over thetow rope in such a situation?
(e) The mass of Car A isalso 1 200 kg. Car A and Car B are now joinedby a solid
tow bar and the total braking force is 9 600 N. Over what distance could the
cars stop from a velocityof 20 m·s−^1?
- [IEB 2001/11 HG1]- Testing the Brakes of aCar
A braking test is carriedout on a car travellingat 20 m·s−^1. A braking distance
of 30 m is measured when a braking force of 6 000 N is applied to stop the car.
(a) Calculate the acceleration of the car when abraking force of 6 000 Nis applied.
(b) Show that the mass of this caris 900 kg.
(c) How long (in s) does it take for this car to stop from 20 m·s−^1 under the braking
action described above?
(d) A trailer of mass 600 kg is attached to thecar and the braking testis repeated
from 20 m·s−^1 using the same brakingforce of 6 000 N. Howmuch longer will
it take to stop the car with the trailer in tow?
- [IEB 2001/11 HG1]A rocket takes off fromits launching pad, accelerating up into
the air. Which of the following statements bestdescribes the reason forthe upward
acceleration of the rocket?
(a) The force that the atmosphere (air) exerts underneath the rocket is greater than
the weight of the rocket.
(b) The force that the ground exerts on the rocket is greater than the weight of the
rocket.
(c) The force that the rocket exerts on the escaping gases is less than the weight of
the rocket.
