12.3 CHAPTER 12. FORCE,MOMENTUM AND IMPULSE
(b) If the acceleration of the block is 1,5 m·s−^2 , calculate the magnitude of the frictional force
on the block.
(c) Calculate the vertical force exerted by the block on the plane.
- A toy rocket of mass 0,5 kg is supported vertically by placing it ina bottle. The rocket is then
ignited. Calculate the force that is required to accelerate the rocket vertically upwards at 8 m·s−^2.
- A constant force of70 N is applied vertically to a block of mass 5kg as shown. Calculatethe
acceleration of the block.
5 kg
70 N
- A student of mass 70 kg investigates the motion of a lift. While he stands in the lift on a bathroom
scale (calibrated in newton), he notes three stages of his journey.
(a) For 2 s immediatelyafter the lift starts, the scale reads 574 N.
(b) For a further 6 s it reads 700 N.
(c) For the final 2 s it reads 854 N.
Answer the following questions:
(a) Is the motion of thelift upward or downward? Give a reason for youranswer.
(b) Write down the magnitude and the directionof the resultant force acting on the student for
each of the stages I, II and III.
(c) Calculate the magnitude of the accelerationof the lift during the first2s.
- A car of mass 800kg accelerates along alevel road at 4 m·s−^2. A frictional force of 700 N
opposes its motion. What force is produced by the car’s engine?
- Two objects, with masses of 1 kg and 2 kgrespectively, are placedon a smooth surface and
connected with a pieceof string. A horizontal force of 6 N is applied with the help of a spring
balance to the 1 kg object. Ignoring friction, what will the force actingon the 2 kg mass, as
measured by a second spring balance, be?
1 kg 2 kg
6 N
?
- A rocket of mass 200 kg has a resultant force of 4000 N upwards onit.
(a) What is its acceleration in space, where it has no weight?
(b) What is its acceleration on the Earth, whereit has weight?
(c) What driving force does the rocket engine need to exert on the back of the rocket in space?
(d) What driving force does the rocket engine need to exert on the back of the rocket on the
Earth?
