Scie 1001
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3. A 4-wheel drive truck has a mass of 2000kg and is at rest. When the driver engages the gear, the four drive wheels EACH push with a force of 1000N against the ground in a backward direction and so the truck moves forward.
(a) What is the name of the forward force causing the truck to move forward?
(b) Explain how the forward force is generated from this backward force.
(c) Calculate the value of the forward force on the truck.
(d) State Newton's Second law.
(e) Calculate the initial acceleration of the truck.
(f)Assuming that the force on the truck is constant, what is the velocity of the truck after 20 seconds? Ignore air resistance.
(g) What happens to the velocity of the car as air resistance increases to equal the forward force on the car?

Fig 1
Answer  

(a) Friction

(b) Newton's 3rd law states that for every action there is an equal and opposite reaction.
The tyre pushes against the ground so the ground pushes back against the tyre (friction) by an equal and opposite reaction.

(c) By Newton's third law the force is 4 X 1000N = 4000N

(d) An object accelerates
(i) in direct proportion to and
(ii) in the direction of a resultant force and
(iii) inversely proportional to its mass.
F = ma

(e) F = ma

a = F/m

a = 4000N/2000kg

a = 2ms-2.

(f) u = 0ms-1
a = 2ms-2
t = 20s

v = u + at
v = 0ms-1+ (2ms-2) (20s)
V = 40ms-1

(f) There is no resultant (or net) force on the truck so by Newton's first law the velocity remains constant.

 
   
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Concept by Kishore Lal. Programmed by Kishore Lal... Copyright © 2015 Kishore Lal. All rights reserved.