A horse pulls a cart along a level road, causing the cart to accelerate. Newton\

Question

A horse pulls a cart along a level road, causing the cart to accelerate. Newton's third law states that the cart exerts an equal and opposite force on the horse. In view of this, how can the cart accelerate? Under w hat conditions does the system (horse plus cart) move with constant velocity? A 5.00 g bullet leaves the muzzle of a rifle with a speed of 320 m/s. The expanding gases behind it exerts what force on the bullet while it is traveling down the barrel of the rifle. 0.820 m long? Assume constant acceleration and negligible friction.

Explanation / Answer

Problem 1 :

The two equal and opposite forces are acting on two different objects .

force by horse on the cart is acting on the cart towards right

and reaction force by cart on the horse acts at horse in left direction

since the two forces do not act on the same object (cart) , hence the forces don't cancel out

there is net force on cart by the horse in right direction

hence the cart accelerate in right direction

the system moves at constant velocity only when the net force on the system becomes 0 .

problem 2 :

Vo = initial speed = 0 m/s

Vf = final speed = 320 m/s

d = distance travelled = 0.820 m

a = acceleration

using the equation

Vf2 = Vo2 + 2 a d

3202 = 02 + 2 a (0.820)

a = 6.2 x 104 m/s2

m = mass = 5 g = 0.005 kg

Force is given as

F = ma = 0.005 x 6.2 x 104 = 310 N

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