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4-2.Friction
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A small child tries to move a large rubber toy placed on the ground. The toy does not move but gets deformed under her pushing force $F$, which is obliquely upward as shown in the figure.Then, 
Athe resultant of the pushing force $F$, weight of the toy, normal force by the ground on the toy and the frictional force is zero
Bthe normal force by the ground is equal and opposite to the weight of the toy
Cthe pushing force $F$ of the child is balanced by the equal and opposite frictional force
Dthe pushing force $F$ of the child is balanced by the total internal force in the toy generated due to deformation
(KVPY-2011)
Solution
(a)
Taking boy, toy and ground as a composite system, we can say that there is no external force acting on the system, net acceleration of the system is zero.
$A_{\text {system }}=0 \Rightarrow\left(F_{\text {net }}\right)_{\text {system }}=0$
$\Rightarrow F +(m g)_{\text {boy }}+(N)_{\text {ground }}+(f)_{\text {friction }}=0$
Taking boy, toy and ground as a composite system, we can say that there is no external force acting on the system, net acceleration of the system is zero.
$A_{\text {system }}=0 \Rightarrow\left(F_{\text {net }}\right)_{\text {system }}=0$
$\Rightarrow F +(m g)_{\text {boy }}+(N)_{\text {ground }}+(f)_{\text {friction }}=0$
Standard 11
Physics
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