Physics
The force which makes a vehicle to stop when break is applied is called

Gravitational force
Covalant force
Frictional force
Vanderwalls force

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Physics
In a railway track, two rails are joined end to end with a gap in between them because

Accidents due to expansion in summer can be avoided
Air-gaps are necessary for bearing the weight of running train
Steel can be saved
Accidents due to contraction in winter can be avoided

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