Explanation
In cycling, the rear wheel moves by the force communicated to it by pedalling while front wheel moves by itself. So, while pedalling a bicycle, the force exerted by rear wheel on ground makes force of friction act on it in the forward direction (like walking). Front wheel moving by itself experience force of friction in backward direction (like rolling of a ball.) However, if pedalling is stopped both wheels move by themselves and so experience force of friction in backward direction,
Statement I is practical experience based; so it is true. Statement II is also true but is not the correct explanation of statement I. Correct explanation is “there is increase in normal reaction when the object is pushed and there is decreases in normal reaction when object is pulled”.
On a rainy day, the roads are wet. Wetting of roads Iowers the coefficient of friction between the tyres and the road. Therefore, grip of car on the road reduces and thus chances of skidding increases.
When a bicycle is in motion, two cases may arise
(i) When the bicycle is being pedalled. In this case, the applied force has been communicated to rear wheel. Due to which the rear wheel pushes the earth backwards. Now the force of friction acts in the forward direction on the rear wheel but front wheel moves forward due to inertia, so force of friction works on it in backward direction.
(ii) When the bicycle is not being pedalled:
In this case both the wheels move in forward direction, due to inertia. hence force of friction on both the wheels acts in baclcward direction.
In sliding down, the entire potential energy of body is converted only into translational energy. While in rolling motion, some part of potential energy is converted into kinetic energy of rotation and rest into kinetic energy of translation. Therefore, in sliding motion, the velocity acquired by the body is more.
Only static friction is a self adjusting force. This is because force of static friction is equal and opposite to applied force (so long as actual motion does not start). Frictional force = µmg i.e., friction depends on mass.
Bearings are used to reduce friction.
Coefficient of friction is constant for two given surfaces in Contact. It does not depend upon the weight or normal reaction.
Horizontal velocity of apple will remain same but due to retardation of train, velocity of train and hence velocity of boy w.r.t. ground decreases, so apple falls away from the hand of boy in the direction of motion of the train.
Horizontal velocity of ball and person are same so both will cover equal horizontal distance in a given interval of lime and after following the parabolic path the ball falls exactly in the hand which threw it up.
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