Q10 of 14 Page 155

What is the cause of heat generation in the following situations: a) we apply brakes on our fast-moving car?

b) People often jump up and down to feel warmer in cold weather?

(a) Cause of heat generation on applying brakes:


When we apply brakes on a fast-moving vehicle a force of friction comes into play, and work is done. The force creates the work is done which in turn emerges as heat.


(b) People often jump up and down to feel warmer in cold weather


Jumping increases, the flow of oxygen in our body and increases the metabolism. Increased metabolism causes burning of fat and carbohydrates creating energy.


This energy comes out as heat. So people in cold country often jumps to keep themselves warm.


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A father and son pushed their car to bring it to the side of a road as it had stalled in the middle of the road. They experienced that they had to push with all their might initially to move the car. But the push required to keep the car rolling was smaller. Explain.

 9

If you try to hold a glass with soapy hands, it slips. Why?

 11

Explain why:

(a) A pencil will write on paper but not on glass.


(b) The handles of motor cycle are covered with a rubber sheet with spikes.


(c) The soles of our shoes wear out gradually.


(d) Tiers of car wear out gradually.

 12

Explain why machines parts are frequently oiled and greased.