The force of friction can be reduced by the help of

A force which occurs when two surfaces come in contact and slows down motion while acting in

A. opposite direction

B. same direction

C. same and opposite direction

D. both a and b

Force can

A. speed up

B. slow down

C. stop an object

D. all of them

Friction between moving parts of machine causes

A. heat

B. light

C. energy

D. oil

When upthrust is equal to the weight of an object, it

A. floats

B. sinks

C. moves

D. stops

The mass of a substance per unit of volume is called its

A. weight

B. density

C. volume

D. gravity