For each case, identify the force (including its direction) that causes the acceleration. (a) a sprinter at the very start of the race. the gravitation force by earth on the sprinter the backward direction the normal force of the block on the sprinter the forward direction (b) a hockey puck skidding freely but slowly coming to rest on the ice. the opposite direction to the velocity the frictional force by the ice on the puck the parallel direction to the velocity the gravitation force by earth on the puck (c) a long fly ball at the top of its arc. the gravitation force by earth on the ball the upward direction the downward direction the frictional force by the air on the ball (d) a bungee jumper at the very bottom of her descent. the upward direction the gravitation force by earth on the jumper the force exerted by the bungee cord on the jumper the downward direction

Explanation:

a ) The sprinter at the very start of the race , pushes hard the ground under his feet in backward direction  . The harder he presses the ground backwards  , the greater reaction force he receives back in forward direction , throwing him forward  direction. This force creates acceleration.

b ) The frictional force acting in opposite direction on the motion of ball creates   - ve acceleration .

c ) The frictional force acting opposite to the motion of the ball creates - ve acceleration. The gravitational force acting on the ball in downward direction creates positive  acceleration on the ball.

d ) The restoring force created in the bungee chord due to its extension  creates acceleration in him in upward direction.

t2 equal the displacement of the particle over the same time period if t2> t/2? hint: consider a sine wave starting at t1=0, then consider a closing wave starting at t1=0, for your explanatio

a and b sometimes have a positive value