(a) As the car is moving with constant velocity, it means the rate change of velocity does not change, therefore the average acceleration of the car is zero.
Thus, there is no acceleration, when velocity is constant.
(b) Average acceleration,
Here, v is final velocity and u is the initial velocity and 
is the time interval.
As twelve seconds later, the car is halfway around the track and traveling in the opposite direction with the same speed, therefore
Thus, the average acceleration of the car is 
in the direction to the left.
Answer:
Gravitational
Tension
Normal
Friction.
Explanation:
The forces acting on the sled are:
Tension: the tension from the rope, this is the force that "moves" the sled.
Friction: kinetic friction between the sled and the ground as the sled moves.
There are another two forces that also act on the sled, but that "has no effect"
Gravitational force: This force pulls the sled down, against the floor.
Normal force: This force "opposes" to the gravitational one, so they cancel each other.
These two forces cancel each other, so they have no direct impact on the movement of the sled. BUT, the friction force depends on the weight of the moving object, and the weight of the moving object depends on the gravitational force, so we need gravitational force in order to have friction force.
Then we can conclude that the forces acting on the sled are:
Gravitational
Tension
Normal
Friction.
