We have to use the Newton's second law of motion here.
F = ma
Since the mass of the object is constant the force time graph will depend on the acceleration.
According to the figure the motion has three stages.
- In the first stage the object goes in an constant accelerations because in the velocity time graph we have a constant gradient. (gradient of a V-T graph gives accelerations) So we have a positive force here.
- Then the object moves with a constant velocity where no external force will act on it.
- At the latter end the object have a constant deceleration where the force will act on the opposite side of the motion.
These points are included in the force time graph in below figure.
The y axis represent force and x represent time.
Yellow line shows the acceleration region
Blue line shows the constant speed region
Red line shows the deceleration region.