You generally can't mix Physics with other behaviours.
From the manual:
Manual movement & other behaviors
If you move objects by events (e.g. set X, set Y) or other behaviors (e.g. also adding 8 direction to a physics object), the physics simulation will do its best to keep up with what you've done. However, it is usually more realistic to achieve the same thing by applying forces and impulses to physics objects. This keeps everything "in the physics world" and realistic.
For example, if you use Set Position to move an object to the other side of the layout instantly, it has effectively teleported. That's not a realistic physical phenomenon, so the result may be unrealistic as well. To keep up, the physics behavior will spot this, and simulate the object suddenly moving extremely fast towards its destination point for one tick (about 1/60th of a second). That's an incredible amount of acceleration, speed, then deceleration. Remember, to keep a simulation stable, you should avoid extremes.
Although you can add the Platform behavior to a Physics object, the two tend not to get along very well. Again, it's better to achieve the same thing by applying forces. See the Physics - rolling platformer example provided with Construct 2 for a demonstration of how you can achieve this.