The angle of the handle does not affect the force required to tow the suitcase at constant speed. The force needed to move an object at a constant speed is determined by its weight and the coefficient of friction between the object and the surface it is being towed on. As long as the angle of the handle does not change, the force needed to tow the suitcase will remain the same.
The weight of the suitcase directly affects the force required to tow it at a constant speed. The greater the weight of the suitcase, the more force is needed to overcome its inertia and move it at a constant speed. This force can be calculated using the formula F = ma, where F is the force, m is the mass (in this case, the weight divided by acceleration due to gravity), and a is the acceleration (in this case, zero since the suitcase is moving at a constant speed).
The coefficient of friction between the suitcase and the surface it is being towed on also directly affects the force required to move it at a constant speed. A higher coefficient of friction means there is more resistance between the suitcase and the surface, thus requiring more force to overcome this resistance and tow the suitcase at a constant speed.
It would be easier to tow a suitcase with a straight handle. As mentioned earlier, the angle of the handle does not affect the force required to tow the suitcase at a constant speed. However, a straight handle would provide a more efficient and direct transfer of force from the person towing the suitcase to the suitcase itself, making it easier to tow.
The surface type does affect the force required to tow the suitcase at a constant speed. Different surfaces have different coefficients of friction, which as mentioned earlier, directly affects the force needed to overcome resistance and tow the suitcase. For example, it would be easier to tow a suitcase on a smooth, flat surface like a concrete floor compared to a rough, uneven surface like gravel.