Let's break this down so it's easy to understand:
Imagine the crate as a big, heavy box sitting on the factory floor. Now, the floor and the bottom of the crate are not perfectly smooth, even if they seem that way to us. If you could zoom in really close, you would see they have lots of tiny bumps and ridges.
Friction is a force that opposes the motion between two surfaces that are in contact, like the bottom of the crate and the floor. It's kind of like these tiny bumps are trying to "grip" each other.
Now let's go through each option:
a. "only if the crate is pushed vertically as well as horizontally"
- This means that friction would only happen if you're pushing down on the crate (vertically) and also to the side (horizontally) at the same time. But that's not quite true. Even if you're not pushing at all, the weight of the crate is pushing it down onto the floor, so friction can still happen.
b. "only if the crate is sliding"
- This means that friction would only happen if the crate is moving across the floor. This is a type of friction called "kinetic friction". But there's also another type of friction called "static friction" which occurs even when the crate is not moving. Static friction is what keeps the crate from sliding around every time there’s a slight disturbance, like a breeze.
c. "even when no pushing occurs"
- This is true! Like we just discussed, even if the crate is just sitting there and not being pushed, there's still friction. The weight of the crate pushes it against the floor, and the tiny bumps and ridges on the surfaces are gripping each other. This type of friction is called "static friction".
d. "when the crate is pushed horizontally, whether sliding or not"
- This means that if you push the crate to the side, there's friction whether it moves or not. This is also true! When you start to push the crate, static friction is what you have to overcome to get it to start moving. Once it's moving, kinetic friction is what opposes the motion.
The most encompassing and accurate answer is c. even when no pushing occurs because it takes into account the friction that is present due to the weight of the crate even when it's just sitting still, which is static friction. However, option d is also true but is more specific to when the crate is being pushed horizontally.