Force is a pull or push exerted by one object on another. You can use a spring scale to measure force. It’s marked off in units called newtons. Some forces oppose the motion of objects in contact with a surface. This is called friction. If you pull a block across a table, you’ll feel a drag along the surface. This is sliding friction acting on the block. Objects remain stationary in slanted surfaces due to static friction. You can skate along a sidewalk because of rolling friction. Friction between the skate wheels and the ground pushes you ahead. Friction acts anytime surfaces of objects are in contact.