B. Friction can be a centripetal force, such as when it keeps a car on the road going around a curve.
C. Gravity can be a centripetal force, such as when it pulls a satellite in its orbit.
Explanation:
The centripetal force is any force that keeps an object moving in circular motion, "pulling" the object towards the centre of the circular trajectory.
Several forces can act as centripetal force. Examples are:
- friction: when a car is going around the curve, is moving by circular motion. The force that keeps the car in circular motion is, in fact, the friction between the tires and the road.
- Gravity: when a satellite moves around the Earth, it is moving by circular motion. The force that keeps the satellite in circular motion is the gravitational attraction between the Earth and the satellite, that pushes the satellite towards the Earth.
The other two options are not correct because:
A) An object can also accelerate if there is no centripetal force (for example, a car speeding up on a straight road is accelerating, but there is no centripetal force since there is no circular motion
D) Centripetal force is not an outward force, since it pushes the object inwards (towards the centre of the trajectory).
