The forces acting on an object can be balanced or unbalanced. When sitting on a chair, the force of gravity pulls you downwards, towards Earth. The chair supports you by pushing upwards. This balances the downwards force of gravity.
Balanced forces don’t always mean that the object is stopped - it might be travelling at the same speed without changing directions. Consider a girl, about to ride her bike. To take off, she must accelerate, pedalling fast and hard to produce a force large enough to push her forwards. She needs to overcome the friction caused by the roughness of the road and by the air that she pushes through. To speed up, she keeps accelerating. Her pedalling needs to provide a driving force that is bigger than the force of friction acting in the opposite direction. As she continues her journey, she may travel at a constant speed without slowing down or speeding up. When this happens, the forwards force from her pedalling is cancelled out by the friction forces pushing her backwards. At this stage, her motion is constant and all of the forces acting on her are balanced.
Whenever the forces acting on the girl are unbalanced, her motion will change. Motion always changes in the direction of the unbalanced force. This means that the forces are unbalanced when she:
- Starts moving (by pedalling fast and hard)
- Speeds up (by pedalling faster and harder)
- Slows down (by using the breaks)
- Comes to a stop (by using the breaks)
- Changes direction (by turning the handlebars)
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