The need for an external resultant force to change an object's state
Method
Through class discussion, point out the need for an external resultant force to change the state of an object in daily activities like the examples below, and fill in the table.
Example
First part (from rest to motion)
Second part (change of direction)
Third part (from motion to rest)
1. Players passing a football on a field
The resultant force of the first player's kick makes the ball roll in the direction of that force.
The ball rolls straight in the direction of the first player's force. The resultant force applied by another player changes its direction.
If no friction force arose between the ball and the ground, the ball would roll on for ever, which is impossible. Friction and the resultant external force applied by other players bring the ball to rest.
2. Driving a bus along a winding road
The effect of the resultant force applied by the engine
The effect of the resultant force applied through the steering wheel
The effect of the resultant force from the brakes and the friction with the ground
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