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## Physics Tutorials

### Ice Hockey and Newton's Third Law

Newton's third law states that whenever one body exerts a force on another body, the second body exerts an oppositely directed force of equal magnitude on the first object.

Explanation: This tells you, if someone pushes horizontally against a wall with a force of 100 N, the wall will push horizontally against the person with a force of 100N. This law is very important because if it was not true, the force of gravity would pull us straight to the center of the Earth. Instead, when our weight pushes against the ground, the ground pushes against us. Normal force is the force that pushes up against our bodies. Therefore, this law keeps us planted to the ground when we walk.

#### Skating and Stopping in Ice Hockey

Newton's third law comes into use when you play ice hockey. Newton's third law tell you that there an equal and opposite reaction force for every action force. When you push with one of your skates backwards on the ice, the ice then pushes forward on the skate, sending you forwards.

Another use of Newton's third law is when you stop. In order to stop, you have to position your body so that your body from waist below is perpendicular to the direction of your travel. This results in placing the sharp part of the blade against the ice surface. As you stop, the sharp edges cut into the ice so that it is deep enough for a barrier to be created. The barrier pushing against you stops you. The reaction force is you pushing against the barrier. The harder you push on the barrier, the harder the barrier pushes against you, stopping faster.

#### Question

When you are hitting a baseball with a bat if we call the force on the bat against the ball the action force, what is the reaction force?

Solution: The reaction force is the ball hitting the bat.

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Michael Jassal