8.4. A Force can Change the State of Motion

Let us now find out what happens when a force acts on an object.

Activity 8.3 Take a rubber ball and place it on a level surface such as a table top or a concrete floor. Now, gently push the ball along the level surface (Fig. 8.6). Does the ball begin to move? Push the ball again while it is still moving. Is there any change in its speed? Does it increase or decrease?

This chunk introduces the concept of force and how it can affect the state of motion of an object. When you apply a force to a rubber ball, it can change from rest to motion. This is observed as the ball begins to move once pushed. Additionally, if you continue to push the ball while it is already in motion, the speed might increase or decrease based on the strength and direction of the applied force.

Imagine you are on a skateboard. When you push off the ground with your foot (the force), you start moving. If you push harder, you speed up; if you try to stop with your foot, you will slow down. This is just like how the ball behaves when you push it.

Next, place your palm in front of the moving ball. Remove your palm as soon as the moving ball touches it. Does your palm apply a force on the ball? What happens to the speed of the ball now? Does it increase or decrease? What would happen if you let your palm hold the moving ball?

This chunk emphasizes the interaction between moving objects and forces. When the palm meets the ball, it applies a force that can change the ball's motion. If the palm is removed when the ball touches it, the ball continues moving with its previous speed. If the palm holds onto the ball, it applies a force in the opposite direction, slowing it down and potentially stopping it.

Think of a game of football where a player tries to stop the ball with their foot. If they hit the ball at the right angle, the ball might still move forward, but if they grab the ball, it stops. Both scenarios demonstrate how exerted forces influence the motion.

You might recall similar situations. For example, while taking a penalty kick in football, the player applies a force on the ball. Before being hit, the ball was at rest and so its speed was zero. The applied force makes the ball move towards the goal. Suppose, the goalkeeper dives or jumps up to save the goal. By his action, the goalkeeper tries to apply a force on the moving ball. The force applied by him can stop or deflect the ball, saving a goal being scored. If the goalkeeper succeeds in stopping the ball, its speed decreases to zero.

This chunk uses the example of a football penalty kick to illustrate the effect of force on speed. When the player kicks the stationary ball, the force they apply changes the ball's state from rest to moving. Conversely, when the goalkeeper applies a force to intercept the ball, they can either stop it or change its direction. This shows how different forces can alter the speed and trajectory of moving objects.

Consider how you push a shopping cart. The harder you push (force), the faster it goes. If someone pulls it back while you are pushing, it slows down or stops completely. This shows a clear relationship between force and the changes in speed.

These observations suggest that a force applied on an object may change its speed. If the force applied on the object is in the direction of its motion, the speed of the object increases. If the force is applied in the direction opposite to the direction of motion, then it results in a decrease in the speed of the object.

This chunk clarifies two important outcomes of applying force: an increase or decrease in speed. If the force acts in the same direction as movement, the object speeds up; if it acts against the motion, the object slows down. Understanding this principle is crucial when analyzing motion in sports, vehicles, and many daily activities.

Imagine riding a bicycle downhill; pedaling faster (force in the same direction) makes you go faster. If you stop pedaling or pull the brake (force in the opposite direction), you will slow down. This easily illustrates how force affects speed.

A change in either the speed of an object, or its direction of motion, or both, is described as a change in its state of motion. Thus, a force may bring a change in the state of motion of an object.

This chunk defines what is meant by a change in the state of motion. It highlights that motion can change in terms of speed or direction due to the application of force. Understanding this concept is vital for grasping the principles of dynamics and how different forces interact with objects to induce motion.

Think of driving in a car. If you turn the steering wheel (applying force), the direction of the car changes. If you press the gas pedal (applying force), the speed increases. Both actions showcase how forces lead to changes in motion.

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