Force and Motion(Continued)

Objective 4.04

Determine that an unbalanced force is needed to move an object or change its direction.

What is Force?(Review)

• A force is a push or pull on an object. Applying a force to an object can change its motion. There are two main categories of forces: balanced and unbalanced.

• Balanced forces cause an object to remain in place. A bench on the street could represent a balanced force. The force of gravity pulling the bench down is balanced by the force of the street pushing the bench up.

• Unbalanced forces can cause an object to start moving, stop moving or change direction. Unbalanced forces change the motion of objects. The change in motion depends on three main factors: 1. direction of force applied 2. strength of the force 3. the mass of the object

Balanced and Unbalanced Forces

• "Unchanging motion" is when the body is at rest or is moving with a steady speed in a straight line. Balanced forces are responsible for unchanging motion.

• Balanced forces are forces where the effect of one force is cancelled out by another. A tug of war, where each team is pulling equally on the rope, is an example of balanced forces. The rope will have an acceleration of zero under the action of these balanced forces. It will therefore remain stationary.

Balanced Forces

• When two forces act upon each other in opposite directions, and they are both the same size, we say they are balanced. You can see balanced forces in a tug-of-war game when the rope is not moving.

Unbalanced Forces

• "Changing motion" includes movement where the body is speeding up or slowing down. It also includes motion where the direction in which the body moves is changing ie the body is moving in a curved path. It is unbalanced forces that cause "changing motion".

• The lift-off of a space shuttle is an example of an unbalanced force in action. The space shuttle accelerates upwards from its launch pad. The thrust T from the rocket engines is greater than the weight W of the rocket system. Because force T is greater than the force W, the effect of one force does not cancel that of the other. The forces acting are unbalanced.

Direction of Application

• The direction in which the force is applied determines the direction of motion. The first scene of Figure 14.1 shows a foot that is just about to kick a ball. The force is applied to the ball by the foot. The direction the force is applied is shown by an arrow. The second scene of Figure 14.1 shows what happens after the foot kicks the ball. The ball moves in the direction of the applied force.

Remember that the angle at which the force is applied is also important. Figure 14.2 shows that the soccer ball could be kicked in two different angles. You see that the motion that results could be in two different directions. Force A results in Motion A. Force B results in Motion B.

Strength of Force

• So, an applied force will move an object in the direction the force was applied. How much the object moves depends on how much force is applied. Kicking the ball hard delivers a lot of force, so the ball moves a long way. Kicking the ball gently delivers less force, so the ball does not move far.

In Figure 14.3, we can see the aggressive player kicks the ball hard, and it moves a long distance. The lazy player kicks the ball softly, and it only moves a short distance.

Mass of Object• Mass also affects the motion of the object.

The mass of an object describes how much matter it contains. What happens if a soccer ball and a bowling ball are each kicked with the same amount of force?

• The soccer ball will move much farther than the bowling ball. Why? Because the bowling ball has more mass. The greater the mass of an object, the more force must be applied to move the object. So, to move a bowling ball the same distance as the soccer ball, you must use more force on the bowling ball. That is one reason we use simple machines: to multiply the force that we apply.

• Moving an object is considered work. Work takes energy. Energy is needed to make objects and simple machines move. Energy is the ability to do work.

Review

Are these pictures examples of balanced forces or unbalanced forces?

Unbalanced forces

The figures show a block of wood on a table. When the block is pulled at point A, it begins to move towards the left and if the block is pulled at the point B it moves towards the right.

An unbalanced force acting on an object changes its speed or direction. The resultant of unbalanced forces is always greater than zero.

Is this picture an example of balanced forces or unbalanced forces?

Balanced forces

If the block is pulled from both sides with the same effort the block remains stationary. The forces are equal and opposite and therefore the block does not move. The resultant of the forces acting on the body is zero.

Is this picture an example of balanced forces or unbalanced forces?

Balanced forces

In a game of tug of war when both the teams pull the rope with equal and opposite forces, then the rope remains stationary as the forces acting on it are equal and opposite and their resultant is zero.

Is this picture an example of balanced forces or unbalanced forces?

Unbalanced forces

In tug of war, usually the weaker team is pulled by the other team, which is stronger. In this case, the forces are not equal and opposite i.e., they are not balanced and hence the weaker team is pulled by the stronger team.

A class puts together the model car ramp shown below. Use the figure to answer questions 1-3.

1. Amy’s car is placed at the top of the ramp. She releases the car and it rolls to Stop A. What could Amy do to help the car move farther?

A. add mass to the carB. remove mass from the carC. clap for the carD. paint the car

2. Which student’s car will stop at Stop D?

A. Amy’s car (10 grams)B. Serge’s car (14 grams)C. Tanisha’s car (16 grams)D. Dora’s car (20 grams)

3. Which of the cars from the previous question will stop at stop B?

A. Amy’s car (10 grams)B. Serge’s car (14 grams)C. Tanisha’s car (16 grams)D. Dora’s car (20 grams)

4. What best describes the direction of Earth’s gravitational forces?

A. It pulls objects closer together.

B. It keeps objects from getting too close.

C. It pulls objects closer to Earth.

D. It pulls objects away from the air.

5. Which of the following will have the least effect on the motion of a ball?

A. the mass of the ballB. the direction of the forceC. the strength of the forceD. the weather conditions

The End!!!

Forces in Motion

http://www.bbc.co.uk/schools/ks2bitesize/science/physical_processes/forces_action/play_popup.shtml

How does changing the gradient affect the car's speed?