Week 1

What is a force?

learning intentions

I can define ‘force’.
I can describe how forces can cause a change in speed, direction or shape.
I am able to distinguish between push forces, pull forces and twist forces.
I can tell the difference between contact forces and non-contact forces.
I can state the units used for measuring forces and explain how forces have both a magnitude and a direction.

Tasks

Knowledge & understanding

🍐 Balanced & Unbalanced forces

Investigation Skills

Prac: Forces Investigation activity

Literacy & research

Unscramble sentences

Knowledge and understanding

The scientific view of forces

In science we would say that a force is a push, or a pull or a twist, or a turn, or a bend - anything applied to an object to try to change its state of motion or to deform it.

In other words, a force can have one or more of the following effects on an object:

- - - - A change in speed.
        A change in direction.
        A change in shape.

Conversely, the only way an object can change speed, direction or shape is if a force is applied to it.

A force can be a push

... a pull

... or a change in shape

Force is measured in newtons (N).

A spring balance (or newton meter) is a type of weighing scale. They are commonly used to measure the force exerted on an object. This force is usually measured in Newtons. The balance contains a spring which provides a measure of the force applied to the balance. The larger the force, the more the spring extends, with the force level indicated by the scale on the balance.

Changes In Speed

Forces can cause objects to move faster or slower. That is, forces can cause either an increase in speed or a decrease in speed.

An increase in speed is known as acceleration. This may involve an object that is initially stationary (not moving) or an object that is already moving.

Example 1

If you jump out of a plane, you will fall with increasing speed.

Example 2

If you start pedalling harder when riding your bike, your speed will increase.

A decrease in speed is known as deceleration. If the force causing the deceleration continues for long enough, it will cause the object to stop moving.

Example 1

If a rolling ball approaches an upwards slope, its speed will decrease.

Example 2

If you apply the brakes on your bike, your speed will decrease. If you keep applying your brakes, you will eventually come to a stop.

Changes In Direction

Forces can change the direction that an object is moving in.

Example 1

If you hit a ball against a wall, the ball will rebound towards you.

Example 2

Wind can cause a moving object, such as a yacht, to change direction.

Changes In Shape

Forces can change the shape of an object.

Depending on the object, the change in shape may be temporary or permanent. An object that returns to its original shape once the force is no longer acting on it is known as an elastic object. An object that retains its new shape once the force is no longer acting on it is known as an inelastic object.

Example 1

Squeezing a rubber ball can change its shape, but when released, the ball will return to it original shape. A rubber ball is therefore an elastic object.

Example 2

Squeezing clay will cause a permanent change in its shape. Clay is therefore an inelastic object.

Balanced vs. Unbalanced Forces

Net force - is the sum of all the forces acting on an object, if there are more than one forces acting on an object.

The direction a force acts is important. They can act in opposite directions and cancel each other out. In this case we say that the net force is 0. If the net force is 0 then there is no change in the motion of the object. We can also describe the forces as balanced.

If the net force is anything other than 0 then there will be a change in motion. We say the forces are unbalanced. We call this change in motion acceleration

🍐 Balanced & Unbalanced Forces & Net Force

Investigation skills

Prac: Forces Investigation Activity

From the worksheet Activity Force Investigation on Google Classroom:

What does the graph tell you about the motion of the cart?

3. As you added more masses to the string, what did you observe about the cart’s motion? Does your answer to this question match your answer to question 2?

4. How could you increase the accuracy of your distance and time measurements?

5. The force on the cart, due to the masses, is unbalanced. What relationship is there between the cart’s speed and the unbalanced force acting on it? Does this match your answer to questions 2 and 3?

Challenge questions

6. For each set of results, calculate the cart’s average speed using the formula: average speed = distance / time. Explain any pattern in your answers.

7. Would the speed of the cart be different if the cart was heavier? (If you have time, test this by adding masses to the cart and repeating some of your earlier test runs).

Questions from Diagram 1

Describe what you think will happen in the experiment above - diagram 2.

William thinks that adding a small mass (10 g) to one side won’t cause the cart to move while Katie thinks it will. Who is correct? Explain your answer.

What will happen to the cart if 50 g is removed from end A? Explain your answer.

What happens to the speed of the cart as more masses are removed from end A? Explain your answer.

If the cart is stationary, are the forces on it balanced or unbalanced? Explain your answer.

Discussion question

Equal masses are placed on ends A and B, then the cart given a gentle push. The cart moves in the direction of the push, then slowly comes to a stop. William thinks the cart comes to a stop because the forces are balanced so there will be no motion. Katie thinks that the cart comes to a stop because of friction. Who is correct? Explain your answer.

Literacy & research

Rewrite these sentences correctly.

You may need to completely restructure them.

A moving object can change its direction of travel without having a force applied to it
Moving objects need a force on them to slow them down.
When an object falls it goes faster without any force being applied to it.
A seesaw only needs two people to work it. No forces are necessary.
Magnets attract each other because of magnetism, not because of any forces.
List five observations that would tell you that a force was being applied to an object.

Click here for the answers

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