1. Motion, Forces & Energy (Mod I)
6th Science U1L3 - Forces!
Motion, Forces & Energy (Mod I)

2. Forces A force is a push or pull. Can cause an object to move.
A force is described by its size
& direction (so it’s a vector quantity!)
Measured in newtons (N)
Measured with a spring scale
Let’s try some demos! And draw arrows on board!

3. Contact force – act only if objects touch.
Pushing a heavy box
Friction (read p. 31 pgh 1)
Distance force - act even if objects do not touch.
Gravity
Electric charges
Magnets

4. The forces acting on an object, called the net force, are added together to determine their overall effect.
You should write down the info on the next slide, and draw the pictures!

5. - The Nature of Force

6. Unbalanced forces on an object cause its motion to change
Read p. 33 & do 9 & 10
Unbalanced forces on an object cause its motion to change
Balanced forces do not change an object’s motion.

7. - The Nature of Force

8. - The Nature of Force

9. Newton’s First Law of Motion Read p 34 & 35
An object at rest stays at rest, and an object in motion remains in motion at the same speed and direction, unless it experiences an unbalanced force.
Why do objects slow down?
(The force of friction and/or gravity. Note: Air resistance is a type of friction.)
Also called the Law of Inertia.
Mythbusters car crash -
Newton’s first law -

10. Inertia
Inertia is the tendency of an object to resist a change in its motion.
A more massive object has greater inertia than a less massive object.

11. Questions:
What happens to an object at rest if 2 balanced forces act on it?
(It remains at rest.)
Which has greater inertia, a beach ball or a bowling ball? Explain.
(The bowling ball because it has greater mass.)
An object moves in a circular path at a constant speed. Are the forces on the object balanced? Explain.
(Unbalanced. If the forces were balanced the object would move in a straight line at a constant speed.)

12. If there was no air friction & no gravity, what would happen when a baseball was thrown?
( The baseball would continue forever in a straight line.)
Why does a crash-test dummy continue to move forward when the car is stopped suddenly?
(The force stopped the car – not the crash-test dummy.)

13. Newton’s Second Law of Motion
The acceleration of an object depends on the mass of the object and the amount of force applied.
Read p. 36
acceleration = force ÷ mass
As force increases, force ÷ mass becomes larger, so acceleration increases.
When the mass increases, force ÷ mass becomes smaller, so acceleration decreases.
Do #16 pg. 36
Newton’s 2nd law -

14. • If a team pulls with a combined force of 9000N on an airplane with a mass of kg, What is the acceleration of the airplane?
• Half the people on the team decide not to pull the airplane. The combined force of those left is 4500N, While the airplane’s mass is still kg. What will be the acceleration?
• A girl pulls a wheeled backpack with a force of 3N. If the backpack has a mass of 6 kg. What is its acceleration?

15. If the same unbalanced force is applied to 2 objects on the ground, the object with the smaller mass has the greater acceleration.
The acceleration of an object increases as the unbalanced force on it increases.
An object accelerates in the direction of the unbalanced force acting on it.

16. Questions:
Suppose you are riding a bike. You stop to place a heavy object on the back of the bike. How will the increased mass affect the bike’s acceleration if you pedal with the same force?
(The acceleration will decreases because an increase in mass results in a decrease in acceleration if force remains constant.)
The same unbalanced force is applied to objects A, B, & C. Object B has the greatest acceleration, and object A the least. Rank the mass of the 3 objects.
(Mass of B is less than the mass of C is less than mass of A.)

17. Newton’s Third Law of Motion
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.
Also known as the Law of Action & Reaction
Read p. 38 pg. 1&2
Newton’s 3rd law -

18. - The Science of Rockets

19. 2 forces occur simultaneously as force pairs Read p. 38 pg. 3 – p. 39
Forces in pairs have equal size, but opposite directions
Forces in pairs can have unequal effects
Forces can act in multiple pairs

20. The swimmer pushing off the wall is the action force. Why?
(because the swimmer makes the first action of the force pair, not the wall)
Why is the wall the reaction force?
(the wall pushes back on the swimmer)
What objects are the swimmer and the wall acting on?
(the force from the swimmer acts on the wall, and the force from the wall acts on the swimmer)

21. Can you name a force pair at work?
(the force of the paddle on the water, and the force of the water on the paddle)
Which is the action and which is the reaction?
(Action: force of paddle on water Reaction: force of water on paddle)

22. What effect does the force of the ball have on the bat?
(It causes the bat to slow down)
What type of force made the ball move toward the batter?
(action force: the arm of the pitcher caused the ball to move)

23. An amusement park is the best place to see all of Newton’s three laws of motion in action.
1st Law  roller coaster
2nd Law  hammer on bell
3rd Law  bumper cars

24. Newton’s Second Law and You!