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

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!

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

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!

- The Nature of Force

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.

- The Nature of Force

- The Nature of Force

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.

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.

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.)

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.)

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

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.

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.)

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

- The Science of Rockets

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

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)

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)

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)

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

Newton’s Second Law and You!